General Equivalence Mappings 2012

Diagnosis Code Set 

General Equivalence Mappings

2012 Version Documentation

Appendix A—File and Format Detail


ICD-9-CM to ICD-10-CM
General Equivalence Mapping (GEM)

FILE FORMAT

FIELD	POSITION	LENGTH	VALUE
ICD-9-CM Code [source]	1 – 5	5	Left justified, blank filled No decimal
Filler	6	1	Blank
ICD-10-CM Code [target]	7 – 13	7	All seven characters used, no decimal
Filler	14	1	Blank
Approximate [FLAG]	15	1	1 = Yes/On 0 = No/Off
No Map [FLAG]	16	1	1 = Yes/On 0 = No/Off
Combination [FLAG]	17	1	1 = Yes/On 0 = No/Off
Scenario	18	1	0 – 9
Choice list	19	1	0 – 9

ICD-10-CM to ICD-9-CM
General Equivalence Mapping (GEM)


FILE FORMAT

FIELD	POSITION	LENGTH	VALUE
ICD-10-CM Code [source]	1 – 7	7	Left justified, blank filled No decimal
Filler	8	1	Blank
ICD-9-CM Code [target]	9 – 13	5	All seven characters used, no decimal
Filler	14	1	Blank
Approximate [FLAG]	15	1	1 = Yes/On 0 = No/Off
No Map [FLAG]	16	1	1 = Yes/On 0 = No/Off
Combination [FLAG]	17	1	1 = Yes/On 0 = No/Off
Scenario	18	1	0 – 9
Choice list	19	1	0 – 9

Glossary

Approximate flag — attribute in a GEM that when turned on indicates that the entry is not considered equivalent

Applied mapping — distillation of a reference mapping to conform to the needs of a particular application (e.g., data quality, research)

Backward mapping — mapping that proceeds from a newer code set to an older code set

Cluster — in a combination entry, one instance where a code is chosen from each of the choice lists in the target system entry, that when combined satisfies the equivalent meaning of the corresponding code in the source system

Choice list — in a combination entry, a list of one or more codes in the target system from which one code must be chosen to satisfy the equivalent meaning of a code in the source system

Combination flag — attribute in a GEM that when turned on indicates that more than one code in the target system is required to satisfy the full equivalent meaning of a code in the source system

Combination entry — an entry in a GEM for which a code in the source system must be linked to more than one code option in the target system to be a valid entry

Complete meaning [of a code] — all correctly coded conditions or procedures that would be classified to a code based on the code title, all associated tabular instructional notes, and all index references that refer to a code

Forward mapping — mapping that proceeds from an older code set to a newer code set

General Equivalence Map (GEM) — reference mapping that attempts to include all valid relationships between the codes in the ICD-9-CM diagnosis classification and the ICD-10-CM diagnosis classification

ICD-9-CM — International Classification of Diseases 9th Revision Clinical Modification (I-9)

ICD-10-CM — International Classification of Diseases 10th Revision Clinical Modification (I-10)

No map flag — attribute in a GEM that when turned on indicates that a code in the source system is not linked to any code in the target system

Reference mapping — mapping that includes all possible valid relationships between a source system and a target system


Reverse lookup — using a GEM by looking up a target system code to see all the codes in the source system that translate to it

Scenario — in a combination entry, a collection of codes from the target system containing the necessary codes that when combined as directed will satisfy the equivalent meaning of a code in the source system

Single entry — an entry in a GEM for which a code in the source system linked to one code option in the target system is a valid entry

Source system — code set of origin in the mapping; the set being mapped ‘from’

Target system — destination code set in the mapping; the set being mapped ‘to’

Step 3: REFINE

Step 3: REFINE

REFINE Select the row(s) of an entry that meet the requirements of the applied mapping.

What is the purpose of the applied mapping? Does the applied mapping require that the code in the source system be mapped to only one “best” alternative in the target system? Will the correct applied mapping vary depending on the documentation in the record?

Once the user has analyzed all rows for an entry in a GEM, it is possible to select the row or rows most appropriate to an applied mapping. We will use two different sample entries—one combination entry from the ICD-9 to ICD-10 GEM and one single entry from the ICD-10 to ICD-9 GEM—in order to discuss the process of refining an entry and deriving an applied mapping.

Sample Entry 1—ICD-9 to ICD-10 GEM
896.2 Traumatic amputation of foot (complete) (partial), bilateral, without mention of complication

ICD-9 Code	ICD-9 Description	ICD-10 Code	ICD-10 Description	Approximate [FLAG]	No Map [FLAG]	Combination [FLAG]	Scenario	Choice list
896.2	Traumatic amputation of foot (complete) (partial), bilateral, without mention of complication	S98.011A	Complete traumatic amputation of right foot at ankle level, initial encounter	1	0	1	1	1
896.2	Traumatic amputation of foot (complete) (partial), bilateral, without mention of complication	S98.012A	Partial traumatic amputation of left foot at ankle level, initial encounter	1	0	1	1	2
896.2	Traumatic amputation of foot (complete) (partial), bilateral, without mention of complication	S98.021A	Complete traumatic amputation of right foot at ankle level, initial encounter	1	0	1	1	1
896.2	Traumatic amputation of foot (complete) (partial), bilateral, without mention of complication	S98.022A	Partial traumatic amputation of left foot at ankle level, initial encounter	1	0	1	1	2

