Simple Method for the Determination of Plasma Corticoids

Abstract

A simple dialysis method of estimating cortisol and corticosterone in human plasma has been described, utilizing the steroid-binding properties of plasma. The addition of increasing amounts of unlabeled cortisol to an equilibrium dialysis system containing standard plasma and a constant amount of cortisol-4-C14 caused a proportional decrease in the percentage of cortisol-4-C14 bound to the plasma protein. Preparations of plasma containing unknown amounts of cortisol also decreased binding; the cortisol content could thus be determined from a standard curve. This method required only one ml of test plasma and gave a standard deviation of ±1 μg over the range of 0–10 μg/100 ml. The technique was highly specific for cortisol and corticosterone in human plasma and was affected neither by hemolysis nor by drugs which interfered with other methods. Values obtained by this method were similar to those obtained using the method of Nelson and Samuels. The mean recovery of cortisol added to plasma was 91 %. Since many serial plasma samples could be readily determined, the method proved very suitable for studies of ACTH stimulation and suppression, diurnal variation, and response to Metopirone.