Low Dose Dexamethasone Suppression Test

INDICATION

Screening test for Cushing's syndrome, especially if the result of the overnight suppression test contradicts other investigations. In women with a high testosterone this test may be used to differentiate PCO and partial hydroxylase deficiencies (CAH) from autonomous androgen secreting tumours.

CONTRAINDICATIONS

Patients on enzyme inducing drugs e.g. anti-convulsants may rapidly metabolise dexamethasone.
Oestrogens (e.g. pregnancy, HRT or COC) may induce cortisol binding protein and artificially increase total cortisol levels.
Care in diabetes mellitus and patients who are psychologically unstable.

PREPARATION

This is usually an inpatient test with no particular patient preparation.
Stop all oral oestrogen therapy 6 weeks prior to test. Patients on sex steroid implants might generate results that are difficult to interpret. Measuring SHBG and CBG might be helpful in this circumstance.

METHOD

1. The patient takes 0.5 mg dexamethasone p.o. at strict 6 hour intervals (i.e. 0900h, 1500h, 2100h and 0300h) for 48 hours.
2. The cortisol is measured at 0900h (before the first dose) on the first day ("2+0") of the test and 48 hours later (6 hours after the last dose) ("2+48"), samples are taken in red top Vacutainers (serum). The same sample can be used to measure SHBG and CBG if needed.

INTERPRETATION

If the 0900h cortisol ("2+48") value is less than 38nmol/l the patient has shown suppression. Failure to suppress is seen in the autonomous secretion of cortisol found in Cushing's syndrome. In virilisation from PCO or partial hydroxylation deficiencies there will be suppression of testosterone. This is not seen in ovarian or adrenal tumours.

SENSITIVITY AND SPECIFICITY

Suppression in patients with Cushing's syndrome is rare (2-5%). Some reported cases metabolise dexamethasone slowly and so achieve higher circulating levels than expected. This test is more specific than the overnight suppression test with a lower false positive rate. Failure of suppression in patients is rarely seen in patients with systemic illness, endogenous depression, or on enzyme inducing drugs e.g. phenytoin or rifampicin.
In virilisation some cases of PCO do not show suppression so imaging and venous sampling is required to exclude ovarian or adrenal tumours.

REFERENCES

Crappo A., Metabolism 28, 955-979 (1979).