28-joint counts invalidate the das28-remission definition due to the omission of the lower extremity joints: A comparison with the original das-remission

Robert BM Landewé ^1*, Désirée MFM van der Heijde ¹, Sjef van der Linden ¹ and Maarten Boers ²

¹ Dept internal medicine/rheumatology, University Hospital Maastricht, Netherlands
² Dept clinical epidemiology & Biostatistics, VU Medical center, Amsterdam, Netherlands

^* To whom correspondence should be addressed. E-mail: rlan{at}sint.azm.nl.

Accepted 2 October 2005

Abstract

Background: Remission is an increasingly important goal in the treatment of rheumatoid arthritis (RA). We compared the performance of DAS28-remission with limited joint counts and DAS-remission with comprehensive joint counts.

Patients and methods: We analysed 620 actually measured paired observations of DAS and DAS28 in 155 patients. Discordant (either DAS or DAS28 below remission cut-off level; 1.6 for DAS and 2.6 for DAS28) and concordant (both DAS and DAS28 below their remission cut-off level) observations were analysed separately.

Results: 93/620 (15%) paired DAS observations were discordant; 89 (in 54 patients) comprised observations in which the DAS28 remission criterion, but not the DAS remission criterion, was met. The reverse was found in only 4 observations, which were therefore omitted. With the original DAS as standard, DAS28 sensitivity was 95% and specificity was 83%. Probability plots showed that a swollen joint count >0 was found in 75% of discordant pairs vs. 48% of concordant pairs. The same was found for TJC (TJC>0 in 90% vs. 40%; median TJC: 0 vs. 6) and patient global, but not for ESR. Individual joint analysis showed that 51% of the discordant observations vs. 18% of the concordant observations (p<0.0005) had involvement of lower extremity joints that are not included in the DAS28.

Conclusions: DAS remission is far more conservative than DAS28 remission. Activity (tenderness and swelling) in joints not included in the reduced joint counts (ankles, feet), but not differences in ESR, account largely for the discrepancy found between DAS remission and DAS28 remission. Therefore, DAS28 remission at a cut-off level of 2.6 has insufficient construct validity and should not be used in clinical practice and clinical trials.

Keywords: Disease activity score, joint count, probability plot, remission, rheumatoid arthritis

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