In this instance the ICD-9 code specifies that the traumatic amputation is bilateral but does not specify whether it is partial or complete. Since both types of information—left or right foot, and whether the amputation is partial or complete—are specified in separate codes in ICD-10, the entry in the ICD-9 to ICD-10 GEM is a combination entry. There are two choice lists in this entry, because two codes in ICD-10 are required to satisfy the equivalent meaning in the ICD-9 combination code. And because the injury can be partial on one side and complete on the other, both sides partial, or both sides complete, there are two choices in each choice list.

After collating the combination entries into their respective choice lists (there is only one scenario), the four valid clusters are:

ICD-9-CM Source	to	ICD-10-CM Target
896.2 Traumatic amputation of foot (complete) (partial), bilateral, without mention of complication	≈ ≈ ≈ ≈	S98.011A Complete traumatic amputation of right foot at ankle level AND S98.012A Complete traumatic amputation of left foot at ankle level
		OR
		S98.011A Complete traumatic amputation of right foot at ankle level AND S98.022A Partial traumatic amputation of left foot at ankle level
		OR
		S98.021A Partial traumatic amputation of right foot at ankle level AND S98.012A Complete traumatic amputation of left foot at ankle level
		OR
		S98.021A Partial traumatic amputation of right foot at ankle level AND S98.022A Partial traumatic amputation of left foot at ankle level

To refine this entry, first the user must decide whether or not it is possible to choose a single cluster—the correct combination of left and right, partial and complete—and if possible, whether or not it is necessary. This decision of course depends on the use of the mapping.

A health information professional or health statistics researcher who is converting a limited number of old ICD-9 records to ICD-10, and has access to the individual medical record, can make use of the increased specificity in ICD-10 codes to re-code the record directly in ICD-10. The user can simply refer to the original record to see the specific nature of the bilateral traumatic amputation and assign the correct pair of ICD-10 codes to the record.

However, a health statistics analyst or data modeler who is translating aggregate ICD-9 data forward to ICD-10, and has no access to individual medical records, cannot make use of the fine distinctions in ICD-10 since they are not present in the old data. In this case, choosing a single cluster that is the closest equivalent cannot be the goal. The user must choose an ICD-10 code or pair of codes to represent all the ICD-10 alternatives, and could choose to fashion a rule by which to map similar cases. Rules specific to the applied mapping would promote consistency and document the decisions made. For example, here the applied mapping could use only the partial traumatic amputation codes.


Sample Entry 2—ICD-10 to ICD-9 GEM:
G92 Toxic encephalopathy

ICD-10 Code	ICD-10 Description	ICD-9 Code	ICD-9 Description	Approximate [FLAG]	No Map [FLAG]	Combination [FLAG]
G92	Toxic encephalopathy	323.71	Toxic encephalitis and encephalomyelitis	1	0	0
G92	Toxic encephalopathy	323.72	Toxic myelitis	1	0	0
G92	Toxic encephalopathy	349.82	Toxic encephalopathy	1	0	0

This same method could be used to translate a record coded in ICD-10 back to ICD-9, and could then be processed by a legacy payment system for reimbursement. The approximate flag is on, indicating that the relationship between the code in the source system and the code in the target system is an approximate equivalent. The approximate flag is on for all three target system ICD-9 code translations of ICD-10 code G92, because the complete meaning of G92 Toxic encephalopathy— as encompassed by the tabular instruction and index entries that refer to G92—includes the clinical concepts toxic encephalitis and encephalomyelitis, toxic myelitis, and toxic encephalopathy specified in three separate ICD-9 diagnosis codes.

To choose among the alternatives in ICD-9 is not possible based on the meaning of the codes themselves. Because the applied mapping is intended to establish general rules for translation rather than deciding on a case-by-case basis, then a consistent method must be derived and documented for resolving the disparity in classification between the two systems. Depending on the applied mapping, the user may choose the closest matching code description or the most frequently recorded of the ICD-9 code alternatives based on ICD-9 data.

For example, for mapping research data on toxic encephalitis during a period that overlaps the use of both ICD-10 and ICD-9, a valid one-to-one mapping from G92 to 323.72 could be derived. But for patient records coded in ICD-10 and mapped internally to an ICD-9 based medical necessity edit system during the transition period, it would depend on the classification of the ICD-9 code alternatives in the edit system. If all three ICD-9 codes were included in the same edit grouping, a valid one-to-one applied mapping to any of the three ICD-9 mapping choices could be derived.

Section 2: How to Use GEM Files

Section 2—How to Use GEM Files

For ease of use, we recommend loading the GEM files into a database along with the code descriptions for both code sets. With roughly 85,000 codes and their descriptions in both code sets, a desktop database like MS Access is adequate.

