Laboratory diagnosis of rheumatoid arthritis. Prospective study of 85 patients.

Because early diagnosis of rheumatoid arthritis is difficult increasingly important, we have assessed the value of laboratory investigation in 85 patients with knee effusions studied from presentation and followed for sufficiently long periods to allow a definite diagnosis. Histopathology on needle biopsy specimens narrowed the differential diagnosis to rheumatoid arthritis and closely related conditions even at an early stage of disease and also allowed recognition of other conditions which would not otherwise have been detected. Immunofluorescence on similar specimens further narrowed differential diagnosis since the presence of IgM was found to be very suggestive of rheumatoid arthritis. Other tests were of less value. It is concluded that laboratory investigation can improve diagnostic sensitivity and specificity in relatively early rheumatoid arthritis.