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example, may benefit more from other tests. Physicians do not ask for HLA-B27 (77%) or SI-MRI (only 24%) in all patients but when clinical symptoms are indicative but not confirmatory.
It is also interesting to look at which factors did not contribute. Apparently, clinicians do not base their behaviour (of asking SI-MRI or HLA-B27) on the conventional appreciation that AS is a disease of males. Gender was not a factor that determined a SI-MRI or HLA-B27 request. That is a promising finding in light of the fact that many still consider females with SpA underdiagnosed in spite of findings suggesting that as many females as males may suffer from axial SpA 22.
It is important to mention that this study has only addressed the performance of a test, and not the result of that test. A recent study from the ESPeranza Cohort 23 has assessed the utility of SpA features to anticipate the presence (not the request) of sacroiliitis on MRI or a positive result of a HLA-B27 test in patients with suspected axSpA. They found that IBP according to the ASAS definition plus alternating buttock pain or IBP according to Calin criteria plus awakening in the second half of the night predicted the presence of sacroiliitis on MRI. Our study assigned IBP as a determinant for ordering SI-MRI. Together, these studies confirm that the presence of IBP according to certain expert-criteria not only evokes an SI-MRI-request, but also that this behaviour is rational in that the likelihood of a positive result increases. In analogy, we have found uveitis being predictive of ordering HLA-B27, while in the ESPeranza Cohort uveitis appeared to be the factor related to a positive HLA-B27 (LR+ of 2.6). This finding is also in line with previous studies reporting that a positive HLA-B27 test in a patient presenting with uveitis plus SpA features may increase the likelihood of a diagnosis of SpA 24, 25.
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