Measurements of disease activity, such as the clinical disease activity score (DAS28) or ultrasound (US) scores, often yield discordant results. This study's objectives were to determine the proportion of disagreements between the two assessment methods in patients with rheumatoid arthritis (RA) and to describe factors associated with discrepancy in assessment.

All RA patients in the Swiss registry for inflammatory arthritides (SCQM) with at least one concomitant DAS28 and US score, were included. Disease activity was categorized as remission, low-to-moderate, and high, based on previously established cut-offs, for both the DAS28 and the US score. A longitudinal analysis was performed among patients who underwent at least two assessments.

Of 2369 assessments included (1091 patients), 1196 (50.4%) were discordant. The US score both over- and under-estimated disease activity compared to the DAS28 score (23.5% and 26.8% respectively). Clinical and demographic factors significantly associated with discordant results were the individual components of the DAS28 score when US was used as the reference and age, disease duration, and the swollen joint count when the DAS28 was used as the reference. The main US-related factor associated with discordance was the presence of US tenosynovitis. In the longitudinal analysis of 1081 patients, the proportion of disagreements remained essentially unchanged.

Rates of disagreement between clinical and US assessments of disease activity among RA patients are high and remain high during follow-up, even when the US assessors were aware of the clinical examination findings. Both clinical- and ultrasound- related factors were associated with discordances.