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Introduction
The unifying concept initially termed seronegative spondarthritides, which first lumped together an interrelated yet heterogeneous group of disorders, was a pivotal step forward in the modern classification of rheumatic conditions.1 However, classification of rheumatic diseases has evolved over the years following progress in knowledge of the clinical features, genetic predisposition, pathophysiology and aetiology of the various disorders. In this issue, Rudwaleit and colleagues2 present new Assessment in Spondyloarthritis International Society (ASAS) classification criteria for peripheral spondyloarthritis (SpA) (see article on page 15). We will discuss the implication of this new set of classification criteria in view of the history and present state of the concept and terminology of SpA, the controversy of lumping and splitting, and the potential for the future use of the new criteria in research and clinical practice. Additionally, we put into perspective the recent elucidation of the frequency of chlamydial infections in undifferentiated SpA (uSpA) and the potential for their cure with combination antibiotic treatment, which might have implications for future classification of SpAs.
History and present state of the concept
The first classification criteria for ankylosing spondylitis (AS) were proposed in 1963 at the European Congress of Rheumatology in Rome based on the clinical experience of European rheumatologists, who viewed AS a disease entity separate from rheumatoid arthritis (RA), in contrast to US rheumatologists, who described rheumatoid spondylitis as a variant of RA.3 As the seminal event in the early 1970s, however, Moll et al1 formulated the unified concept of seronegative polyarthritis, which they termed seronegative spondarthritides under consideration, the prefix ‘spond-’emphasising the strong association between each member of the group and AS. The diseases initially included were AS, psoriatic disease, Reiter's syndrome, the arthropathies of ulcerative colitis and Crohn's disease, Whipple's disease and Behçet syndrome with the suggested common characteristics: absence of rheumatoid factors, absence of …