1 Assessment of compliance in the overnight dexamethasone suppression test.
2 Assessment of the serum concentration of dexamethasone during corticosteroid replacement therapy.
Dexamethasone is administered for the diagnosis of Cushing’s syndrome. Inadequate suppression of the cortisol concentration in serum may be due to non-compliance, Cushing’s disease, endogenous depression or obesity. Non-compliance can be verified or dismissed by measuring the concentration of dexamethasone in serum.
2 Monitoring therapy.
The dose of dexamethasone (plasma half-life approx. 3 – 4h) required for adequate replacement varies widely between patients. Therapy may be monitored and regulated by use of this assay.
The patient is given a 1 mg dexamethasone tablet to be taken between 11 pm and midnight. Blood is taken for cortisol measurement at 09.00h.
In those patients who fail to suppress their cortisol levels after an overnight dexamethasone test, and in whom non-compliance is suspected, send 0.5 mL serum to the SAS laboratory. Record the cortisol result on the request form.
2 Monitoring replacement therapy.
Take blood (5 mL) into a plain tube immediately before and midway between doses of dexamethasone.
Send serum (0.5 mL) to the SAS laboratory. Record on the SAS request form the time of sampling, the serum cortisol concentration and (when monitoring therapy) the timing and dose of dexamethasone.
Compliance (l mg dose): 2-16 nmol/L.
Centre offering this assay
English J, Chakraborty J, Marks V, Parke A. A radioimmunoassay procedure for dexamethasone. Eur J Clin Pharmacol 1975; 9: 239-244.
Tsigos C and Chrousos GP. Differential diagnosis and management of Cushing’s syndrome. Ann Rev Med 1996; 47: 443-461.
Young MC, Cook N, Read GF and Hughes IA. The pharmacokinetics of low-dose dexamethasone in congenital adrenal hyperplasia. Eur J Clin Pharmacol 1989; 37: 75-77.