Background/Purpose: The Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) is the most commonly used tool to assess axial disease in psoriatic arthritis (PsA). However, five of its six questions focus on general disease activity rather than being specific to axial involvement. Furthermore, back pain itself is a non-specific symptom and is potentially affected by many factors. This raises concerns regarding the utility of the BASDAI back pain question for monitoring axial disease in a multidomain disease like PsA. We aimed to evaluate the utility of the BASDAI and its back pain subscore in assessing axial disease in patients with PsA.

Methods: Patients with available BASDAI scores (done every 6 months after 1999) were selected from a large longitudinally assessed PsA cohort initiated in 1978. These patients are followed every six months with comprehensive documentation of demographic data, clinical and patient-reported outcomes, and treatment details. Axial disease was defined as sacroiliitis (modified New York criteria) and/or syndesmophytes. The trend of BASDAI and back pain was studied across axial and peripheral subgroups over the follow-up duration. The association of axial disease with total BASDAI and BASDAI back pain scores was studied using multivariable linear mixed models (LMM) for the whole cohort, adjusting for swollen joint count (SJC), tender joint count (TJC), enthesitis, dactylitis, Psoriasis Area and Severity Index (PASI), modified Steinbrocker score, and drug use. Drivers of BASDAI and BASDAI back pain scores were studied in the peripheral and axial subgroups using multivariable GEE models.

Results: Of 1,059 patients, 610 (57.6%) had peripheral disease, 449 (42.39%) had axial involvement at any time over their follow-up. The mean age was 44.4 years (SD 12.8), with 55.9% being males.The BASDAI and back pain scores were similar between the axial and peripheral subgroups (Figure 1).When compared to patients with peripheral disease, the presence of axial involvement did not show an association with BASDAI [β -0.14 (-0.05 to 0.33)] after adjustment for confounders. However, the presence of axial disease was associated with a small but significant effect on the BASDAI back pain subscore [β 0.3 (0.06 to 0.55)] as compared to peripheral disease. (Figure 2)Among patients with axial disease, higher BASDAI back pain scores were positively associated with TJC [β 0.06 (95% CI: 0.074–0.08)], enthesitis [0.49 (0.25-0.74)], PASI score [0.03 (90.02–0.05)], along with age and sex. In patients with peripheral disease only, BASDAI back pain was significantly driven by SJC [0.05 (0.01-0.08], TJC [0.05 (0.03-0.07)], enthesitis [0.62 (0.40-0.84)] and PASI score [0.04 (0.02-0.07)] along with age and sex.

Conclusion: BASDAI and BASDAI back pain scores are influenced by age, sex, peripheral joint (arthritis, enthesitis), and skin involvement in PsA, regardless of axial involvement. Similar factors drive this association in patients with axial and peripheral disease. These findings highlight that the BASDAI back pain question is not a reliable outcome measure for assessing axial disease in PsA

Figure 1: Smooth trajectories of BASDAI and BASDAI back pain over follow-up

Figure 2: Multivariable linear mixed models for association of axial disease with BASDAI and BASDAI back pain

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/understanding-the-drivers-of-basdai-and-back-pain-scores-in-psoriatic-arthritis/