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in Amor criteria 11 and both (SI-MRI/ HLA-B27) in the ASAS classification criteria. In the ASAS axial- SpA criteria 12, two “anchor criteria” were defined in combination with SpA features: the “imaging arm” requires the evidence of sacroiliitis by imaging and the “clinical arm” requires the presence of the HLA-B27 antigen. In peripheral-SpA 13, sacroiliitis on MRI and HLA-B27 were included as additional SpA features. Second, sacroiliitis by MRI and HLA-B27 positivity have been selected as feasible screening methods for axial-SpA 14, and have been included in the decision tree of the original and modified Berlin diagnostic algorithm advised to be used by rheumatologists in daily practice 15. Recently, these two tests were included in the list of parameters of the ASAS-endorsed recommendation for early referral of patients suspected of having axial-SpA by primary care physicians or non-rheumatologists 16. Moreover, in an international survey about referral, diagnosis and management in axial-SpA 17, rheumatologists reported that they belief strongly in imaging (MRI) and systematically request HLA-B27-typing when evaluating a patient in their daily practice. However, these tests are rather costly, and waiting time for MRI is long in many countries. Therefore, many recommend to pre-select patients for HLA-B27 testing and SI-MRI in order to increase diagnostic yield.
Knowledge about criteria to order MRI and/or HLA-B27 in patients suspected of SpA, is rather limited. Since imaging MRI and HLA-B27 are a fundamental part of the ASAS classification criteria, and are considered important elements in the early referral and diagnosis of patients, it is expected that the decision about whether or not to order one or both of these tests will become increasingly important. Therefore, the aim of the present study was to identify and evaluate the patient´s characteristics associated with the clinical decision to ask SI-MRI and/or HLA-B27 in the diagnostic work-up of SpA in clinical practice.
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