A comparison of the cortisol suppression index and the dexamethasone suppression test

The cortisol suppression index (CSI) (the ratio of pre- to postdexamethasone serum cortisol concentrations) was compared to the dexamethasone suppression test (DST) in endogenously depressed DST suppressors (N = 20) and nonsuppressors (N = 21) and in normal controls (N = 23). The 8 a.m. CSI detected 17.1% and 31.7% of endogenous depressives at the 4.0 and 6.0 thresholds; for the 4 p.m. CSI, rates were 48.8% and 65.9%. The 4 p.m. CSI produced 17.4% and 39.1% false positives in normal controls at the two thresholds, whereas the false positive rate for the DST was 4.3%. These data suggest that the CSI is not as specific as the standard DST for the detection of endogenous depression.