Laboratory diagnosis of rheumatoid diseases.

The diagnosis of most collagen diseases requires particular clinical findings. Rheumatoid factor assays are tests commonly used to detect seropositive rheumatoid arthritis. Latex tests are highly sensitive for rheumatoid factors but relatively nonspecific for rheumatoid arthritis, while sheep cell agglutination tests are moderately insensitive for rheumatoid factors but highly specific for rheumatoid arthritis. These tests are not helpful in following the course of the disease, however, since titers remain stable. Serial observations of elevated C-reactive protein levels or ESR may be more helpful in this regard. Specific antibodies have been identified for some collagen diseases, notably the mixed connective tissue disease syndrome, aiding in differential diagnosis.