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Criteria for classification
Retrospectively each patient who had a clinical diagnosis of SpA was classified according to ASAS axial and peripheral SpA, ESSG and Amor-criteria. If the presenting symptom was ‘back pain’, patients were classified according to the axSpA-criteria. If the presenting symptom was ‘arthritis, enthesitis and/or dactylitis’, patients were classified according to the pSpA-criteria (also if they reported back pain in the past). The ESSG and Amor-criteria were applied using radiographs for the imaging criterion. In these criteria (ESSG and Amor), when the patient had MRI findings of “sacroiliitis on MRI” (active inflammatory changes), this finding was used as a substitute for radiographic damage into the original criteria to fulfill the imaging criterion. If information on a SpA feature or a test was not available, it was considered absent.
Data analyses
Disease characteristics were analysed using descriptive statistics, such as mean (SD) for continuous variables and percentages for categorical variables. The classification rate was defined as the proportion of patients fulfilling a particular criterion (numerator) and the total number of patients in the cohort (denominator). The concordance between the different criteria sets was analysed representing the proportion of patients who were also captured by another criterion. Venn- diagrams were constructed representing patients who met one or more of the ASAS, ESSG, and Amor-criteria sets. In addition, we present patients with overlap between clinical diagnosis and fulfillment in the axial and peripheral ASAS-criteria in a Venn-diagram. Disease characteristics of patients fulfilling each classification criteria set vs. those who did not, were compared by t-test for continuous variables and by Chi-Square test for categorical variables. All analyses were performed using SPSS version 20, p-values ≤0.05 were considered statistically significant.
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