Glucose Tolerance Test Reference Information

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The oral glucose tolerance test (OGTT or GTT ) is a diagnosis method for diabetes and reactive hypoglycemia. In short, the GTT consists of drinking a 100g glucose solution and measuring the blood glucose (bG) values every hour to get a curve. The form of the curve tells us a lot about the body's sugar metabolism. A normal (2 hour) GTT is sometimes used to diagnose diabetes, however, this is not a common practice anymore. In order to diagnose reactive hypoglycemia an extended (5 or 6 hour) test is necessary, as with some hypoglycemics symptoms won't occur until the 5th hour.

These days, the GTT is not considered as the instrument for the diagnosis of reactive hypoglycemia anymore, because if one looks at blood glucose (bG) values only quite some healthy persons would be recognised as hypoglycemics too. Twenty-five percent of the normal population will have hypoglycemic symptoms and a blood glucose content less than 50 mg/dL (2.8 mmol/l) when challenging a GTT. Many have blood glucose levels less than 50 mg/dL but remain asymptomatic. Therefore, not only the bG values are of importance, hypoglycemic symptoms must be experienced in the course of the test in order to diagnose someone as a hypoglycemic.

It is not my advice to try this at home. Because of the severe symptoms that may occur during the test (100 g of glucose is a hell of a load) a GTT is normally carried out in hospital. With the help of a glucometer it may also be carried out in any other clinic, though (handheld finger prick) glucometers are not as accurate as laboratory tests on whole blood samples. In case of fast bG level change, psychotic or neurotic behaviour might be present during the test. Especially patients who take cortico-steroids, the birth control pill, diuretics, salicates (Asperin) or fenytoine (anti-epilepticum) are strongly discouraged to try this at home.


Some instructions on how a normal GTT (used in diagnosing diabetes) is carried out. At least three days before the test a diet rich in carbohydrates has to be followed (230-300 grams per day). During the test, no eating or smoking is permitted. The diabetes test lasts for 3 hours.

As said, this test is not suitable to diagnose reactive hypoglycemia with, because low blood sugar levels as a reaction on the glucose intake often occur after 2 hours or more (up to 5 or 6 hours). Therefore, Harris proposed a 6-hour test with 1-hour interval samples. It is advisable to take additional samples at 0.5 and 3.5 (steep curve-slopes are likely to be seen at these times in case of reactive hypoglycemia) and whenever the patient experiences symptoms. No meals after 20.00h on the day before the test. For the sake of accuracy it is recommended to stop any medication (if possible!) a few days before the test, because of their potential influence on the blood glucose metabolism. Nutritional supplements may influence the test results as well.

It is noted, that the given ranges in the curve that is considered normal (below) are an average. The patient's symptoms are just as important as the actual GTT curve.


With all of the following curves, 100 g of glucose (solution in water) is taken at the start of the test. The average blood glucose level at which urinal glucose is found is 170 mg/dl. This may be an indication for diabetes. All of the tests lasted for 6 hours, with blood samples being taken at 30 min, 1 h, 2 h...6 h. To convert the blood glucose level values in mg/dl to mmol/l, divide by factor 18. The presented curves are used in diagnosing several blood sugar metabolism disorders. It is obvious that real curves may vary from person to person (and even from month to month). If the measured values lie within the ranges of the following 'normal curve' and no typical symptoms are present good glucose tolerancy is allowed to be concluded.

Some typical GTT curves may look as follows:

Early diabetes normal fasting glucose, glucose high during the first 2 hours indicating diabetes, lowplasma glucose hours 3 to 4
Postgastrectomy rapid elevation of glucose by 1 hour, rapid decline with a trough at 2 to 3 hours
Idiopathic hypoglycemia normal plasma glucose hours 1 and 2, low glucose hour 3, return to baseline by hour 5
Idiopathic postprandial syndrome have postprandial adrenergic symptoms with normal GTT
More detailed info can be found below, and at these sites: Stop-Don't-Shoot site, Healthgate Database.

