OBJECTIVES: To apply a previously validated claims-based severity index for rheumatoid arthritis (SIFRA) to prevalent rheumatoid arthritis (RA) groups in Turkey and assess the effect of claims-based RA severity on health care costs and biologic use.  METHODS: The Turkish national health insurance database MEDULA (01JUN2009-31DEC2011) was used for the study. Prevalent RA patients were required to be age 18-99 with two RA diagnoses ≥60 days apart and continuous enrollment 1 year pre- (baseline period) and post-index date (follow-up), which was the first RA claim during the identification period (01JUN2010-31DEC2010). SIFRA was calculated for the baseline period. For the follow-up period, total health care costs and biologic use were examined. To determine health care costs, generalized linear models were applied, and multivariate logistic regression determined the effect of SIFRA on outcome measures for biologic use.  RESULTS: A total of 1,920 RA patients were identified. The mean SIFRA score was 14.21. There was a significant variation in scores across cities. Study results confirmed increased biologic use in more severe patients. After adjusting for differences in age, gender, region and comorbidity index, patients in the high SIFRA tercile were 5.16 times more likely to be prescribed biologics (p<0.001, confidence interval [CI]: 3.46-7.69), and incurred more annual health care costs in the amount of €2,091 (p<0.001, CI: €1,557-€2,625) than those in the low SIFRA tercile.  CONCLUSIONS: This study showed that RA severity is a significant determinant of healthcare costs and biologic therapy use. Biologic use was positively correlated with the severity score. According to severity scores, the total medical costs of RA patients in Turkey ranged from €1,435 to €3,275. Since statistically omitting a variable from population models provides biased and inconsistent estimates, any comparative effectiveness studies on RA treatment should include severity scores in the analysis.