Table 1. Comorbidity classification systems.

Index	Description
Deyo-Charlson	The Deyo-Charlson index places ICD-9 codes into one of seventeen comorbidity categories and assigns weights to each comorbidity category. This index was developed to identify comorbid illnesses that increase the risk of acute hospital mortality.
Elixhauser	The Elixhauser measure was developed for use with ICD-9 data to address some of the perceived shortcomings of the Charlson index. Elixhauser incorporates twenty-nine disease categories to assess the impact of each category on outcomes.
CMS Comorbidity Tiers	The CMS Comorbidity Tiers rely on a four-tiered system for grading medical complexity as part of the prospective payment system of inpatient rehabilitation facilities developed by RAND for Medicare. The most complex tier receives the highest payment for a given diagnosis and age. Comorbidity data are obtained for up to ten ICD-9-CM codes.