Diagnosing axial spondyloarthritis: estimation of the disease probability in patients with a priori different likelihoods of the diagnosis.

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To evaluate the diagnostic value of spondyloarthritis (SpA) parameters and their combination for the diagnosis of axial SpA in patients with an a priori different probability of the diagnosis.A total of 361 patients with chronic back pain and suspicion of axial SpA (181 referred by primary care physicians or orthopaedists, 180 recruited via an online screening tool) received a structured rheumatologic examination, which resulted into a diagnosis or exclusion of axial SpA. The prevalence of axial SpA indicating the pre-test probability was 40% in the physician-referred subgroup and 20% in the online screening subgroup. Sensitivities, specificities, and likelihood ratios for SpA features were determined in both subgroups and the respective post-test probabilities of axial SpA were calculated.The relative diagnostic value of single SpA features varied substantially between the groups with different referral pathways. For instance, HLA-B27 positivity increased the probability of the presence of axial SpA by 35% to 55% in online-screened patients and by 22% to 62% in physician-referred patients. The absence of HLA-B27 resulted in a sharp decrease in the probability of the presence of axial SpA in physician-referred patients (from 40% to 6%). This decrease was less sharp in the online screening group (from 20% to 10%). These differences were especially relevant in patients with a small number (1-2) of positive SpA features.The diagnostic value of SpA features varies in different patient populations, which should be considered in the diagnostic approach.



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Authors: Denis Poddubnyy, Fabian Proft, Laura Spiller, Mikhail Protopopov, Valeria Rios Rodriguez, Burkhard Muche, Judith Rademacher, Murat Torgutalp, Janis Lucas Vahldiek, Joachim Sieper, Imke Redeker

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