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associated with a clinical decision to order a SI-MRI and/or HLA-B27 testing in the diagnostic work- up of SpA. We found that the presence of inflammatory back pain (IBP), enthesitis, the number of symptoms at presentation and uveitis increased the likelihood of ordering MRI and/or HLA-B27
testing. These clinical clues are important, because the presence of these manifestations increase the pre-test probability of axial SpA and therefore the finding of a positive test may significantly contribute to confirming a diagnosis while finding a negative test may help ruling out the disease. This is an appropriate manner of increasing the diagnostic efficiency and optimizing the use of
resources. A recent study from the ESPERANZA cohort has investigated the utility of SpA features associated with a higher likelihood of a positive SI-MRI or a positive HLA-B27 in patients with chronic back pain 5. 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 finding sacroiliitis on MRI. Our findings assigned IBP as a determinant for ordering SI- MRI. Together, these studies confirm that the presence of IBP not only evokes an SI-MRI-order, but also that this behaviour is rational in that the likelihood of a positive result will increase. Our results suggest that clinical reasoning indeed follows the principles of Bayesian theory and that it helps
rheumatologists to improve efficiency of these tests. Moreover, the efficiency of MRI improves when the patients are optimally selected, which relies on training about the appropriate use of SpA
features 6.
In the study presented in chapter 4 we have performed a comparative study with the general population on data from Latin American patients (including Colombia) from the multinational Assessment of SpondyloArthritis international Society of COMOrbidities in SpA (ASAS-COMOSPA) study 7. The most common comorbidities and risk factors were arterial hypertension (AHT), hypercholesterolemia, osteoporosis and gastrointestinal ulcer. The prevalence of AHT (including
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