The dexamethasone suppression test in normal control subjects: comparison of two assays and effect of age

Abstract

Competitive protein binding assay and radioimmunoassay were used to measure cortisol levels in 38 normal control subjects 3 times before and 3 times after administration of 1 mg of dexamethasone. Significant interassay differences were found at 2300 h before dexamethasone and at all 3 postdexamethasone times. Analysis of variance revealed significant overall positive relationships between age and cortisol levels measured by both techniques. Age correlated significantly with postdexamethasone cortisol levels measured by radioimmunoassay but not when measured by competitive protein binding assay. Clinicians should obtain data from their laboratories as to appropriate cutoffs for cortisol suppression on the specific assay used.