Interference by rheumatoid factor with the detection of C-reactive protein by the latex agglutination method.

The concentration of the acute phase serum protein C-reactive protein (CRP) is used as an index of disease activity in patients with rheumatoid arthritis and with other disorders associated with rheumatoid factor (RF). When CRP was determined by the latex agglutination method, RF caused both increased titers for CRP and false positive reactions for CRP. These effects of RF were abrogated by pretreatment of test samples with 2-mercaptoethanol. Detection of CRP by radial immunodiffusion was not affected by RF. Therefore, the clinical relevance of CRP detected in RF positive sera by latex agglutination can be ascertained only following pretreatment of sera with 2-mercaptoethanol or by the use of radial immunodiffusion to circumvent interference by RF.