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signs of inflammation are present, even though characteristic features of SpA may be lacking or insufficiently present.
AS as a clinical diagnosis implied the fulfillment of more criteria sets. Patients with AS had the highest availability of important tests (imaging and HLA-B27), as compared with other subtypes. Apparently, physicians who suspect a patient of having AS often test not only HLA-B27 but also ask for MRI, even though both a HLA-B27 and an MRI result are formally redundant. This finding regarding additional testing was similar when analyzing patients who had chronic back pain. Interestingly, these tests were far less frequently ordered if a patient had ReA or uSpA, while in the context of current thinking, they would have been most useful to corroborate a clinically made diagnosis. Explanations could be that in case of a patient with ‘clinical AS’ the physician wants to have supportive information, for example because the pelvic radiograph is often equivocal 18, which may lead to the fulfillment of more criteria sets. In case of ReA, on the other hand, the physician may not consider the disease as part of the (axial) SpA-spectrum and does not require supportive tests, which will lead to the fulfillment of less criteria sets. In support of this thesis, we could delineate a considerable number of patients with a clinical diagnosis of SpA who fulfilled only one set of classification criteria: these patients often had a diagnosis of uSpA or ReA.
Our results are in concordance with other studies reporting on the validity of the ASAS-criteria in a clinical rheumatology setting using the clinical diagnosis as reference standard. The DECLIC-study 19 performed in patients with chronic back pain, the SPACE-cohort 20, which is a cohort of patients younger than 45 years with chronic back pain lasting between 3 months and 2 years, and the Spanish ESPERANZA-program 21, which includes patients referred to clinics for early SpA, all found that the ASAS-criteria had a higher sensitivity when tested against a clinical diagnosis of axSpA than
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