OBJECTIVES: Compare four methods of calculating disease-related cost. METHODS: This study utilized the Truven Marketscan® claims database and two disease cohorts, Rheumatoid Arthritis (RA) and Ulcerative Colitis (UC). Patients were age 18+, had two diagnoses for either RA or UC, no other autoimmune conditions and were continuously enrolled in 2015. Disease cohorts were matched to controls using age, gender, region and insurance type. In cost methods 1, 3, and 4, disease-related inpatient costs were attributed using the primary diagnosis field. In addition, method 1 included outpatient cost if the disease was on the claim and pharmacy cost using a pre-determined drug list. Method 2 included outpatient/pharmacy cost for procedures/medications occurring at increased frequency in the disease cohorts versus controls (1.5x and 3.5x were both tested). Method 3 used the difference in total cost between disease cohorts and matched controls. Method 4 (RA only) served as our internal standard method and utilized a separate stand-alone clinician review of codes received by the RA cohort. Outpatient/pharmacy claims flagged as RA-related were included in the Method 4 analysis. RESULTS: