A direct radioimmunoassay of serum cortisol with in-house 125I-tracer and preconjugated double antibody.

We describe a radioimmunoassay of serum cortisol in which 125I-tracer and preconjugated double antibody are used. This simpler method involves direct serum assay, and the majority of reagents are prepared in-house. Within-assay precision (coefficient of variation) of assay of Ortho Diognostic RIA Cortisol I and II was 3.2% (mean = 131 microgram/L, n = 10) and 2.2% (mean = 405 microgram/L, n = 10), respectively. Between-assay precision (coefficient of variation) for the same controls was 6.7% (mean = 128 microgram/L, n = 62) and 5.6% (mean = 395 microgram/L, n = 62), respectivley. By using automatic pipetters and a gamma counter with a data-reduction package, a technologist can complete 100 assays in less than 3 h. The operating cost is approximately 10 cents per tube.