Background: Rheumatoid factor (RF) and anti-cyclic citrullinated peptide (CCP) antibody tests are often measured at the time of rheumatoid arthritis (RA) diagnosis but may not be repeated and therefore not available in electronic health record (EHR) data; lab test results are unavailable in most administrative claims databases. ICD10 coding allows discrimination between rheumatoid factor positive (M05) (“seropositive”) and seronegative (M06) patients, but the validity of these codes has not been examined. Methods: Using the ACR’s Rheumatology Informatics System for Effectiveness (RISE) EHR-based registry and U.S. MarketScan data where some patients have lab test results, we assembled two cohorts. Seropositive RA was defined having a M05 diagnosis code on the second rheumatologist encounter, M06 similarly identified seronegative RA, and RF and anti-CCP lab test results were the gold standard. We calculated sensitivity (Se) and positive predicted value (PPV) of the M05/M06 diagnosis codes. Results: We identified 43,581 eligible RA patients (RISE) and 1185 (MarketScan) with RF or anti-CCP lab test results available. Using M05 as the proxy for seropositive RA, sensitivity = 0.76, PPV = 0.82 in RISE, and Se = 0.73, PPV = 0.84 in MarketScan. Results for M06 as a proxy for seronegative RA were comparable in RISE, albeit somewhat lower in MarketScan. Over 3 consecutive visits, approximately 90% of RA patients were coded consistently using either M05 or M06 at each visit. Conclusion: Under ICD10, M05 and M06 diagnosis codes are reasonable proxies to identify seropositive and seronegative RA with high sensitivity and positive predictive values if lab test results are not available.