Radioimmunoassay for cortisol using antibodies against prednisolone conjugated at the 3-position

A radioimmunoassay for the measurement of cortisol in plasma was developed. The procedure requires a one-step precipitation of the proteins in plasma or a one-step extraction with methylene chloride in urine. The method is precise, accurate, sensitive, and very specific. The primary antibody was prepared against prednisolone-3- carboxymethoxime-bovine serum albumin. The method is sensitive to 12.5 pg/ml of plasma. The antiserum has little interference from other endogenous steroids and metabolites of cortisol. 0830 hour plasma levels of cortisol in twenty normal human adults averaged 150 +/- 33 ng/ml.