Introduction

The Charlson Comorbidity Index contains 19 categories of comorbidity, which are primarily defined using ICD-9-CM diagnoses codes (a few procedure codes are also employed). Each category has an associated weight, taken from the original Charlson paper (Charlson et al., 1987), which is based on the adjusted risk of one-year mortality. The overall comorbidity score reflects the cumulative increased likelihood of one-year mortality; the higher the score, the more severe the burden of comorbidity.

Use the Charlson Comorbidity Index with caution - the ICD-9-CM codes used in the program can refer either to complication or comorbidity diagnoses. Unless these can be reliably differentiated it is possible that the burden of comorbid disease might be overestimated. The best way to avoid the confusion of complications with comorbidities is to identify and separate the diagnoses that are complications / comorbidities, and remove the complications. For hospital datasets this can be done using the diagnosis type variable.

When this is not possible the use of this macro is not precluded: the effect of complications on the comorbidity estimates provided by this macro is limited since individual diagnoses are not used directly in calculating the comorbidity score. Rather, these determine attribution to comorbidity categories which are scored as binary variables. This binary attribution within a comorbidity category limits the effect of complications since both the comorbidity and complication codes often fall in the same disease category. (Romano et al., 1993),, (Roos et al., 1997),

How the Index Works

The Index may be calculated either for a single record (separation) or over a defined period of time prior to an index event. Every diagnosis and procedure code is analyzed to see if it falls within one of the 19 comorbid conditions. If one of these is found, a flag (1) for that condition is set. These flags are weighted appropriately and summed to generate values:
• PCHRLSON - Charlson variable computed from admissions prior to and including the event.

• XCHRLSON - Charlson variable computed from the event admission only.

• PCOMP - Component variables for the Charlson variable observed on admissions prior to the event. (Note: there are 61 component variables)

• XCOMP - Component variables for the Charlson variable which were observed on the event admission. (Note: There are 61 component variables)

• AGGREGATES - Combinations of PCOMP and XCOMP variables which are used for final determination of the Charlson score.

SAS CODE

A SAS macro is available from MCHP that will compute the Index. This SAS code corresponds to the original Charlson work. This SAS code is based on 1993 ICD-9-CM codes for various conditions. If you are using the index on other years you should check to confirm there have been no major changes in ICD codes.

The following diagnoses and procedure codes are included in the SAS program.

A description of the macro, including the requirements and output, and an example call of the macro are provided in charlsn.docs

The original SAS macro code is available internally at: SAS macro: charlsn.txt

A modified version (1998) of the SAS macro is also available internally at: SAS macro: charlsn_index.txt

Cautions

• The Charlson Comorbidity Index should only be used with the Hospital Discharge Abstract data and NOT with the Medical Services (Physician Claims) data. The reason for this is because the Index, in some instances, requires the degree of specificity only found in the 4- and 5-digit ICD-9 codes provided in the Hospital Discharge Abstracts data. Medical Services (Physician Claims) data only contains 3-digit ICD-9-CM codes.

• Charlson ME, Pompei P, Ales KL, MacKenzie CR. A new method of classifying prognostic comorbidity in longitudinal studies: development and validation. J Chronic Dis 1987;40(5):373-383. [PubMed Abstract]
• Romano PS, Roos LL, Jollis JG. Adapting a clinical comorbidity index for use with ICD-9-CM administrative data: differing perspectives. J Clin Epidemiol 1993;46(10):1075-1079. [PubMed Abstract]
• Roos LL, Stranc L, James RC, Li J. Complications, comorbidities, and mortality: improving classification and prediction. Health Serv Res 1997;32(2):229-238. [PubMed Abstract]