Background: RF is commonly tested in primary care to support or exclude a diagnosis of RA. We investigated the utility of RF in the diagnosis of RA in primary care using the Information System for the Development of Research in Primary Care (SIDIAP) in a retrospective cohort study. SIDIAP is an electronic database with clinical information from the primary care records of more than five million people representing 80% of the Catalonian population aged (cid:2) 14 years. Methods: We included patients (cid:2) 18 years who were tested for RF from 1 January 2006 to 31 December 2011.From this population, we identified patients with an incident GP-coded diagnosis of RA in the year following RF testing (from 1 January 2006 to 31 December 2012). We excluded participants with RA diagnosed > 1 year after RF testing. We calculated the sensitivity, specificity, likelihood ratios and predictive values for RF, ESR and CRP. We used receiver operating characteristic(ROC) curves for RF, ESR and CRP against an incident diagnosis of RA made in the following year and estimated the areas under the ROC curve (95% CI) to identify the best theoretical thresholds. We estimated age and gender-adjusted odds ratios (OR) for a diagnosis of RA in the year following RF testing using the Mantel–