Glucose Tolerance Testing Reference Range

Page content

The Cost Of Diabetes

It is estimated that a staggering 7.8% of the American population has diabetes, approximately 23 million people. Diabetes is recognised as a leading cause of death and disability (due to complications associated with the disease) in the USA. The disease costs an estimated $174 billion per year in the USA according to estimates from the National Institute of Health.

Diabetes is a disease of glucose management. When people consume food, the body breaks it down into simpler compounds which it needs to thrive through the process of digestion. One of these metabolic products is glucose, the simplest form of sugar, which is the energy source used to power the body. For glucose to be useful, it needs to pass from the digestive system into the bloodstream where it can be carried throughout the body. Once glucose is in the bloodstream, it can supply the body’s cells with the energy they need to function. However, for this to happen, the glucose must be able to pass from the bloodstream and into the cell. A molecule called insulin is required to regulate the transfer of glucose from the bloodstream and into cells. Insulin is a hormone which is produced by the pancreas. In healthy people, the body will produce enough insulin to allow the transfer of the glucose produced by eating, from the bloodstream and into the cells.

In people with diabetes, the body does not produce enough insulin to regulate the amount of glucose within the bloodstream. The level of glucose in blood will rise, but the cells cannot access it. Eventually, some of the glucose will excreted into the urine via the kidneys.

There are three main types of diabetes:

Type 1 diabetes (an auto-immune disease)

Type 2 diabetes (accounts for 90 to 95% of all diabetic patients)

Gestational diabetes (occurs in late pregnancy and usually stops after the child is born)

High and Low Blood Glucose

Hyperglycemia is the name given to the condition of a high blood glucose level (>180 mg/dl). In the absence of enough insulin to transfer glucose into cells, the body is starving and will catabolise (break-down) fat and protein stores. A bi-product of this process is ketosis (sweet-smelling breath) when ketones produced in the body cannot be excreted. In extreme cases, hyperglycemia may result in a diabetic coma and can be fatal.

The condition of an abnormally low blood sugar level is known as hypoglycemia

(< 70 mg/dl). If blood glucose levels drop too far, the patient may suffer from fits or pass out. In diabetic patients, the condition usually results from not eating regularly (skipping a meal) or taking too much insulin. The condition is quickly treated by taking easily absorbed sugars or a glucose tablet.

The Glucose Tolerance Test

As we have seen, diabetes is a disease of uncontrolled blood glucose levels. The definitive test for diabetes is the glucose tolerance test. In this test, the patient is asked to fast for eight hours prior to its administration, then they are given a drink of water in which a specific quantity of glucose (75 to 100g) has been dissolved. The rate at which the glucose is cleared from the body is determined by taking blood samples at regular intervals. There is a glucose tolerance testing reference range which enables clinicians to interpret the results of the test.

An oral glucose tolerance test takes between two and three hours to perform. Blood samples are typically drawn at the outset (baseline value) at 30, 60, 120 and 180 minutes (depending on the dosage of glucose administered). The glucose tolerance testing reference values (from NIH, see 4 below) are as follows:

Normal blood values for a 75-gram oral glucose tolerance test used to check for type 2 diabetes:

  • Fasting: 60 -100 mg/dL
  • 1 hour: less than 200 mg/dL
  • 2 hours: less than 140 mg/dL. Between 140 - 200 mg/dL is considered impaired glucose tolerance (sometimes called “prediabetes”). This group is at increased risk for developing diabetes. Greater than 200 mg/dL is a sign of diabetes mellitus.

Normal blood values for a 50-gram oral glucose tolerance test used to screen for gestational diabetes:

  • 1 hour: equal to or less than 140 mg/dL

Normal blood values for a 100-gram oral glucose tolerance test used to screen for gestational diabetes:

  • Fasting: less than 95 mg/dL
  • 1 hour: less than 180 mg/dL
  • 2 hour: less than 155 mg/dL
  • 3 hour: less than 140 mg/dL

Note: mg/dL = milligrams per deciliter

For information on determining glucose in blood, please read the companion article.


  1. American Diabetes Association:
  2. Mayo Clinic:
  3. National Diabetes Information Clearinghouse:
  4. National Library:
  5. National Institutes of Health: