in AIC values >4 between models (17). We also examined the fit of our models using deviance values for negative binomial regression models and c statistics for logistic regression models. All analyses were conducted using SAS for microcomputers (SAS Institute, Cary, NC). Among the 306 participants in this study, 96% were men, the mean ± SD age was 59.8 ± 14.2 years, 49% were white, and 32% were African American (with the remainder of participants having no race indicator in the electronic medical record). The RxRisk-V yielded the highest mean number of drug/disease indicator categories (4.3), followed by the Elixhauser method (2.2) and the Charlson Score (1.0). (Note that the RxRisk-V also involved the greatest possible number of indicators [45], followed by Elixhauser [30] and Charlson [19].) The proportions of participants with each drug or disease indicator for all comorbidity measures are presented in Table 1. These results show that the RxRisk-V captures information on a larger number of disease categories than the other 2 measures; however, 15 of these disease indicators were present for ≤1% of the sample. All comorbidity scores were significantly correlated (Pearson's r = 0.42–0.60, P < 0.001). Values are the percentage except where otherwise indicated. CHF = congestive heart failure; HTN = hypertension; IHD = ischemic heart disease; HIV = human immunodeficiency virus; AIDS = acquired immunodeficiency syndrome. There were notable differences among the comorbidity indices in the measurement of musculoskeletal and pain-related conditions (Table 1). The Elixhauser method and the Charlson Score include broad indicators for rheumatic diseases, but not for OA or other pain-related conditions specifically. The Elixhauser method identified a larger number of participants as having a rheumatic disease than the Charlson Score (18.4% compared with 5.6%) because the Elixhauser method includes a broader spectrum of diseases. However, neither score includes an ICD-9-CM code for OA in the rheumatic disease category. The RxRisk-V does not contain specific indicators for any rheumatic conditions except gout. However, the RxRisk-V includes indicators for pain medications (opioid and nonopioid analgesics) and pain/inflammation medications (nonsteroidal antiinflammatory drugs [NSAIDs]), which were present for a large proportion of this sample (38.9% and 63.4%, respectively). Although the RxRisk-V comprehensively assesses analgesics and NSAIDs, it does not include indicators for some rheumatoid arthritis (RA) medications, including disease-modifying antirheumatic drugs and biologic response modifiers. Furthermore, methotrexate and cyclosporine (both disease-modifying antirheumatic drugs) are included as indicators of other disease categories (malignancies and transplants, respectively). The RxRisk-V does include an indicator for steroids (i.e., steroid responsive conditions category; 8% of this sample), which are used to treat RA and a variety of other conditions. There are also notable differences among the indices in the measurement of some other conditions of interest among individuals with arthritis. Both the RxRisk-V and the Elixhauser method include indicators for depression, which is a common comorbidity among individuals with arthritis (18). However, the proportion of participants identified as having this condition was higher using the RxRisk-V (24.5% compared with 13.8%). Only the Elixhauser method includes an indicator for obesity (a risk factor for OA