Classification and diagnosis of axial spondyloarthritis--what is the clinically relevant difference?

The Assessment of Spondyloarthritis international Society (ASAS) classification criteria for axial spondyloarthritis (axSpA) have added nonradiographic axSpA (nr-axSpA) to the classic ankylosing spondylitis (AS) as defined by the modified New York criteria. However, some confusion remains about differences between classification and diagnosis of axSpA. Our objective was to analyze differences between classification and diagnostic criteria by discussing each feature of the classification criteria based on real cases.

The clinical features of the ASAS classification criteria were evaluated in relation to their significance for an expert diagnosis of axSpA. Twenty cases referred to our tertiary center outpatient clinic were selected because of an incorrect diagnosis of axSpA: 10 cases in which axSpA had been excluded initially because the classification criteria were not fulfilled, and 10 patients who had been previously diagnosed with axSpA because the classification criteria were fulfilled. Upon reevaluation, the former were diagnosed with axSpA while the latter had other diseases.

All items that are part of the classification criteria show some variability related to their relevance for a diagnosis of axSpA. There are clinical features suggestive of axSpA that are not part of the classification criteria. Misinterpretation of imaging procedures contributed to false-positive results. Rarely, other diseases may mimic axSpA.

Because the sensitivity and specificity of the axSpA classification criteria have been around 80% in clinical trials, some false-positive and false-negative cases were expected. It is hoped that their detailed description and discussion will help to increase the understanding of diagnosing axSpA in relation to the ASAS classification criteria.