The oral glucose tolerance test can be useful in diagnosing patients with diabetes, by monitoring glucose levels following an oral glucose challenge.


Test Preparation:

(1) Starting 3 days prior to test, the patient receives a diet containing 150 g of carbohydrate per day.

(2) The patient must not be stressed by illness prior or during the test.

(3) All nonessential medications should be discontinued at least 3 days prior to testing; many medications can impair glucose tolerance.

(4) A 10-16 hour fast is recommended (minimum 8 hours)..

(5) Undue exercise before or during the test is to be avoided.

(6) No coffee or smoking is allowed once the fast has started or during the test.


Initial evaluation:

(1) If the fasting glucose is < 95 mg/dL, then diabetes is excluded

(2) If the fasting glucose is 95 - 125 mg/dL, then an oral glucose tolerance test should be done

(3) If the fasting glucose is >= 126 mg/dL, then the diagnosis of diabetes is supported.


Dose of glucose for test:

(1) 75 g orally (1.75 grams of glucose per kilogram of ideal body weight, up to 75 grams)

(2) Oral glucose solutions come in 10 US fluid ounces (296 mL) bottles containing 50, 75 or 100 grams of glucose (5, 7.5 and 10 g per fluid ounce).


The diagnosis of diabetes requires that at least 2 glucose values exceed the cutoffs for the various specimen collection times.



Whole Blood


95 mg/dL

86 mg/dL

1 hour

180 mg/dL

162 mg/dL

2 hour

155 mg/dL

140 mg/dL


Previous criteria for the interpreting the oral glucose tolerance test:


Nonpregnant adult



impaired glucose tolerance


70-105 (70-115 if > 60 y.o.)

>= 140 mg/dL

< 140 mg/dL

30 minute (0.5 hour)




60 minute (1 hour)




90 minute (1.5 hour)




120 minute (2 hour)


> 200 mg/dL

140-200 mg/dL

** at least one of these results must be >= 200 mg/dL


The abnormalities in glucose tolerance test must be present on at least 2 occasions before the diagnosis of diabetes mellitus is made. An oral tolerance test need not be done

(1) if a fasting glucose is >= 126 mg/dL at least twice

(2) a random glucose level is > 200 mg/dL and classic symptoms (polyuria, polydipsia, ketonuria, weight loss) are found


Drugs that may cause a decreased glucose tolerance include beta-adrenergic blocking agents, catecholamines, corticosteroids, diuretics, estrogens, oral contraceptives, phenothiazines, phenytoin, thyroid hormones, and several others.


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