Reliability of plasma cortisol determination by a simple competitive protein binding method.

The method investigated used (3H) cortisol as the ligand, diluted human plasma as the binding protein, and florisil to separate free and bound cortisol. The precision and specificity were found to be adequate for routine use. The effects of such factors as delay in separation of plasma, storage of plasma, provenance of the binding protein, venepuncture, and venous occlusion have been studied. The extent and significance of short term, inter-daily, and longer term variation in plasma cortisol level are discussed.