OBJECTIVES:  Controlling for disease severity in observational studies is crucial to get an estimate with no selection bias. However, outcomes research studies using claims data, contain no information about disease severity. Therefore, comorbid scores are used for a proxy for the disease severity. There exists no severity score specific for rheumatoid arthritis (RA).  The goal of this study was to develop a severity index for rheumatoid arthritis (SIFRA) for private health care claims data.   METHODS: We extracted the following variables related to rheumatoid arthritis from the claims data: total number of synthetic disease-modifying anti-rheumatic drugs (DMARDs), total number of biological DMARDs, tests for C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) ordered, rehabilitation visits, rheumatology visits, Felty’s syndrome and Sjogren’s syndrome, pulmonary, soft tissue nodules, joint surgery, number of platelet counts and chemical panels ordered, and rheumatoid factors testing. A linear regression model was used to create the severity score. The severity score was compared with the rheumatoid arthritis medical records-based index of severity (RARBIS) and currently-used comorbidity scores to proxy severity in outcomes research studies related with rheumatoid arthritis. RESULTS:   According to the Akaike Information Criterion (AIC), Bayesian Information Criterion (BIC), log likelihood function, R-squared values and average squared prediction error, SIFRA performed better than RARBIS, Charlson Comorbidity Score (CCI),  Elixhauser comorbidity score and Chronic disease score. Spearman correlation with RARBIS was 0.65 and significant. However, the correlation with the Charlson Comorbidity Index (0.1, p=0.6521), Elixhauser Index (0.15, p=0.5312) and Chronic disease score (0.13, p=0.6011) were low and insignificant. CONCLUSIONS:  Comorbidity scores (Charlson, Elixhauser or Chronic Disease Scores) commonly used in outcomes research are inadequate to be proxy variable for RA patients. SIFRA, at least for rheumatoid arthritis, controls for disease severity better than any other commonly used measure.