Study on diagnostic value of three kinds of measuring methods for rheumatoid factor in rheumatoid arthritis

Objective Rheumatoid factor (RF) is one of the important bases in diagnosing rheumatoid arthritis (RA). There are three kinds of measuring methods in the common use. They are latex agglutination test (LAT), enzyme linked immunosorbent assay (ELISA) and turbidimetric assay. This study was aimed to compare the diagnostic value of the three methods with receiver operating characteristic (ROC) curve and to obtain the optimal cut off value of turbidimetric assay in diagnosing RA. Methods Tests for rheumatoid factor were performed by the three methods in our rheumatologic clinic among the outpatients and inpatients with swollen or painful joint in Shanghai Huashan hospital from July to December 2001. The gold standard of diagnosing RA is based on the presence of four or more of the revised 1987 ARA criteria. According to the sensitivity and specificity, ROC curves were drawn. Results The areas under ROC of LAT, ELISA (IgG RF, IgA RF, IgM RF) and turbidimetric assay were 0.732, 0.739, 0.774, 0.901, 0.786 respectively. The difference between the areas under ROC of IgM RF and that of the others was significant ( P 0.05). The optimal cut off value of turbidimetric assay in diagnosis of RA was 45 U/ml. Conclusion ELISA method for IgM RF has higher diagnostic value in RA, which would be as an alternative for LAT. The optimal cut off value of turbidimetric assay in diagnosis of RA is 45 U/ml.