Twenty-eight-joint counts invalidate the DAS28 remission definition owing to the omission of the lower extremity joints:: a comparison with the original DAS remission

Objective: To compare 28 joint disease activity score (DAS28) remission with comprehensive joint count DAS remission in rheumatoid arthritis. Methods: 620 actually measured paired observations of DAS28 and DAS were analysed in 155 patients. Discordant observations ( either DAS or DAS28 below remission cut off level: 1.6 for DAS and 2.6 for DAS28) and concordant observations ( both DAS and DAS28 below their remission cut off level) were analysed separately. Results: 91 of 620 paired DAS observations (15%) were discordant; 87 ( in 53 patients) comprised observations in which the DAS28 remission criterion, but not the DAS remission criterion, was met. The reverse was found in only four observations, which were therefore omitted. With the original DAS as standard, DAS28 sensitivity was 95% and specificity 84%. Probability plots showed a swollen joint count >0 in 75% of discordant pairs v 48% of concordant pairs. The same was found for total joint count (TJC >0 in 90% v 40%; median TJC, 0 v 6) and patient global assessment, but not for ESR. Individual joint analysis showed that 51% of discordant v 18% of concordant observations ( p< 0.0005) had involvement of lower extremity joints that are not included in the DAS28. Conclusions: DAS remission is more conservative than DAS28 remission. Activity ( tenderness and swelling) in joints not included in the reduced joint counts ( ankles, feet) mainly account for the discrepancy between the two assessments. DAS28 remission at a cut off level of 2.6 has insufficient construct validity and should be used with caution in clinical practice and clinical trials.