To compare the DAS28 (Disease Activity Score including a 28-joint count) values of rheumatoid arthritis (RA) and fibromyalgia (FM) patients, and to establish whether high pain levels and impaired mood influence DAS28 values.
DAS28 values were calculated in 62 consecutive patients with RA and in 26 patients suffering from FM. Values for DAS28 scores as well as for the single items of the patient cohorts were compared using Student's t-tests. To evaluate the item weighting and internal consistency of the total score factor analysis was performed and Cronbach's alpha calculated.
RA patients showed a mean DAS28 score of 4.23 (+/-1.2; range 0.77-7.46) and in FM patients the mean DAS28 came to 4.04 (+/-1.13; range 1.19-6.28). DAS28 values of RA and FM patients were not significantly different statistically. Comparing the single components of the score, however, highly significant differences (P<0.0005) occurred between RA and FM patients. Cronbach's alpha for the DAS28 in RA patients amounted to 0.7329, indicating high internal consistency, whereas in FM patients it was 0.4832.
The DAS28, as expected, proved to be inappropriate to express disease activity in FM patients. DAS28 values for expressing disease activity in RA patients may be flawed by coexisting FM and should therefore be regarded with caution as high pain levels more than impaired mood may lead to higher total scores.