Validation of the ASAS criteria and definition of a positive MRI of the sacroiliac joint in an inception cohort of axial spondyloarthritis followed up for 8 years

Background The new Assessment of SpondyloArthritis international Society (ASAS) criteria classify axial spondyloarthritis (SpA) into human leucocyte antigen-B27 and/or imaging-based arms. To aid implementation, ASAS has proposed a definition of a positive MRI for active sacroiliitis. Objective The authors aimed to test the diagnostic and predictive value of the ASAS criteria and definition of a 'positive' MRI. Methods Baseline MRI scans on 29 patients with early inflammatory back pain and 18 controls were read independently by four experienced rheumatologists. Both arms of the criteria were tested against a 'gold standard' of physician diagnosis of SpA. MRI abnormalities were assessed according to a global assessment of MRI and the ASAS definition. Sensitivity, specificity and likelihood ratios for individual and concordant reader data were calculated for axial SpA diagnosis at baseline and the development of radiographic sacroiliitis, fulfilling the modified New York criteria at 8 years. Results All patients were classified as having axial SpA, with more patients fulfilling the imaging arm (83%, n=24/29) than the human leucocyte antigen B27 arm (62%, n=18/29). Concordant reader data showed that the baseline MRI had high diagnostic utility for SpA according to global assessment (sensitivity/specificity: 66%/94%, LR+ (positive likelihood ratio) 11.8, LR- (negative likelihood ratio) 0.4) and ASAS definition (sensitivity/specificity: 79%/89%, LR+ 7.1, LR- 0.2). Likewise, a positive baseline MRI had 100% sensitivity for subsequent radiographic sacroiliitis by either assessment, although specificity was lower (56% for global assessment and 33% for ASAS definition). Conclusion Both arms of the ASAS criteria have good diagnostic utility in early SpA, although they are of limited value for the prediction of radiographic progression. This may be due to the definition of a positive MRI for sacroiliitis that lacks specificity at baseline.