What is Glucose Tolerance Test?

A glucose tolerance test is used to determine the body’s ability to metabolize glucose/sugar obtained from food an individual eats. 

Glucose tolerance test results depends on :

  • How effectively the body produces insulin and
  • Uses produced insulin to bind and move sugar from the blood to cells.

It uses the standardized measured amount of glucose solution to determine Type-1 Diabetes, Type-2 diabetes, and Gestational Diabetes.

It involves measuring blood sugar at predetermined intervals of time.

The results are classified as below depending on glucose levels at that particular interval of time.

  1. Normal,
  2. Impaired and
  3. Abnormal

Who needs to take Glucose Tolerance Test?

  1. If you are overweight.
  2. Have a family history of Diabetes.
  3. Has polycystic Ovarian Syndrome (PCOS).
  4. Has High Blood Pressure.
  5. Was having Gestational Diabetes during pregnancy.
  6. Has delivered a baby weighing 9 pounds or more.

What are the pre-requisites for Glucose Tolerance Test?

  1. The patient should take normal food containing at least150 gms of carbohydrates.
  2. No activity not involved in the usual lifestyle should be performed.
  3. The patient should come fasting for the test in morning and should not take any intake after 10 pm a day before the test.

How is Glucose Tolerance Test performed?

  1. A first fasting blood sample is drawn to establish a baseline glucose level.
  2. Then glucose solution for consumption with a ratio of 1.75 gm of glucose/kg patient weight in 8 ounces (approx. 250 ml) of water. But it should be noted that in no case the concentration of glucose should be higher than 75 gms per 250 ml.
  3. Patients are made to drink the glucose solution.
  4. Further, the samples are drawn at various time intervals.

What are different ways of performing Glucose Tolerance Test ?

Generally, the test is further carried out in 3 different ways depending on the objective and time of sample collection.

  1. In the first case, the Baseline sample is drawn and the next sample is drawn 60 Min. after consumption of the Glucose solution. This test is generally carried out to determine Gestational diabetes in a pregnant woman from 24 to 28 weeks gestation. This test can also be conducted non-fasting.
  2. In the second case, the Baseline fasting sample is drawn and the next sample is collected at 120 min. after consumption of glucose solution. This is generally done to confirm diabetes diagnosis if the glucose levels are abnormal at baseline and 120 min level.
  3. In third type sample is collected at baseline and at 30 mins., 60 mins., 30 mins., and 90 min. and 120 mins. to determine impaired tolerance to glucose and delayed reaction in excretion of insulin from the pancreas or delayed absorption of glucose in the liver.

What are the factors which may affect Glucose Tolerance Test?

Following factors may role in glucose tolerance test.

  1. Diabetic or Pregnancy medication if consumed before the test.
  2. Excessive water consumption and urination during test.
  3. Allergy to sugar and Dextrose.
  4. Just recovered from surgery, trauma or infection.

What are the results of the Glucose Tolerance Test?

The result of the test varies from individual to individual. Further, these results are dependent on various factors enlisted above. Generally, the below representative results are obtained.

Following are the results of tests for Type-1 and Type-2 Diabetes:

Sr. No. Time/Interval of testNormal ResultsImpaired ResultsAbnormal Results
1Fasting glucose level60 to 100 mg/dL100 to 125 mg/dLGreater than 126 mg/dL
2One-hour glucose levelLess than 200 mg/dL
3Two-hour glucose levelLess than 140 mg/dL140 to 200 mg/dLGreater than 200 mg/dL
Results of Glucose Tolerance Test for Type-1 and Type-2 Diabetes.

Following are the probable results of the test for Gestational Diabetes:

Sr. No.Time/Interval of testNormal ResultsAbnormal Results
1Fasting glucose levelLess than 90 mg/dlGreater than 95 mg/dL
2One-hour glucose levelless than 140 mg/dLGreater than 140 mg/dL
3Two-hour glucose levelless than 120 mg/dLGreater than 120 mg/dL
Results of Glucose Tolerance Test for Gestational Diabetes.
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