OP0048 DIAGNOSING AXIAL SPONDYLOARTHRITIS: ESTIMATION OF THE DISEASE PROBABILITY IN PATIENTS WITH A PRIORI DIFFERENT LIKELIHOODS OF THE DIAGNOSIS

Background: The diagnostic approach in axial spondyloarthritis (SpA) relies on a estimation of the post-test disease probability that is based on evaluation of positive and negative results of diagnostic tests in the context of the pre-test disease probability. Objectives: To evaluate the diagnostic value of SpA parameters and their combination for the diagnosis of axial SpA in patients with an a priori different probability of the diagnosis. Methods: 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 as a part of the OptiRef study [1], 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 (LRs) for SpA features were determined in both subgroups and the respective post-test probabilities of axial SpA were calculated. Results: The relative diagnostic value of single SpA features varied substantially between the groups with different referral pathways – see the online disease probability calculator http://www.axspa.de/calculator.html. It can be seen that the diagnostic values of the SpA parameters vary substantially between the groups. 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. Furthermore, 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%). Furthermore, combinations of parameters performed differently in the studied subgroups. Figure 1 illustrates that the observed differences in the diagnostic values of the SpA parameters in different subgroups were only clinically relevant in the presence of a low number of positive test results. For instance, combining IBP with anterior uveitis increased the post-test probability for axial SpA to 78% in the online screening group and to 87% in the physician-referred group, whereas using HLA-B27 positivity and active sacroiliitis on MRI in combination with IBP resulted in a surge in the post-test probability of the presence of axial SpA to around 95% in both groups. Conclusion: The diagnostic value of a single diagnostic test in the clinical practice is not fixed and a number of factors including the referral pathway can affect it. Fluctuation of the diagnostic values is especially relevant when the number of positive parameters is low (1-2). References: [1]Proft F, et al. Semin Arthritis Rheum. 2020;50:1015-1021. Acknowledgements: The OptiRef study was supported by a research grant from Novartis. Disclosure of Interests: None declared