Midnight serum cortisol: comparison of healthy volunteers and hospitalized patients with Cushing's syndrome

Diurnal variation in serum cortisol is nearly always absent in patients with Cushing's syndrome (CS), as shown by elevated levels of midnight serum cortisol (MSC). The sensitivity of MSC in the diagnosis of CS has been shown to be 96-100% measured on an inpatient basis. The purpose of this study was to state reference values for MSC measured on an outpatient basis in healthy non-sleeping controls, and to calculate the sensitivity and specificity of the test in patients with verified CS based on these data. Thirty-six healthy volunteers (11 males, 25 females) with a median age of 40.5 (range 22-60) years and a mean body mass index (BMI) of 27.0 +/- 5.9 kg/m(2) were included in the study. Serum cortisol was measured at 24.00h. In 35 CS patients (5 males, 30 females) with a median age of 44.5 (range 23-79) years and mean BMI of 28.5 +/- 6.9 kg/m(2), MSC was measured on the first night after admission to hospital, in a non-sleeping state. All controls, with the exception of one, had MSC values below 200 nmol/L. One CS patient showed an MSC level below 200 nmol/L. Based on these observations, the sensitivity and specificity of the test were 97.1% and 97.2%, respectively, when 200 nmol/L was used as the cut-off limit. Non-sleeping state does not seem to compromise the sensitivity or the specificity of the test.