To the Editor.
—The dexamethasone suppression test (DST) is of great current interest in psychiatry because of the large body of evidence that some depressed patients fail to suppress cortisol levels to 5 μg/dL or less after an oral dose of 1.0 mg of dexamethasone.1 Many psychiatrists have adopted this procedure as the first laboratory test of significance for clinical practice. Carroll et al pointed out a number of pitfalls in applying this test.1 We wish to call attention to the need to assess rigorously the method of cortisol determination used by the clinical laboratories that carry out the test. Routine application of the 5-μg/dL cutoff could lead to a number of false-positive results, based on the results to be described.Research laboratories, eg, those of Carroll, Sachar, and ourselves, use the competitive protein-binding assay (CPBA) method to measure plasma cortisol levels, whereas most clinical laboratories use commercial radioimmunoassay (RIA)kits.
Fang VS, Warenica B, Meltzer HY. Dexamethasone Suppression Test: Technique and Accuracy. Arch Gen Psychiatry. 1982;39(10):1217. doi:10.1001/archpsyc.1982.04290100073012