The Comorbidity Software is one in a family of databases and software tools developed as part of the Healthcare Cost and Utilization Project (HCUP), a Federal-State-Industry partnership sponsored by the Agency for Healthcare Research and Quality. HCUP databases, tools, and software inform decision making at the national, State, and community levels.

Contents:

• Overview of Comorbidity Software
• Data Elements and Coding Conventions
• Format Program
• Analysis Program
• Diagnoses and DRGs
• Variables Created
• Downloading information
• Questions?

Comorbidity software assigns variables that identify comorbidities in hospital discharge records using the diagnosis coding of ICD-9-CM (International Classification of Diseases, Ninth Edition, Clinical Modifications). This document describes the software that creates the comorbidity measures reported by Elixhauser et al. ("Comorbidity Measures for Use with Administrative Data." Medical Care, 1998;36:8-27).

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The comorbidity software consists of two SAS computer programs for PCs. Although these programs are written in SAS, they are being distributed in ASCII so that they can be readily adapted to other programming languages.

The first program, Creation of Format Library for Comorbidity Groups, creates a SAS format library that maps diagnosis codes into comorbidity indicators. Additional formats are created to exclude conditions that may be complications or that may be related to the principal diagnosis.

The second SAS program, Creation of Comorbidity Variables, applies these formats to a data set containing administrative data.

This documentation describes three topics:

• Data elements required for the programs.
• The SAS programs.
• How to use the SAS programs on an IBM-compatible PC.

The input data file must contain certain elements that are coded in specific ways. These elements are required for the assignment of the comorbidity flags. The flags are 0/1 indicators that note whether individual records include each comorbidity.

The input data set must have the following two variables:

1. Diagnosis-related groups (DRGs).
2. Diagnostic codes (ICD-9-CMs).

The required elements and coding are detailed further below.

Required Elements of Input File

Element	Type	Length	Label and Uniform Coding
DRG	Num	3	DRG in effect on discharge date, assigned by the DRG Grouper algorithm of the Health Care Financing Administration (HCFA)
DX1	Char	5	Principal diagnosis, annnn (blanks indicate missing)
DX2-DXn	Char	5	Secondary diagnoses 2-n, where n varies by data set
NDX	Num	3	Number of diagnoses on this discharge. (Note: A macro variable defines this element; it is currently set as 2.) NDX should always be equal or less than the macro variable called NUMDX.

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Creation of Format Library for Comorbidity Groups
The format program defines a format library that contains the diagnosis screens necessary for the comorbidity analysis. It will be used by Program 2, the comorbidity analysis program.

• Input: None
• Output: Permanent SAS format library called \FMTLIB\formats.sc2.
• Changes: The code points to a directory called FMTLIB on the c: drive for the format library output file. If you use another directory or drive, this code must be changed. Initially, this directory should be defined.

Creation of Comorbidity Variables

The analysis program assigns to the inpatient records 0/1 indicators for the comorbidity variables of interest. This program assumes that the input data file conforms to specific variables names, attributes, and coding conventions, as described above.

• Input: SAS inpatient data (CORE) conforming to HCUP coding conventions (described above), and SAS format library (FMTLIB), created from the included format program.
• Output: SAS data set (ANALYSIS) containing inpatient records with their comorbidity indicators. The contents of the ANALYSIS file and the means for the comorbidity variables are output as hard copy. The output file is called ANALYSIS; if your file name differs, the DATA statement needs to be changed.
• Changes: The code points to a directory called DATA on the c: drive for the input and output files. If you use another directory or drive, this code must be changed. Initially, the DATA directory needs to be defined. The macro variable (NUMDX) that defines the number of diagnoses on your data file needs to be defined (change the "2" to the appropriate number).

The macro variable (CORE) that defines the input file (SASname) needs to be defined; change the XXXXXX to the appropriate name.

There is an options statement that defines page and line size preferences; the settings currently are linesize=159 and pagesize=56. These settings can be changed, depending upon whether you prefer portrait or landscape style output.

ICD-9-CM and DRG coding changes through September 30, 2004, are incorporated into this software. Changes to the Comorbidity Software for FY2004, Version 2.0 are summarized in Table 1. (PDF file, 39 KB)

Thirty comorbidity variables are created using the comorbidity software, based on the format library. The library contains formats for the ICD-9-CM codes and DRG screens. Construction of these variables is summarized in Table 2. (PDF file, 24 KB)

The original table appeared in the paper by Elixhauser et al (1998). This table has been updated to reflect the ICD-9-CM and DRG updates in the software.

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• Format Program: Creation of Format Library (ASCII file, 11 KB).
• Analysis Program: Creation of Comorbidity Variables (ASCII file, 15 KB).

Though they are written in SAS, both files are being distributed in ASCII so they can be readily adapted to other programming languages.

Questions regarding the CCS may be directed to HCUP User Support through the following channels:

• e-mail: hcup@ahrq.gov
• Phone (toll free): 1-866-290-HCUP.