Development of diagnostic criteria for psoriatic arthritis: methods and process.

Psoriatic arthritis is an inflammatory arthritis associated with psoriasis but this simple definition obscures the difficulty in defining the condition for epidemiologic purposes. Since the original description by Moll and Wright, a number of other classification criteria have been published. None of these have been fully evaluated and only one has been developed using patient derived data. Methodologic problems in developing classification criteria--their purpose, utility, and accuracy--are discussed in relation to a new International collaboration, the International Multicentre Validation of Diagnostic Criteria for Psoriatic Arthritis (CASPAR).