A general process for using the GEMs consists of three basic steps. In most cases it is expected these steps will be performed by software designed to integrate the GEMs content and translate codes or lists of codes from I-9 to I-10 or vice versa. In that case that a small number of records need to be translated, and the user has access to the original medical record, it is more efficient and accurate to look the codes up directly in the respective ICD-9-CM or ICD-10-CM book.

The code we will use for purposes of demonstration is I-9 code 599.72, Microscopic hematuria.

I-9 to I-10 GEM:
599.72 Microscopic hematuria

The illustrations at left and below display the I-9 code 599.72 as it appears in the I-9 to I-10 GEM. At left is the entry in text file format with its adjacent GEM entries, and below is the same information as it would appear in a desktop database. Note that the codes do not contain decimals in the GEMs.

59972 R311 10000 59972 R312 10000 59981 N3641 00000 59982 N3642 00000 59983 N368 10000 59984 N368 00000 59989 N398 00000 5999 N369 10000 5999 N399 10000

The code in the source system is listed first, followed by the code in the target system. Here the source system is the I-9 code and the target system is the I-10 code. The final group of digits is used to indicate additional attributes for entries in the map. The first three digits are called flags. The last two digits are used in combination entries, and will be discussed later. The GEM entry contains a flag characterizing the degree of correspondence between codes in one row (“approximate” flag), a flag for codes with no correspondence in the target system (“no map” flag) and a flag indicating the row is part of a combination entry (“combination” flag). If the digit is 1, the flag applies (is “turned on”) to that entry in the GEM. If the digit is 0, the flag does not apply (is “turned off”) to that entry in the GEM. In other words, 1 means “yes,” the flag applies to the entry in a GEM and 0 means, “no,” the flag does not apply. There are two rows in the I-9 to I-10 GEM for code 599.72. The entry is of the single type, meaning that each row—code 599.72 linked to one of two I-10 code alternatives—is considered a valid entry.

I-9 Code	I-9 Description	I-10 Code	I-10 Description	Approximate [FLAG]	No Map [FLAG]	Combination [FLAG]
599.72	Microscopic hematuria	R31.1	Benign essential microscopic hematuria	1	0	0
599.72	Microscopic hematuria	R31.2	Other microscopic hematuria	1	0	0

Step 2: ANALYZE

Is the “approximate” flag turned on? If yes, the correspondence is not a precise equivalent. Is the “no map” flag turned on? If yes, there is no corresponding code in the target system. Is the “combination” flag turned on? If yes, more than one code is the target system is required to satisfy the meaning of the code in the source system.

In the GEMs, there are three flags:

Approximate	indicates that the entry is not considered equivalent
No Map	indicates that a code in the source system is not linked to any code in the target system
Combination	indicates that more than one code in the target system is required to satisfy the full equivalent meaning of a code in the source system

The Approximate Flag

I-9 Code	I-9 Description	I-10 Code	I-10 Description	Approximate [FLAG]	No Map [FLAG]	Combination [FLAG]
599.72	Microscopic hematuria	R31.1	Benign essential microscopic hematuria	1	0	0
599.72	Microscopic hematuria	R31.2	Other microscopic hematuria	1	0	0

The approximate flag is turned on when no one code in the target system or linked combination of codes in the target system expresses the same essential meaning as the code in the source system. The difference between the two systems is typically in level of detail between the codes, and in nearly all cases the I-10 code is more detailed than the I-9 code.

The approximate flag is on for both rows in the source system GEM entry for I-9 code 599.72. The level of detail differs here—the type of hematuria is specified in I-10 and not in I-9. Although 599.72 Microscopic hematuria in I-9 and R31.2 Other microscopic hematuria in I-10 could be said to be equal, in fact they are not, because the I-9 code represents all varieties of microscopic hematuria and the I-10 code represents only microscopic hematuria not classified in the other code. The approximate flag is turned on to indicate no single code in I-10 expresses the same meaning as 599.72.

The approximate flag is on for the majority of entries in the GEMs. This may include code pairs that have the same description in both code sets. In such cases, neighboring codes in a subcategory are more specific in one code set than another, and so the number of clinical conditions included in a code is different—hence it does not express the same essential meaning. Codes containing the word “other” in their description are a common example.

I-10 Code	I-10 Description	I-9 Code	I-9 Description	Approximate [FLAG]	No Map [FLAG]	Combination [FLAG]
B37.41	Candidal cystitis and urethritis	112.2	Candidiasis of other urogenital sites	1	0	0
B37.42	Candidal balanitis	112.2	Candidiasis of other urogenital sites	1	0	0
B37.49	Other urogenital candidiasis	112.2	Candidiasis of other urogenital sites	1	0	0

In this example, the body sites included in the “other candidiasis” code differs between code sets, so the approximate flag is on for all entries in this subcategory. I-10 has specific codes for cystitis/urethritis and balanitis. I-9 does not. In I-9, balanitis is listed as an “includes” note under the code 112.2 Candidiasis of other urogenital sites, and cystitis or urethritis have no specific entry in tabular.

