Oral Glucose Tolerance Test (OGTT)

ORAL GLUCOSE TOLERANCE TEST (OGTT)


DESCRIPTION
A blood sample to determine the fasting (baseline) blood glucose for the patient is drawn first. Then, the patient drinks a highly concentrated glucose solution (75 gm/300 mL for nonpregnant adults and 100 gm/300 mL for pregnant women). Subsequently, a timed series of blood glucose tests is performed at 30, 60, 90, and 120 minutes for nonpregnant adults and 1,2, and 3 hours for pregnant women to determine the rate of removal of glucose from the bloodstream. This test is not performed if the fasting blood sugar is > 140 mg/dL since virtually all patients will have blood glucose determinations that meet or exceed the diagnostic criteria for diabetes mellitus.

PURPOSE
OGTT is used to diagnose [or rule (R/O)] overt diabetes, glucose intolerance, cushing’s syndrome, and acromegaly.

FINDINGS

A.Normal
Adult, Non-pregnant :
Fasting blood glucose 115 < mg/dL
After 75 gm of oral glucose : [30 min < 200 mg/dL], [60 min < 200 mg/dL], [90 min < 200 mg/dL], [120 min < 140 mg/dL]

B.Abnormal
Adult :

a.Diabetes Mellitus
Sustained elevated blood glucose levels during at least 2 OGTTs.
The 2 hour sample and at least one other between 0 and 2 hr > 200 mg/dL.

b.Impaired Glucose Tolerance :
[2 hr OGTT blood glucose level of 140 to 200 mg/dL],
[0 to 2 hr OGTT blood glucose level > 200 mg/dL]

c.Gestational Diabetes
This diagnosis may be made if 2 blood glucose values equal or exceed : [Fasting : 105 mg/dL], [1 hr : 190 mg/dL], [2 hr : 165 mg/dL], [3 hr : 145 mg/dL]

PHARMACY IMPLICATIONS

1. Patient should be instructed to fast overmight (12 hr)
2. 75 gm glucose (Glucola) is given to nonpregnant women on morning of the test.
3. Insulin or oral hypoglicemics should not be given until after test is completed.
4. The following drugs should be discontinued at least 3 days before the test: hormones (including oral contraceptives), alcohol, salicylates, indomethacin, diuretics (especially thiazide), guanathidine, hypoglycemic agents, propranolol, corticosteroids, MAOIs, lithium, nicotinic acid, phenothiazines, and ascorbic acid.

Source : Boh, L,E (ed), 1996, Clinical Clerkship Manual, Applied Therapeutics,Inc. Washington, 6-19