This test is indicated if the patient cannot tolerate a large oral carbohydrate load or if the patient has a gastrointestinal condition (disease, surgery) that could affect the rate of glucose absorption from the GI tract. The test is done only infrequently and shows a poor correlation with the oral GTT.


NOTE: This data reflects recommendations prior to the 1997 American Diabetes Association revised criteria.


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.

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


Dose of glucose for test: 0.5 gram per kilogram body weight up to a maximum of 35 grams, given IV as 25 g glucose per 100 mL solution within 1-2 minutes


Specimen Collection:

Glucose levels are drawn fasting, and then 3, 5, 10, 20, 30, 45 and 60 minutes after the infusion is complete. Occasionally plasma insulin levels are drawn at 2, 3 and 5 minutes after the infusion is complete.


Shortly after the infusion, transient glucose concentrations up to 250 mg/dL can be seen in normal individuals, but fasting glucose levels will be achieved by 90 minutes, with subfasting levels at 120 minutes and return to fasting levels at 180 minutes. Transient glucosuria can be seen right after the infusion since the renal threshold for glucose will be exceeded.


The rate in decrease in glucose levels is then determined. Blood glucose levels tend to decrease exponentially.

(1) Normal adults under 50 years of age show a mean rate of glucose disappearance of 1.5% per minute

(2) Normal adults over 50 years of age will show a mean rate of glucose disappearance which declines with the age over 50 (about 0.09% per decade)

(3) Diabetics show a mean rate of glucose disappearance of < 1% per minute.


The rate of decrease can be measured in several ways. Methods include:

(1) The rate of disappearance of glucose expressed as a percent per minute of the 10 minute level (take 10 minute level as 100%, and then the subsequent levels as percentages).

(2) A semilogarithmic rate of decline of glucose from 10 to 30 minutes.


A commonly example of the first method is the equation:


rate of disappearance of glucose in % per minute =

= 70 / (number of minutes required for the blood glucose level to decrease to half of the 10 minute level)


<see documentation file. A plot of glucose levels vs log10(minutes after infusion) is linear, as is a plot of the percent of 10 minute level vs log10(minutes after infusion). >


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