Low-dose
dexamethasone suppression testing has been recommended for biochemical screening when
Cushing's syndrome is suspected. The criterion for normal suppression of
cortisol after
dexamethasone is controversial. To assess diagnostic utility (sensitivity), we report the results of low-dose
dexamethasone suppression testing in 103 patients with spontaneous
Cushing's syndrome. There were 80 patients with
Cushing's disease (78%), 13 with the
ectopic ACTH syndrome (13%), and 10 with
cortisol-producing
adrenocortical adenomas (10%). Fourteen (18%) of 80 patients with
Cushing's disease suppressed serum
cortisol to less than 5 micro g/dl (<135 nmol/liter) after the overnight 1-mg test, whereas six patients (8%) actually showed suppression of serum
cortisol to less than 2 micro g/dl (<54 nmol/liter). In addition, the 2-d, low-dose
dexamethasone suppression test yielded false-negative results in 38% of patients when urine
cortisol was used and 28% when urinary
17-hydroxycorticosteroids were used. Serum
cortisol after the 1-mg test correlated with baseline urinary free
cortisol (r = 0.705, P < 0.001), plasma
ACTH level (r = 0.322, P = 0.001), and urinary free
cortisol after the 2-d test (r = 0.709, P = 0.001). This study provides evidence that low-dose
dexamethasone may suppress either plasma
cortisol or urinary
steroids to levels previously thought to exclude
Cushing's syndrome and that these tests should not be used as the sole criterion to exclude the diagnosis of endogenous
hypercortisolism.