Spondyloarthritis (SpA) refers to a group of HLA-B27-positive associated rheumatic diseases that share clinical and genetic features. The diseases and conditions that constitute the SpA group are defined by signs, symptoms, and radiographic findings, and consist of ankylosing spondylitis (AS), reactive arthritis (ReA), the SpA subsets psoriatic arthritis (PsA), Crohn’s disease (CD) and ulcerative colitis, and a subgroup of undifferentiated forms1. Traditionally, patients with SpA may be classified according to Amor, et al2 and the European Spondylarthropathy Study Group (ESSG) 3 criteria. Patients fulfilling such criteria may then be classified or diagnosed according to specific criteria, clinical picture, or diagnostic tests for AS, ReA and SpA subsets PsA, CD and ulcerative colitis, for example. In general, patients with SpA that do not fulfill such classification or diagnostic-specific criteria remain unclassified and comprise the subgroup known as undifferentiated SpA (USpA). USpA accounts for a significant but variable proportion of patients with SpA in the open population4, 5, in a group of relatives of probands with AS6, 7, and in hospital-based registries of SpA8-10. As a group, USpA are mostly characterized by isolated episodes of peripheral arthritis and enthesitis, axial symptoms, and by a lower incidence of HLA-B27 versus AS.

As a group, SpA are characterized by overlapping of clinical manifestations over the disease course11. For example, patients with AS may develop psoriasis and CD, patients with ReA may develop psoriasis, and patients with psoriasis may developed CD and AS. Similarly, patients with USpA may develop any specific clinical manifestation and fulfill specific diagnostic criteria some time later. In this context, it is clear that patients with SpA fulfilling specific criteria of more than 2 SpA constitute a “true SpA overlap,” while patients presenting with nonspecific or isolated manifestations of SpA without fulfilling specific SpA diagnostic or classification criteria constitute the group of USpA.