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A moderately high degree of concordance (50-70%) was observed among the different criteria sets using the clinical diagnosis as the external reference, adding to the credibility of the concept that these criteria classify a considerable proportion of patients with similar and rather homogeneous characteristics. While none of the single SpA feature was particularly important in explaining differences in the fulfilment of several sets of classification criteria, chronic back pain, IBP, enthesitis and alternating buttock pain were the parameters that particularly differed between patients with a clinical diagnosis of SpA fulfilling and those not fulfilling the criteria. This means that Colombian physicians in this clinic tend to consider a diagnosis of SpA if patients present with these characteristics. These features were remarkably similar across all criteria, which suggests that they are among the core characteristics of SpA, which is entirely in line with the ‘Gestalt’ -the ‘picture’- of this disease.
The results of this study allow interesting interpretations about how rheumatologists in this clinic in Colombia make a clinical diagnosis of SpA. The rate of HLA-B27 positivity in the group of patients with a clinical diagnosis of SpA who did not fulfill the ASAS-SpA criteria was low, and was lower than in those patients with a clinical diagnosis of SpA not fulfilling other criteria sets for SpA. This low rate of HLA-B27 in those patients contrasts with the prominent role of HLA-B27 as the entry criterion in the clinical arm of axSpA-criteria, and as a SpA feature in both axial and peripheral ASAS SpA-criteria. Apparently, rheumatologists did not pay too much attention to the presence or absence of HLA-B27 when considering a clinical diagnosis of SpA, which is in line with the widely disseminated message in the past that many healthy individuals may be HLA-B27 positive without having AS 17. Furthermore, patients who did not fulfil any of the criteria sets, had a higher CRP level than patients who fulfilled at least one the criteria sets. Apparently, the rheumatologist still diagnosed them as having SpA. It seems that a clinical diagnosis of SpA is more easily made when
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