IV dexamethasone suppression test

Indication

The diagnosis of Cushings is often considered in subjects with obesity and polycystic ovary syndrome. Although it would seem that there is no deterioration in the O/N dexamethasone test in obesity (Cronin et al), this test has been evaluated in subjects with simp,le obesity, PCOS and Cushings and shown to be diagnostically accurate. It may be useful in cases where a O/N test is considered to be a false positive, or in subjects who have lost substantial amounts of weight Cushings (Edelstein et al).

Principle

In normal subjects, dexamethasone suppresses ACTH and therefore cortisol secretion. In Cushing’s syndrome, there is incomplete suppression.

Preparation

Patients should be admitted to day case unit for the test but no special preparation is requried.

Requirements

1. i.v. canula and saline to flush
2. 3 plain blood sample bottles
3. Infusion pump
4. Dexamethasone dissolved in 0.9% (normal) saline.The total dose should be 5 µg/kg/h given for 5 hours. For example, a 70 kg person would receive 350 µg per hour so that a total of 1.75 mg is administered over the 5 hour period.

Procedure

0830h    insert canula into forearm vein

0900h    take 3 mL blood sample for cortisol

               Commence pumped infusion of dexamethasone for 5 hours

1400h    stop infusion

1600h    take 3 mL blood sample for cortisol

1800h    take 3 mL blood sample for cortisol

Interpretation

In simple obesity, the cortisol concentration in samples taken at both 1600 and 1800 will be <50 nmol/L.

References

Atkinson AB, McAteer EJ, Hadden DR, Kennedy L, Sheridan B, Traub AI. A weight-related intravenous dexamethasone suppression test distinguishes obese controls from patients with Cushing's syndrome. Acta Endocrinol (Copenh) 1989;120:753-9.

Cronin C, Igoe I, Duffy MJ, Cunningham SR, McKenna TJ. The overnight dexamethasone test is a worthwhile screening procedure. Clin Endocrinol 1990;33:27-33.

Edelstein CK, Roy-Byrne P, Fawzy FI, Dornfield L. Effects of weight loss on the dexamethasone suppression test. Am J Psychiat 1983;140:338-41.

JHB 23 Nov 2009