The Assessment of SpondyloArthritis international Society (ASAS) classification criteria for axial spondyloarthritis (SpA) allow SpA classification of HLA–B27–positive patients if ≥2 specific clinical SpA features are present. We investigated the performance of these clinical ASAS criteria in a population with a high prevalence of HLA–B27.
A total of 807 persons reporting chronic back pain (CBP) lasting for >4 weeks during a population survey underwent a clinical, laboratory, and radiologic evaluation. The ASAS criteria for axial SpA were then used to determine classification status.
Only 332 patients (41% of all CBP patients) fulfilled the prerequisite ASAS definitions for CBP (duration of ≥3 months and onset at age <45 years). In this ASAS-defined CBP cohort (51% women, CBP onset at age 27.2 years, 17% HLA–B27 positive), ASAS classification criteria for axial SpA were met by 8.4% of patients. Radiographic SpA by the modified New York criteria was present in 2.4%, while 6% fulfilled the clinical arm of the ASAS SpA criteria only. One-fifth of patients with clinical SpA developed radiographic evidence of SpA after a median of 8 years.
Application of the clinical ASAS classification criteria in an area with a high prevalence of HLA–B27 leads to significant increases in the prevalence of axial SpA compared to radiologic SpA among CBP patients. This increase in the prevalence of disease is likely to have significant ramifications for patient management and health care systems.