Quantitative determination of rheumatoid factor by an enzyme-labeled immunoassay

The presence and quantity of rheumatoid factor (RF) in human serum were determined by an indirect enzyme-linked immunoabsorbent assay (ELISA). A human rheumatoid factor control serum, standardized against the WHO reference rheumatoid arthritis serum preparation, was used to derive a standard curve in each assay. The results of unknowns were estimated from the standard curve and reported in International Units (IU) per milliliter (ml). The ELISA assay was compared with the bentonite flocculation test. The overall coefficient of correlation between the two assays was 87%; it was 93.3% for patients with rheumatoid arthritis but only 67.5% for patients with undiagnosed conditions. The error of the ELISA assay (coefficient of variation) was generally less than 10% at both high and low concentrations of rheumatoid factor. The quantitative reproducible nature of the assay allows the detection of small variations of rheumatoid factor level and could be useful in the serial evaluation of patients.