An enzyme immunoassay design using labelled antibodies for the determination of haptens. Application to methotrexate assay.

A competitive enzyme immunoassay with labelled antibodies has been developed for methotrexate (MTX). Methotrexate in the sample and a constant quantity of this hapten physically absorbed to polystyrene spheres through a methylated bovine albumin carrier were allowed to compete for a limiting amount of peroxidase labelled antibody. After washing, the residual enzyme activity bound to the solid phase was measured. This test was able to detect 10 fm of MTX per sample. A comparative study of this test with a commercial radioimmunoassay kit using the same antiserum and a high pressure liquid chromatography method showed that the sensitivity, specificity and precision of this test were as good as of the radioimmunoassay. The high pressure liquid chromatography method was 500 times less sensitive. Good agreement was found among the 3 methods on 83 serum samples from patients receiving methotrexate therapy.