Classification Criteria in Axial Spondyloarthritis: What Have We Learned; Where Are We Going?

Spondyloarthritis (SpA) is a chronic inflammatory condition that can have a predominately peripheral or axial presentation. Axial SpA (axSpA) affects the axial skeleton with either radiographic (r-axSpA) or nonradiographic (nr-axSpA) changes. Radiographic changes are a late disease feature and earlier disease stages can be identified by incorporating other imaging methods. The diagnosis of axSpA is a clinical diagnosis and classification criteria are not aimed to be diagnostic tools. The split between r-axSpA and nr-axSpA is artificial and we should move toward the unifying concept of axSpA. Our understanding of genetics, biomarkers, and immunopathophenotypes will drive further refinement of classification criteria.