Adult-onset Still's disease with a significant rheumatoid factor: examining proper use of diagnostic and classification criteria.

A case is presented of a 32-year-old man with classic clinical adult-onset Still's disease, who had an initially elevated (1:320) but not persistently high rheumatoid factor. Since lack of a high rheumatoid factor is one feature in the proposed classification criteria for adult-onset Still's disease, the patient was given a diagnosis of rheumatoid arthritis. The faulty reasoning behind this diagnosis of rheumatoid arthritis is discussed, focusing on the inappropriate use of classification criteria for individual clinical diagnosis, as well as the occasional need for longitudinal diagnosis.