Re-evaluation of ELISA and latex agglutination test for rheumatoid factor detection in the diagnosis of rheumatoid arthritis.

An indirect ELISA for the determination of each isotype (IgM, IgG, IgA, IgD, IgE) of rheumatoid factors (RF) was performed with sera obtained from 77 patients with either classical or definite rheumatoid arthritis (RA) and 319 controls, using rabbit IgG as the antigen. The results were compared with those of a commercial latex agglutination test, using denatured human gamma globulin as the antigen for rheumatoid factor determination. At the cut-off level at which positive results were found in less than 5% of normal controls, ELISA for IgM RF determination had sensitivity, specificity, efficiency, positive predictive value and negative predictive value of 46.75%, 98.12%, 88.13%, 85.71%, 88.41%, while those for IgA RF were 46.75%, 93.42%, 84.34%, 63.16%, 87.91% and for IgG RF were 59.74%, 92.16%, 85.86%, 64.78%, 90.46%, respectively. These indices by latex agglutination test were 83.11%, 93.73%, 91.67%, 76.19% and 95.83%, respectively. IgD RF titre greater than or equal to 1:5 was detected in 19/77 RA patients and 4/200 normal controls while IgE RF titre greater than or equal to 1:5 was detected only in 7/77 RA patients. Thus, ELISA did not appear to have any advantage over latex agglutination test for diagnosis of RA.

Dot-blot ELISA for the detection of IgM RF and IgA RF.

Dot-blot ELISA was developed for the detection of IgM RF and IgA RF. Normal rabbit IgG (NRIgG), concentration 100 micrograms/ml, was used as the antigen for dotting on the 0.45 microns pore size nitrocellulose membrane. Serum, conjugate and substrate incubation conditions were at room temperature for 1 hour, 1 hour and 3 minutes, respectively. The membrane with NRIgG dot could be sotred for 6 weeks before use in the assay. Positive results of IgM RF, at the serum dilution 1:800, were found in 31/51 patients with either classical or definite rheumatoid arthritis and 3/68 normal healthy individuals. Positive IgA RF, at the serum dilution 1:100, was found in 27/51 of the former and none of the latter. Significant concordance with high agreement index was found between the results of the dot-blot ELISA developed and those obtained from ELISA performed in microtitre plate (Kappa greater than or equal to 0.78 for IgM RF and 0.83 for IgA RF, p less than 0.001).