OBJECTIVES: To psychometrically test a generic health status scale in patients with osteo-(OA) and rheumatoid arthritis (RA) using Rasch analysis and; compare health status impairment and identify differential item functioning across two arthritis conditions. METHODS: Data from 101 OA, and 78 RA patients from the Household Component of 1996 Medical Expenditure Panel Survey (MEPS) was analyzed. Rasch Rating Scale model was used. Fit statistic and PC analysis of residuals evaluated unidimensionality of 21-item scale for each group. Item and rating scale functioning were compared across groups by independent t-tests and correlations. RESULTS: Person separation was 1.93 in OA patients and 2.40 in RA. For both groups, the item on perceived mental health had high INFIT/OUTFIT MNSQ (>1.4), while activity limitation item displayed overfit (INFIT/OUTFIT MNSQ <0.6). Analysis demonstrated congruent item hierarchies in both conditions. Invariance in item difficulties and rating scale step measures was revealed by non-significant (alpha=0.05) t-statistic. CONCLUSIONS: Instrument functioned satisfactorily in both groups. Nineteen items showed adequate fit with unidimensional requirements. No conclusive difference in interpretation of items or response categories was observed. Patients with OA and RA demonstrated identical health functional impairments. Therefore, scale appears to be applicable and measures equivalently in OA and RA.