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The influence of peripheral arthritis on disease activity in ankylosing spondylitis patients as measured with the Bath Ankylosing Spondylitis Disease Activity Index.

OBJECTIVES
To assess the differences in disease activity as measured by the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) between patients with and without peripheral arthritis/enthesitis. To investigate whether scores on the BASDAI change by omitting the 2 questions on peripheral disease.


METHODS
Disease activity was assessed on a 10-cm visual analog scale and by BASDAI. Alternative BASDAIs were constructed by omitting the peripheral joints question and/or the enthesitis question. Correlations between the alternative BASDAIs and other measures of disease activity were calculated. Generalized estimating equations (GEE) were used to assess whether having peripheral arthritis influenced BASDAI and alternative BASDAI scores, and to assess whether peripheral arthritis influenced the score of the individual questions of the BASDAI.


RESULTS
At baseline, the BASDAI was calculated in 214 patients. In patients with peripheral arthritis (n = 56), the mean (SD) BASDAI score was 4.4 (2.3) as compared with 3.1 (1.9) (P < 0.0001) in the patients without peripheral arthritis (n = 158). The relationship between arthritis and the BASDAI score appeared to be truly longitudinal (GEE regression coefficient beta = 0.64; 95% confidence interval 0.28-1.00). Peripheral arthritis was significantly longitudinally associated with all separate item scores of the BASDAI. Omitting the peripheral joints and/or enthesitis question from the BASDAI questionnaire only partially explained the difference in BASDAI score between the 2 groups.


CONCLUSION
Disease activity measured by the BASDAI is higher in patients with concomitant peripheral disease compared with patients with disease restricted to the axial skeleton. The increased BASDAI score in patients with peripheral arthritis is partially explained by increased overall disease activity as well as by a disproportionate contribution of the peripheral joints question to the overall score.