The No Map Flag

I-9 Code	I-9 Description	I-10 Code	I-10 Description	Approximate [FLAG]	No Map [FLAG]	Combination [FLAG]
V64.41	Laparoscopic surgical procedure converted to open procedure	NoDx	No Description	0	1	0
V64.42	Thoracoscopic surgical procedure converted to open procedure	NoDx	No Description	0	1	0
V64.43	Arthroscopic surgical procedure converted to open procedure	NoDx	No Description	0	1	0

In the I-9 to I-10 GEM, the “no map” flag is on for a subset of I-9 codes. In this example, the I-9 codes do not identify a diagnosis, but instead further specify the reason why a procedure was performed differently than planned. The recommendation was made that I-10 diagnosis codes do not contain information regarding procedures, so I-10 does not contain an equivalent group of codes. Therefore, the I-9 codes cannot be linked to I-10 at all. In the I-9 to I-10 GEM they are listed without a corresponding I-10 entry, and with the “no map” flag on.

The Combination Flag

The combination flag is turned on when a code in the source system must be linked to more than one code in the target system to be a valid entry. When the combination flag is on, the scenario and choice list fields in a GEM file contain a number. They appear last in a GEM file, after the flags. These numbers allow the user to collate the combination entries in the GEM.

T422X1A 9662 10111 T422X1A E8558 10112 T422X1A 9660 10121 T422X1A E8558 10122 T422X1D V5889 10000 T422X1S 9090 10111 T422X1S E9292 10112 T422X2A 9662 10111 T422X2A E9504 10112 T422X2A 9660 10121 T422X2A E9504 10122 T422X2D V5889 10000 T422X2S 9090 10111 T422X2S E959 10112

I-10 to I-9 GEM:
T42.2X1A Poisoning by succinimides and oxazolidinediones, accidental (unintentional),initial encounter

The illustrations at left and below display the I-10 to I-9 GEM entry for I-10 diagnosis code T42.2X1A Poisoning by succinimides and oxazolidinediones, accidental (unintentional), initial encounter. At left is the entry in text file format, and below is the same information as it would appear in a desktop database. The I-10 combination code T42.2X1A specifies both the diagnosis and the external cause, so it requires a combination entry in the GEM. A combination entry is subdivided hierarchically on two levels: 1) By scenario, the number of variations of diagnosis combinations included in the source system code, and 2) By choice list, the possible target system codes that combined are one valid expression of a scenario.

Each of the two types of drug listed in the I-10 code T42.2X1A is a unique I-9 code, so there are two scenarios from which to choose an applied mapping: one that specifies poisoning by succinimides and one that specifies poisoning by oxazolidinediones. Because each drug type listed in the I-10 combination code requires its own diagnosis code in I-9, each variation of the diagnosis is assigned a separate scenario number in the GEM entry.

A scenario designates one variation of the meaning of the source system diagnosis as specified in a combination code. In other words, it identifies one roughly equivalent expression of the source system code. In this example, scenario 1 contains the I-9 codes needed to satisfy the equivalent meaning of “Poisoning by succinimides, accidental (unintentional), initial encounter” Scenario 2 contains all the I-9 codes needed to specify “Poisoning by oxazolidinediones, accidental (unintentional), initial encounter.”

A scenario is subdivided into two or more choice lists of codes in the target system. These are the codes that must be linked together as a unit in an applied mapping to satisfy the equivalent meaning of the combination code in the source system. A choice list contains one or more codes in the target system that express a portion of the meaning of the code in the source system. A code must be included from each choice list in a scenario to satisfy the equivalent meaning of the code in the source system.

Scenario 1

I-10 Code	I-10 Description	I-9 Code	I-9 Description	Approximate [FLAG]	Combination [FLAG]	Scenario	Choice list
T42.2X1A	Poisoning by succinimides and oxazolidinediones, accidental (unintentional), initial encounter	966.2	Poisoning by succinimides	1	1	1	1
T42.2X1A	Poisoning by succinimides and oxazolidinediones, accidental (unintentional), initial encounter	E855.8	Other specified drugs acting on central and autonomic nervous systems	1	1	1	2

Scenario 2

I-10 Code	I-10 Description	I-9 Code	I-9 Description	Approximate [FLAG]	Combination [FLAG]	Scenario	Choice list
T42.2X1A	Poisoning by succinimides and oxazolidinediones, accidental (unintentional), initial encounter	966.0	Poisoning by oxazolidine derivatives	1	1	2	1
T42.2X1A	Poisoning by succinimides and oxazolidinediones, accidental (unintentional), initial encounter	E855.8	Other specified drugs acting on central and autonomic nervous systems	1	1	2	2

In this example there are two I-9 choice lists in scenario 1 and two I-9 choice lists in scenario 2, with one I-9 code in each list.

This is a comparatively simple example of a combination entry because each choice list contains only one code. The user does not need to choose among alternatives beyond the scenario.

