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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.

Several screening tools have been developed to facilitate early diagnosis of psoriatic arthritis (PsA); however, their performance varied greatly across different studies. In this study, we validated and compared the performance of four screening tools in detecting undiagnosed PsA Chinese patients with psoriasis, and determined the key questions and their weights. The four screening tools were the Early Arthritis for Psoriatic Patients (EARP) questionnaire, Psoriatic Arthritis Screening and Evaluation (PASE) questionnaire, Psoriasis and Arthritis Screening Questionnaire (PASQ), and Psoriasis Epidemiology Screening Tool (PEST). The receiver-operator curve (ROC) with area under curve (AUC) was used to determine sensitivity, specificity, and accuracy. Least absolute shrinkage and selection operator and logistic regression were utilized to retrieve key questions, and a nomogram was utilized to visualize their weights. Of 482 psoriasis patients from dermatology clinics, 77 were newly diagnosed with PsA. Another 68 patients with newly diagnosed PsA from rheumatology clinics were incorporated in the analysis. ROC analysis indicated that the optimal cut-off values for EARP, PASE, PASQ, and PEST were 3, 40, 7, and 3, with corresponding sensitivities of 91.4%, 88.6%, 86.2%, and 88.5%, and specificities of 88.6%, 75.2%, 80.2%, and 83.6%, respectively. The AUC of EARP (0.925) was higher than those of PASE (0.885), PASQ (0.905), and PEST (0.827). However, none of them were sufficiently sensitive to identify pure axial PsA (sensitivities of EARP, PASQ, and PASE were 25.0%, 36.8%, and 42.1%, respectively). Twelve key questions were retrieved from these four tools to establish a nomogram with a high discrimination (C-index = 0.993) and a good calibration (mean absolute error = 0.014). In conclusion, to screen undiagnosed PsA, EARP has slightly better balanced sensitivity and specificity, and higher accuracy. The retrieval of key questions and nomogram signify the necessity of attributing different scores to differently weighted questions when developing a new screening tool to make it function more efficiently.

To define the equivalent cut-off values of Bath ankylosing Spondylitis Disease Activity Index (BASDAI) for discriminating disease activity corresponding to Ankylosing Spondylitis Disease Activity Score (ASDAS) cut-off values, and to determine the equivalent change units for determining the clinically improvement between ΔBASDAI and ΔASDAS-CRP.475 patients with axial spondyloarthritis (axSpA) whose data on BASDAI and ASDAS were available were included. Among them, 154 (32.4%) patients whose data on ΔBASDAI and ΔASDAS-CRP were available. Receiver-operator curve (ROC) with area under curve (AUC) was used to determine the BASDAI cut-off values that best corresponded to ASDAS-CRP. The Cohen's kappa was utilised to assess the degree of agreement between disease activity states based on BASDAI and ASDAS cut-off values, and clinically improvement between ΔBASDAI and ΔASDAS-CRP.According to the ASDAS-CRP, 88 (18.6%), 130 (27.4%), 191 (40.1%) and 66 (13.9%) patients were classified as inactive, moderate, high and very high disease activities, respectively. ROC revealed that BASDAI values 1.6 (AUC: 0.948), 2.9 (AUC: 0.790) and 3.8 (AUC: 0.875) best corresponded to ASDAS-CRP values 1.3, 2.1 and 3.5, respectively. The degree of agreement between them was moderate (kappa: 0.527). The ΔBASDAI 1.6 (AUC: 0.745) and 2.0 (AUC: 0.708) best corresponded to the ΔASDAS-CRP 1.1 (minimal clinically important improvement) and 2.0 (major improvement), respectively. The degree of agreement was good (kappa: 0.685).The BASDAI values 1.6, 2.9 and 3.8 correspond to ASDAS-CRP values 1.3, 2.1 and 3.5, respectively. The ΔBASDAI 1.6 and 2.0 best correspond to the ΔASDAS-CRP 1.1 and 2.0, respectively.

Additional file 1: Table S1. Primary antibodies information. Table S2. Primers used in PCR. Figure S1. C3aR is expressed dominantly in microglia, lowly in neurons, but not in astrocytes.