Blood Glucose Graph


1. Normal curve 
2. Mild diabetes 
3. Severe diabetes 
4. Continuous low 
5. Pre-hypoglycemia 
6. Mild hypoglycemia 
7. Severe hypoglycemia I 
8. Severe hypoglycemia II 
9. Flat curve 
10. Diabetes and hypoglycemia 
Note: 18 mg/dl = 1 mmol/l, so to convert (US) mg/dl to (European) mmol/l simply divide by a factor 18.

1. Normal curve according to Seale Harris

Time [hours]  0 0.5 1 2 3 4 5 6
bGmin [mg/dl] 80 90 105 90 80 80 80 80
bGmax [mg/dl] 120 135 160 130 110 100 110 105

2. Curve with mild diabetes (Source: Hypoglycemia, Dr. P. Airola)

Time [hours]  0 0.5 1 2 3 4 5 6
bG [mg/dl] 115 145 180 160 120 130 130 130

3. Curve with severe diabetes (Source: Hypoglycemia, Dr. P. Airola)

Time [hours]  0 0.5 1 2 3 4 5 6
bG [mg/dl] 200 235 265 280 300 295 280 270

4. Continuous low values (Source: Orthomoleculair, 3, 1988, G.E. Schuitemaker, in Dutch)

Time [hours]  0 0.5 1 2 3 4 5 6
bG [mg/dl] 60 80 100 60 60 60 60 55
The curve stays under normal levels during the entire test. A rare tumor, called insulinomia, may be the cause. In hospitals, a 3-hour GTT is used in diagnosis.

5. Pre-hypoglycemia (Source: Hypoglycemia, P. Airola)

Time [hours]  0 0.5 1 2 3 4 5 6
bG [mg/dl] 90 115 140 100 85 80 70 75
The curve is typical for a prestage of hypoglycemia. However, a range of mild symptoms may be present at this stage. A 3-hour GTT would not have been long enough to diagnose this type of hypoglycemia.

6. Mild hypoglycemia (Source: Hypoglycemia, P. Airola)

Time [hours]  0 0.5 1 2 3 4 5 6
bG [mg/dl] 80 120 80 60 80 75 80 80
This curve represents a mild form of hypoglycemia. Within the hour, the bG level drops to normal value. During the second hour, the value is far too low, this is typical in case of reactive hypoglycemia. Consequently, the curve rises until the normal value are reached. Because the bG level drops 40 mg% (mg/dl) within half an hour, severe symptoms may occur.

7. Severe hypoglycemia I (Source: Hypoglycemia, P. Airola)

Time [hours]  0 0.5 1 2 3 4 5 6
bG [mg/dl] 95 110 120 105 100 60 40 60
During the first 3 hours, the curve is fully normal. However, this curve depicts a form of hypoglycemia that often occurs and causes severe symptoms. The fact that the curve drops more than 20 mg% (mg/dl) below normal value indicates already that severe symptoms might well possible be present.

8. Severe hypoglycemia II (Source: Hypoglycemia, the disease your doctor won't treat; Saunders, Ross)

Time [hours]  0 0.5 1 2 3 4 5 6
bG [mg/dl] 100 170 110 130 170 125 100 100
This 'sawtooth-curve' indicates a severe form of hypoglycemia, though values do not actually drop below normal values. However, severe symptoms may occur.

9. Flat curve (Source: Hypoglycemia; P. Airola)

Time [hours]  0 0.5 1 2 3 4 5 6
bG [mg/dl] 90 90 90 100 90 100 80 90
This is a flattened glucose tolerance curve. This curve may be characteric for people who conduct a dull, monotonous life. They complain about chronic fatigue, boredom, disinterest and loss of libido.

10. Diabetes and hypoglycemia (Source: Hypoglycemia; P. Airola)

Time [hours]  0 0.5 1 2 3 4 5 6
bG [mg/dl] 100 160 220 160 85 60 50 85
This is a curve of a patient that is both diabetic and hypoglycemic. With such a curve, severe symptoms are likely to be expected, e.g. asthma.

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