The result is that for this I-10 combination entry, there are only two applied mapping alternatives:

Scenario 1

ICD-10-CM Source	to	ICD-9-CM Target
T42.2X1A Poisoning by succinimides and oxazolidinediones, accidental (unintentional), initial encounter	≈	966.2 Poisoning by succinimides AND E855.8 Other specified drugs acting on central and autonomic nervous systems

OR


Scenario 2

ICD-10-CM Source	to	ICD-9-CM Target
T42.2X1A Poisoning by succinimides and oxazolidinediones, accidental (unintentional), initial encounter	≈	966.0 Poisoning by oxazolidine derivatives AND E855.8 Other specified drugs acting on central and autonomic nervous systems

Section 1 Mapping And The GEMs

Mapping and the GEMs

Translating Between the ICD-9 and ICD-10 Diagnosis Code Sets

Mappings between ICD-9 and ICD-10 attempt to find corresponding diagnosis codes between the two code sets, insofar as this is possible. In some areas of the classification the correlation between codes is fairly close, and since the two code sets share the conventions of organization and formatting common to both revisions of the International Classification of Diseases, translating between them is straightforward. Many infectious disease, neoplasm, eye, and ear codes are examples of fairly straightforward correspondence between the two code sets. In other areas—obstetrics, for example—whole chapters are organized along a different axis of classification. In such cases, translating between them the majority of the time can offer only a series of possible compromises rather than the mirror image of one code in the other code set.

ICD-10 Description	Correlation	ICD-9 Description	Unequal Axis of classification
A02.21 Salmonella meningitis	=	003.21 Salmonella meningitis	None
C92.01 Acute myeloid leukemia, in remission	=	205.01 Myeloid leukemia, acute, in remission	None

ICD-10 Description	Correlation	ICD-9 Description	Unequal Axis of classification
O26.851 Spotting complicating pregnancy, first trimester	≠	649.50 Spotting complicating pregnancy, unspecified episode of care 649.51 Spotting complicating pregnancy, delivered 649.53 Spotting complicating pregnancy, antepartum	Stage of pregnancy (ICD-10) vs. Episode of care (ICD-9)
O26.852 Spotting complicating pregnancy, second trimester
O26.853 Spotting complicating pregnancy, third trimester
O26.859 Spotting complicating pregnancy, unspecified trimester

A sentence translated from English to Chinese may not be able to capture the full meaning of the original because of fundamental differences in the structure of the language. Likewise, a code set may not be able to seamlessly link the codes in one set to identical counterparts in the other code set. For these two diagnosis code sets, it is often difficult to find two corresponding descriptions that are identical in level of specificity and terminology used. This is understandable. Indeed, there would be little point in changing from the old system to the new system if the differences between the two, and the benefits available in the new system, were not significant.

There is no simple “crosswalk from ICD-9 to ICD-10” in the GEM files. A mapping that forces a simple correspondence—each ICD-9 code mapped only once—from the smaller, less detailed ICD-9 to the larger, more detailed ICD-10 defeats the purpose of upgrading to ICD-10. It obscures the differences between the two code sets and eliminates any possibility of benefiting from the improvement in data quality that ICD-10 offers. Instead of a simple crosswalk, the GEM files attempt to organize those differences in a meaningful way, by linking a code to all valid alternatives in the other code set from which choices can be made depending on the use to which the code is put.

It is important to understand the kinds of differences that need to be reconciled in linking coded data. The method used to reconcile those differences may vary, depending on whether the data is used for research, claims adjudication, or analyzing coding patterns between the two code sets; whether the desired outcome is to present an all-embracing look at the possibilities (one-to-many mapping) or to offer the one “best” compromise for the application (one-to-one mapping); whether the desired outcome is to translate existing coded data to their counterparts in the new code set (“forward mapping”) or to track newly coded data back to what they may have been in the previous code set (“backward mapping”), or any number of other factors. The scope of the differences varies, is complex, and cannot be overlooked if quality mapping and useful coded data are the desired outcomes. Several common types of differences between the code sets will be examined here in detail to give the reader a sense of the scope.

Diagnosis Codes and Differences in Classification

ICD-10-CM has been updated to reflect the current clinical understanding and technological advancements of medicine, and the code descriptions are designed to provide a more consistent level of detail. It contains a more extensive vocabulary of clinical concepts, body part specificity, patient encounter information, and other components from which codes are built.

For example, an ICD-9 code description containing the words “complicated open wound” does not have a simple one-to-one correspondent in ICD-10. The ICD-9 description identifies the clinical concept “complicated,” but according to the note at the beginning of the section, that one concept includes any of the following: delayed healing, delayed treatment, foreign body or infection. ICD-10 does not classify open wound codes based on the general concept “complicated”. It categorizes open wounds by wound type — laceration or puncture wound, for example — and then further classifies each type of open wound according to whether a foreign body is present. ICD-10 open wound codes do not mention delayed healing or delayed treatment, and instructional notes advise the coder to code any associated infection separately. Therefore, depending on the documentation in the record, the correct correspondence between and ICD-9 and ICD-10 code could be one of several.

Diagnosis Codes and Levels of Specificity

ICD-9 and ICD-10 Code Sets Compared:

Code Length and Set Size

Comparison	ICD-9-CM	ICD-10-CM
# of Characters	3-5 Numeric (+V and E codes)	3-7 Alphanumeric
# of Codes	~14,500	~70,000

As shown in the table above, ICD-10 codes may be longer, and there are about five times as many of them. Consequently, in an unabridged ICD-9 to ICD-10 mapping, each ICD-9 code is typically linked to more than one ICD-10 code, because each ICD-10 code is more specific.

