Application of 125I-labelled second antibody to the solid-phase immunoassay of norethisterone in plasma

The concept of employing 125I-labelled second antibodies for making quantitative measurements of small molecules in biological fluids was evaluated. In a model system 125I-labelled second antibodies were used to monitor the binding of purified norethisterone antibodies to albumin-norethisterone complexes immobilised upon plastic tube surfaces. Conditions were established to provide an assay response similar to other immunoassays for norethisterone using the same primary antiserum. The specificity and accuracy of the assay were comparable to the conventional solid and liquid-phase assays, but precision tended to be less at low levels of norethisterone (less than 200 pg/tube). A reduction in assay variance may be achieved through more efficient tube-washing procedures.