Analysis of combined rheumatoid factor determinations by the rheumatoid arthritis latex and sheep cell agglutination tests and the American Rheumatism Association criteria for rheumatoid arthritis.

The clinical records of randomly selected patients receiving both the sheep cell agglutination test (SCAT) and the latex agglutination test (RA latex) for rheumatoid factor (RF) were analyzed for the presence of American Rheumatism Association (ARA) criteria for rheumatoid arthritis (RA). When both tests were positive there was a 3-fold increase compared to only one test positive in the relative risk that a patient met ARA criteria for RA, and there was a 2-fold increase in the probability that a patient with 2 positive tests had classical RA compared to only a positive RA latex. The occurrence of RF reactive with both human and rabbit IgG identifies a population of patients likely to have more ARA criteria for RA and classical disease.