ICD-10 is much more specific than ICD-9, and, just as important for purposes of mapping, the level of precision in an ICD-10 code is more consistent within clinically pertinent ranges of codes. In ICD-9, on the other hand, the level of detail among code categories varies greatly. For example, category 733, Other disorders of bone and cartilage, contains the codes:

• 733.93 Stress fracture of tibia or fibula
• 733.94 Stress fracture of the metatarsals
• 733.95 Stress fracture of other bone
• 733.96 Stress fracture of femoral neck
• 733.97 Stress fracture of shaft of femur
• 733.98 Stress fracture of pelvis

Five of the six codes specify the site of the fracture. The third code is an “umbrella” code for all other bones in the body. In practical terms this means that the general ICD-9 code 733.95 must represent a whole host of disparate fracture sites. Diagnoses that are identified by umbrella codes lose their uniqueness as coded data. When only the coded ICD-9 data is available, it is impossible to tell which bone was fractured. On the other hand, in many instances ICD-10 provides specific codes for all likely sites of a stress fracture, including more specificity for the bones of the extremities, the pelvis and the vertebra. Stress fracture data coded in ICD-10 possesses a consistent level of specificity.

One might expect an ICD-10 to ICD-9 mapping never to contain one-to-many mappings, since ICD-10 is so much larger and more specific. However, there are cases where ICD-9 contains more detail than ICD-10, especially where a clinical concept or axis of classification is no longer deemed essential information. Aspects of some individual ICD-9 code descriptions, such as information about how a diagnosis was confirmed, were intentionally not included in ICD-10. This means a single ICD-10 code could be linked to more than one ICD-9 code option, depending on the purpose of the mapping and the specific documentation in the medical record.

Below are two examples where a distinction made in ICD-9 is not made in ICD-10. The result is that the ICD-10 code could be linked to more than one ICD-9 code, because a particular area of the ICD-9 classification contains detail purposely left out of ICD-10.

Specificity in ICD-9 and not in ICD-10:
Method of Detection

ICD-9 contains

ICD-10 contains

010.90 Primary tuberculous infection, unspecified examination 010.91 Primary tuberculous infection, bacteriological/histological exam not done 010.92 Primary tuberculous infection, bacteriological/histological exam unknown (at present) 010.93 Primary tuberculous infection, tubercle bacilli found by microscopy 010.94 Primary tuberculous infection, tubercle bacilli found by bacterial culture 010.95 Primary tuberculous infection, tubercle bacilli confirmed histologically 010.96 Primary tuberculous infection, tubercle bacilli confirmed by other methods

3-5 Numeric (+V and E codes)

Specificity in ICD-9 and not in ICD-10:

Legal Status and completeness of procedure

ICD-9 contains	ICD-10 contains
635.50 Legally induced abortion, complicated by shock, unspecified 635.51 Legally induced abortion, complicated by shock, incomplete 635.52 Legally induced abortion, complicated by shock, complete 636.50 Illegal abortion, complicated by shock, unspecified 636.51 Illegal abortion, complicated by shock, incomplete 636.52 Illegal abortion, complicated by shock, complete	O04.81 Shock following (induced) termination of pregnancy

Diagnosis Codes in Combination

One ICD-9 or ICD-10 code can contain more than one diagnosis. For purposes of mapping, these are called combination codes. A combination code consists of more than one diagnosis. For example, a combination code can consist of a chronic condition with a current acute manifestation, as in ICD-9 code 250.21 Diabetes with hyperosmolarity, type I (juvenile type), not stated as uncontrolled. Or a combination code can consist of two acute conditions found together, as in ICD-10 code R65.21 Severe sepsis with septic shock. Or a combination code can consist of an acute condition and its external cause, as in ICD-10 code T58.01 Toxic effect of carbon monoxide from motor vehicle exhaust, accidental (unintentional).

If a combination code in one code set has a corresponding combination code in the other code set, then the two entries are linked in the usual way. It is only when a combination code in one set is broken into discrete diagnosis codes in the other set that another method of mapping is needed.

Mapping in cases where a combination code in one set corresponds to two or more discrete diagnosis codes in the other set requires that the combination code be linked as a unit to two or more codes in the other code set. Each discrete diagnosis code is a partial expression of the information contained in the combination code and must be linked together as one GEM entry to fully describe the same conditions specified in the combination code. Entries of this type are linked using a special mapping flag that indicates the allowable A+B+C choices.

