Attempts to diagnose ankylosing spondylitis (AS) earlier based on clinical manifestations and based on MRI as a new imaging modality which is able to depict inflammation without structural damage has led to the concept of axial spondyloarthritis (axSpA) with its two subentities non-radiographic axSpA (nr-axSpA) and AS. The subentity nr-axSpA is defined by the absence of definite radiographic sacroiliitis that is usually required for the diagnosis of AS. Many subsequent studies including the ASAS classification criteria cohort that analyse the entire spectrum of axial SpA revealed that patients with nr-axSpA are similar to patients with AS in many aspects but are not identical. In these cohorts it was found that nr-axSpA patients were consistently more often females, had less often elevated CRP and had a slightly lower prevalence of HLA-B27 as compared to AS patients. This had led to speculations that male gender and elevated CRP are predictors for progression in axial SpA, including the progression from nr-axSpA to AS. Regarding male gender it was know for a long time that among AS patients males have more severe radiographic damage in the spine than females, and recent studies have indeed confirmed a role for CRP as a predictor of progression in AS. Available data suggest that after 2 years about 10% of patients with nr-axSpA progress to AS. In this context, the implication of the lower prevalence of HLA-B27 among nr-axSpA as compared to AS is less well understood. Potentially, nr-axSpA may harbour a proportion of patients with rather mild disease. Misdiagnosis of nr-axSpA as a result of misinterpretation of clinical symptoms and manifestations or of misinterpretation of imaging findings (MRI) cannot be entirely excluded either. This once more underlines the importance of a correct clinical diagnosis based on clinical judgement and experience and should remind us of the potential misuse of the ASAS classification criteria for axial SpA as diagnostic criteria in patients with back pain.
Disclosure of Interest M. Rudwaleit Shareholder of: none, Grant/Research support from: none, Consultant for: Abbvie, BMS, MSD, Pfizer, Roche, UCB