Interference of 21-deoxycortisoi with cortisol assay methods

21-deoxycortisol has been identified by others (1) in the plasma of a patient with the 21-hydroxylase deficiency variant of the adrenogenital syndrome. Because 21-deoxy cortisol* is similar in chemical structure to cortisol, ** we tested the relative ability of this steroid to react as “cortisol” in three commonly employed methods of assaying cortisol: competitive protein binding, fluorometric and the phenylhydrazine (Porter-Silber) reaction. On an equal weight basis, 21-deoxycortisol gave 87% of the value of cortisol in the competitive protein binding assay; and 27% of the value of cortisol in the fluorometric assay. The phenylhydrazine reaction with 21-deoxycortisol was negative so that this steroid did not interfere in the Porter-Silber assay. It is concluded that falsely elevated values of plasma "cortisol" may be found in patients with 21- hydroxylase deficiency when cortisol is determined by the fluorometric or competitive binding methods.