Stability of fatigue, pain, patient global assessment and the Bath Ankylosing Spondylitis Functional Index (BASFI) in spondyloarthropathy patients with stable disease according to the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI).

The study objective was to examine natural variation of the patient-reported outcome measures fatigue, pain, patient global assessment (PaGl) and the Bath Ankylosing Spondylitis Functional Index (BASFI) in patients with stable axial spondyloarthropathy (ax-SpA) defined on the basis of the Bath Spondylitis Ankylosing Disease Activity Index (BASDAI). 107 TNF-inhibitor treated stable ax-SpA patients were identified in the Danish rheumatology registry (DANBIO). According to the Assessment of SpondyloArthritis international Society (ASAS) response criteria, stable disease was defined as a change in BASDAI < 20 between two consecutive visits. Data on BASDAI, fatigue, pain, PaGl and BASFI (0-100) from such two visits were extracted for each patient. Lower and upper 95% limits of agreement (LLoA;ULoA) and the mean of intra-individual differences (the bias) were computed for each measure. Associations were described by linear correlations and standard errors of estimation. Mean BASDAI was 35.6 ± 23.8, mean BASDAI change 0.0 ± 9.7 (range - 19 to 19) and mean inter-visit time duration 16 ± 13 weeks. LLoA;ULoA [bias] for fatigue was - 37.4;36.2 [- 0.6], for pain - 34.1;32.5 [- 0.8], for PaGl - 35.7;32.9 [- 1.4] and for BASFI - 23.2;22.6 [- 0.3]. Intra-individual differences in fatigue, pain, BASFI and PaGl were not correlated with the inter-visit time duration, were poorly inter-correlated and were poorly correlated with baseline values and with changes in BASDAI. In conclusion, natural variation of patient-reported outcome measures was substantial and unpredictable in individual ax-SpA patients in steady state defined on the basis of BASDAI. Consequently, observed changes in the daily clinic should be interpreted with caution.