Background/Purpose: The use of patient-reported outcome measures has become routine in clinical practice and in clinical trials. In order to use any outcome measure in the daily clinic, the natural variation of the outcome measure must be known. Natural variation may also be characterized as measurement error and is assessed in individuals who are considered to be in “steady state”. The objective of this study was to examine natural variation of fatigue, pain and patient global assessment (PaGl) in patients with stable spondyloarthropathy (SpA) defined on the basis of BASDAI (0-100).

Methods: 107 SpA patients treated in the daily clinic with a TNF-inhibitor and characterized by stable disease were identified in the Danish rheumatology registry (DANBIO). According to ASAS response criteria for biological treatment in SpA [1] and to previous reports on BASDAI measurement errors [2], stable disease was defined as a change in BASDAI ≤ 20 between two consecutive visits. Paired data from a single set of such two visits were extracted for each patient. Data comprised BASDAI, BASFI, PaGl, pain and fatigue scored on 0-100 visual analogue scales (VAS). Natural variation was examined using the Bland-Altman method with calculations of lower and upper 95% limits of agreement (LLoA;ULoA) between two consecutive assessments and the corresponding bias (mean of individual differences). Associations between intra-individual inter-visit differences (∆) were described by linear correlation (r) and stepwise multiple regression analyses (partial regression coefficients (rp) and standard errors of estimation (SEE)).

Results: Mean age was 44±14 years, mean BASDAI 35.6±23.8, mean BASFI 33.0±24.8, mean fatigue 49.3±28.2, mean pain 35.6±26.9, mean PaGl 40.8±27.9, mean inter-visit time duration 16±13 weeks (range 3 – 91 weeks) and mean ∆BASDAI 0.0±10.5 (range -20 – 20, NS). Biases were close to 0 for all the variables indicating stable conditions on the group level between the two consecutive visits. On the individual level, however, differences were more pronounced: LoA;ULoA [bias] for fatigue was -37.4;36.2 [-0.6, NS], for pain -34.1;32.5 [-0.8, NS], for PaGl -35.7;32.9 [-1.4, NS] and for BASFI -23.2;22.6 [-0.3, NS]. No significant correlations were found between the absolute ∆values of BASDAI, BASFI, fatigue, pain or PaGl and the inter-visit time duration (r range -0.1 – 0.2, ns). ∆fatigue, ∆pain, ∆PaGl and ∆BASFI were only weakly correlated with ∆BASDAI (r range 0.30 – 0.60, p < 0.01). In multiple regression analyses, ∆fatigue, ∆pain and ∆PaGl were best predicted by ∆BASDAI, and ∆BASFI by ∆pain (rp range 0.40 – 0.49, p < 0.0001). Although these associations were highly statistically significant, SEEs were high (range 10.8 – 15.6) illustrating poor predictability in individuals.

Conclusion: Independently of time duration, fatigue, pain and PaGl fluctuated substantially and unpredictable in individual SpA patients who were considered to be in steady state. A change in these outcome measures of 35 or less (VAS 0-100) may be interpreted to reflect natural variation or measurement error which should be taken into account when managing individual patients in the daily clinic. References: 1. Braun J et al. Ann Rheum Dis 2003; 62: 817-24. 2. Madsen OR et al. Clin Rheumatol 2010; 29: 849-54.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/fatigue-pain-and-patient-global-assessment-fluctuate-substantially-in-spondyloarthropathy-patients-with-stable-disease-according-to-basdai/