Comparison of two rapid cortisol radioimmunoassays for use in the fetal sheep

Cortisol radioimmunoassays (RIA's) utilizing highly specific antisera combined with a simple ethanol protein precipitation procedure (ETOH-PPT) are widely utilized to measure cortisol in human plasma. This same type of RIA has been assumed specific for measurement of cortisol in the plasma of several different species of experimental animals. In order to test this assumption as applied to fetal ovine plasma, we compared an ETOH-PPT cortisol RIA with another rapid cortisol assay which utilizes a dichloromethane extraction (DM-E) step. The DM-E assay in turn was compared with a chromatographic assay previously shown to be highly specific for measurement of fetal plasma cortisol in this species. Fetal ovine plasma cortisol concentrations determined by the DM-E method were nearly identical to the concentrations obtained by the specific chromatographic RIA procedure. On the other hand, the ETOH-PPT RIA grossly overestimated cortisol concentrations when compared with the DM-E RIA. While the rapid DM-E RIA appears to be suitable for use in fetal ovine plasma, the widely used ETOH-PPT RIA yields spuriously high and unpredictable values and must be considered unreliable. These comparisons demonstrate the need for careful reassessment of steroid assays prior to their application in experimental animals even though they have been previously documented as specific in human plasma.