ICD-9 to ICD-10 mapping, combination entry:

Histoplasma duboisii meningitis

ICD-9-CM Source	to	ICD-10-CM Target
115.11 Histoplasma duboisii meningitis	≈	B39.5 Histoplasmosis duboisii AND G02 Meningitis in other infectious and parasitic diseases classified elsewhere

ICD-10 to ICD-9 mapping, combination entry:

Atherosclerosis of autologous vein coronary artery bypass graft(s) with unstable angina pectoris

ICD-9-CM Source	to	ICD-10-CM Target
I25.710 Atherosclerosis of autologous vein coronary artery bypass graft(s) with unstable angina pectoris	≈	414.02 Coronary atherosclerosis of autologous vein bypass graft AND 411.1 Intermediate coronary syndrome

Introduction to the GEMs

The ICD-10 and ICD-9 GEMs are used to facilitate linking between the procedure codes in ICD-9 volume 3 and the new ICD-10 code set. The GEMs are the raw material from which providers, health information vendors and payers can derive specific applied mappings to meet their needs. This is covered in more detail in section 2.

The ICD-9 to ICD-10 GEM contains an entry for every ICD-9 code. Not all ICD-10 codes are contained in the ICD-9 to ICD-10 GEM; the ICD-9 to ICD-10 GEM contains only those ICD-10 codes which are plausible translations of the ICD-9 codes. As with a bi-directional translation dictionary, the translations given are based on the code looked up, called the source system code.

The ICD-9 to ICD-10 GEM can be used to migrate ICD-9 historical data to a ICD-10 based representation for comparable longitudinal analysis between ICD-9 coded data and ICD-10 coded data. It can be used to create ICD-10 based test records from a repository of ICD-9 based test records. The ICD-9 to PCS GEM can also be used for general reference.

The ICD-10 to ICD-9 GEM contains an entry for every ICD-10 code. Not all ICD-10 codes are contained in the ICD-10 to ICD-9 GEM; the ICD-10 to ICD-9 GEM contains only those ICD-9 codes which are plausible translations of the ICD-10 codes. The translations given are based on the ICD-10 code looked up, the source system code in the ICD-10 to ICD-9 GEM.

The ICD-10 to ICD-9 GEM can be used to convert ICD-9 based systems or applications to ICD-10 based applications, or create one-to-one backwards mappings (also known as a crosswalk) from incoming ICD-10 based records to ICD-9 based legacy systems. This is accomplished by using the ICD-10 to ICD-9 GEM, but looking up the target system code (ICD-9) to see all the source system possibilities (ICD-10). This is called reverse lookup. For more information on converting ICD-9 based systems and applications to ICD-10, see the MS-DRG conversion project report at: Link.

The word “crosswalk” is often used to refer to mappings between annual code updates of ICD-9. Crosswalk carries with it a comfortable image: clean white lines mark the boundary on either side; the way across the street is the same in either direction; a traffic signal, or perhaps even a crossing guard, aids you from one side to the other. Please be advised: GEMs are not crosswalks. They are reference mappings, to help the user navigate the complexity of translating meaning from one code set to the other. They are tools to help the user understand, analyze, and make distinctions that manage the complexity, and to derive their own applied mappings if that is the goal. The GEMs are more complex than a simple one-to-one crosswalk, but ultimately more useful. They reflect the relative complexity of the code sets clearly so that it can be managed effectively, rather than masking it in an oversimplified way.

One entry in a GEM identifies relationships between one code in the source system and its possible equivalents in the target system. If a mapping is described as having a direction, the source is the code one is mapping from, and the target is the code being mapped to.

Source	Target	a.k.a.
From ICD-9-CM	To ICD-10-CM	“forward mapping”
From ICD-10-CM	To ICD-9-CM	“backward mapping”

The correspondence between codes in the source and target systems is approximate in most cases. As with translating between languages, translating between coding systems does not necessarily yield an exact match. Context is everything, and the specific purpose of an applied mapping must be identified before the most appropriate option can be selected.

The GEMs together provide a general (many to many) reference mapping that can be refined to fit the requirements of an applied mapping. For a particular code entry, a GEM may contain several possible translations, each on a separate row. The code in the source system is listed on a new row as many times as there are alternatives in the target system. Each correspondence is formatted as a code pair. The user must choose from among the alternatives a single code in the target system if a one-to-one mapping is desired.

The word “entry,” as used to describe the format of a GEM, refers to all rows in a GEM file having the same first listed code, the code in the source system. The word “row” refers to a single line in the file, containing a code pair—one code from the source system and one code from the target system—along with its associated attributes. An entry typically encompasses multiple rows.

There are two basic types of entries in the GEM. They are “single entry” and “combination entry.” In special cases, a code in the source system may be mapped using both types of entries.

• Single entry — an entry in a GEM for which a code in the source system linked to one code option in the target system is a valid entry

An entry of the single type is characterized by a single correspondence: code A in the source system corresponds to code A or code B or code C in the target system. Each row in the entry can be one of several valid correspondences, and each is an option for a “one to one” applied mapping. An entry may consist of one row, if there is a close correspondence between the two codes in the code pair.

An entry of the single type is not the same as a one-to-one mapping. A code in the source system may be used multiple times in a GEM, each time linked to a different code in the target system. This is because a GEM contains alternatives from which the appropriate applied mapping can be selected. Taken together, all rows containing the same source system code linked to single code alternatives are considered one entry of the single type.

