From the Guidelines
During an overnight dexamethasone suppression test, the expected morning cortisol level should be less than 1.8 μg/dL (50 nmol/L) in normal individuals, and the standard dose of dexamethasone used for this test is 1 mg, as recommended by the most recent and highest quality study 1. The test works because dexamethasone, a synthetic glucocorticoid, suppresses ACTH secretion from the pituitary gland in healthy individuals, which subsequently reduces cortisol production by the adrenal glands. Some key points to consider when interpreting the results of this test include:
- A serum cortisol level < 1.8 μg/dL (50 nmol/L) at 0800 h in the morning after 1 mg dexamethasone given between 2300 h and midnight is considered a normal response 1.
- Failure to suppress morning cortisol levels below the threshold suggests abnormal hypothalamic-pituitary-adrenal axis function, which may indicate conditions such as Cushing's syndrome.
- Patients should avoid alcohol and medications that affect dexamethasone metabolism (such as phenytoin, carbamazepine, or rifampin) for at least one week before testing, as these can lead to false results, as noted in 1.
- Measuring dexamethasone concomitantly with cortisol, using laboratory-specific ranges of expected values, can reduce the risk for false-positive results, as recommended in 1. The blood sample should be collected between 8:00 AM and 9:00 AM the following morning, and patients should be advised that this is a screening test, and abnormal results typically require further diagnostic evaluation. It's also important to consider the potential for false positive or false negative results, and to take steps to minimize these risks, such as measuring dexamethasone levels and using laboratory-specific ranges, as discussed in 1.
From the Research
Expected Morning AM Cortisol Levels
- The expected morning AM cortisol levels during overnight dexamethasone suppression testing can vary depending on the dose of dexamethasone used and the specific criteria for suppression 2, 3.
- A study found that using a cut-off for a morning cortisol following overnight dexamethasone of > 5 microg/dL, only three of 17 patients failed to suppress to a value less than this cut-off 2.
- Another study compared the 1 mg and 2 mg overnight dexamethasone suppression tests and found that suppression of serum cortisol to <50 nM (1.8 microg/dL) after dexamethasone administration was chosen as the cut-off point for normal suppression 3.
Dose of Dexamethasone
- The dose of dexamethasone used in overnight suppression testing can vary, with common doses including 1 mg 2, 3 and 2 mg 3.
- A study found that the 2 mg overnight dexamethasone suppression test was more convenient and accurate than the 1-mg test as a screening test for excluding Cushing's syndrome in subjects with simple obesity 3.
- Other studies have used higher doses of dexamethasone, such as 4 mg, in intravenous suppression testing 4 and 8 mg in high-dose dexamethasone suppression testing 5.