Calculating measures of comorbidity using administrative data

The development of a Stata program to calculate published measures of comorbidity will be of value to researchers working with inpatient discharge data coded in ICD-9-CM or ICD-10. The “comorbid” module calculates the weighted sum of comorbidities, as well as comorbidity scores based on the Charlson Index, which reflects the cumulative increase in likelihood of one-year mortality from comorbidities. This module allows for the calculation of three different comorbidity measures: ICD-9-CM, Enhanced ICD-9-CM or ICD-10 (Quan et al, 2005). Exclusion of less severe comorbidities can occur using an optional hierarchical method which excludes from the calculations a mild comorbidity when a patient has also exhibited a more severe form of the same diagnosis. The comparable “elixhauser” command calculates the sum of this alternate set of comorbidity measures which may be associated with negative hospital outcomes (Elixhauser et al, 1998). Both Stata algorithms can handle patients or visits as the observational unit. Options allow for a choice of summary output.