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Clinical clues that guide the request of a SI-MRI or HLA-B27 can be useful in clinical practice, because of costs and waiting time for SI-MRI. A practice based on inexpensive and widely available clues may be cost-and time-efficient, and their importance may increase over time.
This study has limitations. First, the design was retrospective. Other factors than those that have been measured could have influenced the likelihood to request a SI-MRI or a HLA-B27-test. Second the study may have suffered from missing data and from the necessary exclusion of variables with too many missing data. There was, for instance, an indication that SI-MRI and/or HLA-B27 has been preferentially requested in patients with increased acute-phase reactants (CRP and/or ESR). A univariate comparison was highly statistically significant, but the variables were missing in too many patients and were excluded from multivariable analysis. The high level of missingness for CRP and ESR suggests anyway that decisions of requesting additional tests are certainly not only based on CRP/ESR. Third, the study only addresses one side of the entire spectrum: patients with a diagnosis of SpA. Patients in whom a diagnosis of SpA was not established, with or without a SI-MRI or a HLA- B27 test, were not taken into consideration. This study can therefore never be interpreted as a diagnostic experiment that tests the value of SI-MRI and HLA-B27. This study only gives insight into the circumstances in which SI-MRI or HLA-B27 are requested. It is very reasonable to assume that expensive and scarce diagnostics such as SI-MRI and HLA-B27 tests are only considered if they may help the clinician to increase the likelihood of a suspected clinical diagnosis, but not if the clinician considers the likelihood of SpA as sufficiently high. In that situation, he may rather refrain from asking expensive and time-consuming tests. These Bayesian principles form the basis of proper clinical reasoning. Finally, all patients of this study came from the same country and even the same centre, which importantly limits the external validity and the extrapolation of the findings to other countries and clinics. The results of this study should be interpreted in this context.
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