Variability in Cortisol Level Assay Methods

To the Editor.—  The review by Meltzer and Fang in the May 1983 issue of the Archives,¹ and the letter by Wood et al in the same issue,² highlighted the variability of assay methods for cortisol levels in the dexamethasone suppression test (DST). Both reports were concerned with the accuracy of commercially available radioimmunoassay (RIA) kits for cortisol level determination; it is not surprising that there has been little overall agreement among such kits for cortisol values in the concentrations at which the DST is considered positive or negative (approximately 50 ng/mL). Commercial RIA kits for cortisol level testing generally have reagents apportioned such that a cortisol concentration of 50 ng/mL falls at the low end of the standard curve, and thus is imprecise.³ On the other hand, given the excellent specificity of currently available antibodies for cortisol, the RIA should be a better measurement technique than