Here is an entry of the single type, consisting of two rows. The rows can be thought of as rows A or B. Each row of the entry is considered a valid applied mapping option if a one-to-one mapping is desired.

ICD-9 to ICD-10 GEM:

Single type entry for ICD-9-CM code 599.72

ICD-9-CM Source	to	ICD-10-CM Target.
599.72 Microscopic hematuria	≈	R31.1 Benign essential microscopic hematuria
599.72 Microscopic hematuria	≈	R31.2 Other microscopic hematuria

Because ICD-10 codes are for the most part more specific than ICD-9 codes, an entry of the single type in the ICD-9 to ICD-10 GEM is typically linked to multiple ICD-10 codes. The user must know, or must model, the level of detail contained in the original medical record to be able to choose one of the ICD-10 codes. The ICD-9 code itself cannot contain the answer; it cannot be made to describe detail it does not have. The same is occasionally true for the ICD-10 to ICD-9 GEM as well. An ICD-10 code may be linked to more than one ICD-9 code because detail in ICD-9 was purposely left out of ICD-10, as discussed earlier.

Both ICD-9 and ICD-10 contain what we refer to as “combination codes.” These are codes that contain more than one diagnosis in the code description. An example is ICD-10 code R65.21 Severe sepsis with septic shock. In this case, ICD-9 does not have an equivalent combination code, so in order to link the ICD-10 code to its ICD-9 equivalent, a combination entry must be used in the GEM.

• Combination entry—an entry in a GEM for which a code in the source system must be linked to more than one code option in the target system to be a valid entry

An entry of the combination type is characterized by a compound correspondence: code A in the source system must be linked as a unit to code A and code B and code C in the target system to be a valid correspondence. Attributes in a GEM file clearly signal these special cases.

Stated another way, it takes more than one code in the target system to satisfy all of the meaning contained in one code in the source system. As discussed in this section, the situation occurs both when ICD-9 is the source system and when ICD-10 is the source system.

Here is an entry of the combination type, consisting of two rows in the format of a GEM file. The rows can be thought of as rows A and B. The rows of the entry combined are considered one complete translation.

ICD-10 to ICD-9 GEM:
Combination type entry for ICD-10-CM code R65.21

ICD-10-CM Source	to	ICD-9-CM Target.
R65.21 Severe sepsis with septic shock	≈	995.92 Severe sepsis AND 785.52 Septic shock

Linking a code in the source system to a combination of codes in the target system is accomplished by using conventions in the GEMs called scenarios and choice lists.

• Scenario — in a combination entry, a collection of codes from the target system containing the necessary codes that combined as directed will satisfy the equivalent meaning of a code in the source system
• Choice list—in a combination entry, a list of one or more codes in the target system from which one code must be chosen to satisfy the equivalent meaning of a code in the source system

Here is the combination type entry for R65.21, Severe sepsis with septic shock as it is depicted in the “flat text” GEM format, and repeated below in table format with the code descriptions and attributes labeled.

R6521 99592 101 1 1 R6521 78552 101 1 2

There are two rows in the ICD-10 to ICD-9 GEM for combination code R65.21. The entry is of the combination type, meaning that each row—code R65.21 linked to both of the two ICD-9 codes—is considered a valid entry. The combination flag is the third attribute in a GEM file. The scenario number is 1, because there is only one variation of the diagnoses specified in the combination code. There are two choice lists in this entry, and only one code in each choice list.

ICD-10 Code	ICD-10 Description	ICD-9 Code	ICD-9 Description	Approximate [FLAG]	No Map [FLAG]	Combination [FLAG]	Scenario	Choice list
R65.21	Severe sepsis with septic shock	995.92	Severe sepsis	1	0	1	1	1
R65.21	Severe sepsis with septic shock	785.52	Septic shock	1	0	1	1	2

It is important to make the distinction between a single row in a combination entry and an entry of the single type. An entry of the single type is one code in the source system linked to multiple one-code alternatives in the target system. It presents the option of linking one code in the source system to code A or B or C in the target system. Each code correspondence is considered a viable option. Each row of the source system code entry linked with target code A or B or C is one valid entry in an applied map.

An entry of the combination type is one code in the source system linked to a multiple-code alternative in the target system. If the source system is ICD-10, for example, the user must include ICD-9 codes A and B and C in order to cover all the diagnoses identified in the ICD-10 code. Further, there may be more than one multiple-code alternative. If a GEM contains a range of ICD-9 code alternatives for each partial expression of the ICD-10 code, then the number of solutions increases. Each instance of the ICD-10 combination code paired with one code of the allowed range A and one code of the allowed range B and one code of the allowed range C is sometimes referred to as a “cluster,” and is considered a valid entry. The combination flag in a GEM will clearly signal an entry of the combination type.

The two entry types and their main features are summarized in the table below.

Entry Type	Summary Description	Approximate [FLAG]	No Map [FLAG]	Combination [FLAG]	Scenario	Choice list
Single	Source system code has one or more single target code alternatives	On or Off	N/A	Off	0	0
Combination	Source system code has one or more multiple target code alternatives	On	N/A	On	1-9	1-9

Source: Link