The Diagnosis Related Group, Version 18 (DRG18) is assigned by the HCFA DRG Grouper algorithm during HCUP processing. DRG18 is available on the HCUP databases from 1998 to 2005.

Diagnosis and Procedures Used for DRG Assignment

Beginning in the 1996 data, the DRG grouper can handle a maximum of 50 diagnosis and 50 procedure codes. Only diagnoses and procedure that are valid on the date of discharge are used by the grouper for DRG assignment.

In the 1988-1995 data, the DRG grouper cannot handle more than 15 diagnoses and 15 procedures. Therefore, the following rules were used when more than 15 diagnoses or 15 procedures were available:

• the principal diagnosis/procedure (regardless of validity) is retained in DX1/PR1. No secondaries are shifted into the principal position.
• the first 14 valid (by HCUP standards) additional diagnosis or procedure codes are passed to the HCFA DRG grouper and 3M Mapper software.

Logically Mapping ICD-9-CM Codes for DRG Version 18

The diagnoses or procedures selected by the above rules are first passed to the 3M Mapper software so that each ICD-9-CM code can be logically translated into codes in effect during fiscal year 2000, the period associated with DRG Version 18. The translated codes are then passed to the DRG Version 18 HCFA Grouper software.

Different Definitions of Diagnosis and Procedure Validity

HCUP validation of diagnosis and procedure codes allows a window of time around the official ICD-9-CM coding changes (usually October 1), for anticipation of or lags in response to official ICD-9-CM coding changes. During the 1988-1997 HCUP data processing, a six-month window (three months before and three months after) was allowed. Beginning in the 1998 data, a year window (six months before and six months after) was allowed.

The DRG Grouper rules differ in two ways:

• diagnosis and procedure codes must be valid on the date of discharge to be used for assigning the DRG; and
• some valid diagnoses (E-codes) are ruled by the DRG Grouper to be invalid if entered as a principal diagnosis.

This inconsistency between the definition of a valid diagnosis or procedure is obvious when a discharge has a valid principal diagnosis under HCUP standards, but the assigned DRG is 470 "Ungroupable." Consider a discharge with DX1="V300" on October 1, 1989. The diagnosis code "V300" is considered valid by HCUP standards because until September 30, 1989 "V300" is a valid ICD-9-CM code. The DRG Grouper does not recognize the "V300" code on October 1, 1989 and therefore groups the record to "Ungroupable," DRG=470 and MDC=0.

Changes in DRG Grouper Logic

Until the eighth version (before October 1, 1990), the first step in the determination of the DRG had been the assignment of the appropriate MDC based on the principal diagnosis. Beginning in October 1990, there are two types of exceptions:

• The principal diagnosis is not the initial data element in DRG assignment when the initial step in DRG assignment is based on a procedure. If a patient has a liver transplant (DRG 480), a bone marrow transplant (DRG 481) or tracheostomy (DRG 482 and 483), then the patient is assigned to these DRGs independent of the MDC assigned from the principal diagnosis.
• Assignment to MDC 24 (multiple trauma) and MDC 25 (patients with HIV infection) is based on BOTH principal diagnosis and procedure.

Labels

Labels for the DRGs are provided as an ASCII file in HCUP Tools: Labels and Formats.

Formats

A format to label DRG18 is documented in HCUP Tools: Variable Labels and Formats.