Oral Glucose Tolerance Test for Acromegaly

INDICATION

Used where a clinical diagnosis of acromegaly is suspected.

PREPARATION

Fasting from midnight.
19g cannula.
6 Red top Vacutainers.
6 grey top fluoride oxalate tubes

METHOD

1. Take blood sample for GH and IGF-1 (into a red top Vacutainer) and glucose (into grey top tube) at T = 0.
2. Administer 75 grams oral glucose in 300 ml water over about 10 minutes.
3. Take blood for GH and glucose at t=30, 60, 90 and 120 minutes.
4. A synacthen test can be carried out at the end of this test, with samples for cortisol taken at t=120, 150 and 180. Synacthen 250 mcg is administered at t=120.

INTERPRETATION

In normal individuals, GH levels fall following oral glucose, and at least one of the samples during the test should have undetectable GH levels. Failure of suppression or a paradoxical rise in GH suggests acromegaly. Following treatment, "cure" is not an appropriate word. A "safe level" of GH has been thought to be less than 10mU/l (3.8 ng/ml). More recently, this has been revised to 5mU/l (1.9ng/ml).

SENSITIVITY AND SPECIFICITY

False positives sometimes occur in patients with anorexia nervosa, or other causes of chronic starvation, although the IGF-1 level is usually normal.