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The objective of this study is to characterize the association between platelet to albumin ratio (PAR) and disease activity in patients with ankylosing spondylitis (AS) and axial psoriatic arthritis (axPsA). Baseline platelet count, albumin, PAR, C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), Bath ankylosing spondylitis disease index (BASDAI), Bath ankylosing spondylitis functional index (BASFI), and ankylosing spondylitis disease activity score (ASDAS) were collected from patients with a definitive diagnosis of AS or axPsA. Spearman's correlation analysis, quantile regression, and receiver operating characteristic (ROC) curves were performed. Four hundred forty-six patients with AS and 68 patients with axPsA were included. AS patients had higher CRP, ASDAS-CRP, and ASDAS-ESR than axPsA patients (median: CRP, 6.8 vs. 3.5 mg/L, p = 0.02; ASDAS-CRP, 2.32 vs.1.93, p = 0.001; ASDAS-ESR, 2.57 vs.1.97, p = 0.007; respectively). Platelet count, albumin, PAR, ESR, BASDAI, and BASFI did not significantly differ between the two populations (all p > 0.05). In AS patients, PAR was positively correlated with BASDAI (r = 0.204, p < 0.01), BASFI (r = 0.24, p < 0.01), ASDAS-CRP (r = 0.475, p < 0.01), and ASDAS-ESR (r = 0.483, p < 0.01), while these coefficients were not significant in axPsA patients. The quantile regression further confirmed that, in AS patients, PAR was independently associated with BASDAI, BASFI, ASDAS-CRP, and ASDAS-ESR at their individual quantiles (all p < 0.01). However, in axPsA patients, PAR was not significantly associated with these disease activities. The optimal cut-off value of PAR for AS disease activity was 5.87, with an AUC of 0.745, a sensitivity of 72.4%, and a specificity of 71%. PAR could serve as an alternative indicator for AS disease activity. Key Points • Platelet to albumin ratio is independently associated with ankylosing spondylitis disease activity. • Platelet to albumin ratio could serve as an alternative indicator for ankylosing spondylitis disease activity. [ABSTRACT FROM AUTHOR]