Table 1.

Description of comorbidity measures

Measure	Definition	Data Source	Variable Categories for the Predictive Modela
Number of diagnoses	Identifies number of diagnosis codes, to the third digit. Minimum score is 0 and there is no maximum	Hospital records and physician billing claims	0−2 (reference), 3–5, 6–9, 10 or more
Charlson index	Summarizes diagnoses for 17 conditions. Total score can range from 0 to 32	Hospital records and physician billing claims	0−1 (reference), 2–3, 4–5, 6 or more
Elixhauser index	Captures diagnoses for 31 conditions. No total score is produced	Hospital records and physician billing claims	Dummy variables (0 = absent; 1 = present) are used to define the 31 conditions
Number of drugs	Identifies number of prescription drugs using six-digit AHFS codes. Total score can range from 0 to 125	Prescription drug records	0−1 (reference), 2–3, 4–5, 6 or more
Chronic Disease Score (CDS)	Uses AHFS codes to identify number of different prescriptions for treating 17 conditions. Total score can range from 0 to 35	Prescription drug records	0−1 (reference), 2–3, 4–5, 6–7, 8 or more

^aVariable categories were defined based on previous research and by examining the frequency distributions for each of the comorbidity measures. Cut-points for the categories were created to minimize sparse cell sizes, AHFS American Hospital Formulary System.