LAUSR

The last decade has seen the development or renewal of classification criteria in sundry rheumatic diseases, including systemic lupus erythematosus (SLE),1 rheumatoid arthritis2 and axial spondyloarthritis (axSpA).3 These criteria seek the laudable aim of standardising the populations included in clinical trials and observational cohorts for research purposes. But to what extent have the benefits of classification criteria been realised? Have there been unintended consequences as their profile has grown? And could we better use criteria to achieve the desired end of facilitating the implementation and interpretation of research findings to enable their translation into clinical practice? We are all familiar with the refrain of key opinion leaders when they present their update on the management of a disease—‘These are classification criteria, not diagnostic criteria …’ But how often have we heard the same speaker move smoothly on to state, ‘… but I find them helpful in clinical practice too’? Indeed, the abstract of the original paper describing the validation and final selection of the classification criteria for axSpA concluded with the statement that ‘The new Assessment of SpondyloArthritis international Society (ASAS) classification criteria for axial spondyloarthritis (SpA) … may help rheumatologists in clinical practice in diagnosing axial SpA in those with chronic back pain’.3 However, the dangers of applying classification criteria to clinical practice for the purpose of diagnosis are easily demonstrated. The criteria are invariably developed and validated in specialist centres where there is a high prior (pretest) probability of the disease, and they are evaluated based on a clinical diagnosis of the disease in question made by experts in that condition, after alternative causes or explanations for the patient’s symptoms have been excluded. When the criteria are applied to a different population—for instance, in primary care—where there is a low prior (pretest) probability of …