Oral Glucose Tolerance Test for Diabetes - Why it is Done?

Oral Glucose Tolerance Test for Diabetes

Authored by DesiMD Doctor on 30 Oct 2015 - 12:30


Diabetes Mellitus is a condition affecting millions of patients the world over and the numbers are rising. This developing trend is basically due to unhealthy lifestyle including bad diet habits accompanied by sedentary lifestyle. Other risk factors include age, family history, gestational diabetes (during pregnancy), high BP, Polycystic Ovary Syndrome, high cholesterol etc. Most often people are unaware they have diabetes unless its checked for. Therefore a regular health-checkup including the oral glucose tolerance test is a must, to prevent diabetes, especially if you are prone to the risk factors.

The Glucose Tolerance Test (GTT), or the Oral Glucose Tolerance Test (OGTT), is a test to measure the body’s ability to use glucose (sugar), the main source of energy. Those who have diabetes have a problem breaking down glucose because the body does not have adequate supply of insulin.

OGTT is done to test for pre-diabetes and diabetes. This test is normally done to check for diabetes during pregnancy (gestational diabetes).

Glucose tolerance tests are of two types:

a)    Oral glucose tolerance test (OGTT) which is more common

b)    The intravenous glucose tolerance test (IGTT), also known as Fasting Blood Sugar test (FBS).

Why is an OGTT done?

It is done to diagnose:

Preparing for the test:

  • Eat a normal balanced diet in the days leading up to the test. Make sure you eat sufficient carbohydrates (150gms) Starchy vegetables such as potatoes, beans, and corn are good sources of carbohydrate.
  • You are recommended fast, not exercise or smoke for at least 8 hours before your first blood sample is taken.
  • If you are on any medication (OTG drugs or prescription medicines), inform your doctor about it. You may be instructed to stop taking certain medicines before the test.

The glucose tolerance diagnostic test may take up to 4 hours. You are not supposed to eat or drink any beverages, except water during this time, as it may interfere with the results.

During the test:

A sample of blood will be taken after which you will asked to drink a sweetened solution (concentrated glucose solution) within about 5 minutes of time. Post this you will have to wait until your glucose levels will be tested again. Depending on the type of test you are going for, your blood sample will be taken at various times:

a) For the 50-gram or one-hour test, blood sample will be taken after one hour.

b) For the 75-gram or two-hour test, blood sample will be taken every hour for two hours.

c) Similarly if you are taking the 100-gram, or three-hour test, blood sample will be taken every hour for three hours.

These samples of your blood will indicate how quickly your body can process the sugar, which will be explained to you by your healthcare provider.

The likely results of OGTT should be:

i)     People without diabetes:

  • Fasting value (before test): under 6 mmol/L
  • At 2 hours: under 7.8 mmol/L

ii)  People with impaired glucose tolerance (IGT)

  • Fasting value (before test): 6.0 to 7.0 mmol/L
  • At 2 hours: 7.9 to 11.0 mmol/L

iii) Diabetic levels

  • Fasting value (before test): over 7.0 mmol/L
  • At 2 hours: over 11.0 mmol/L

Depending on the results, your doctor will advise a course of treatment. Diet and exercise are very important to follow if you have abnormal levels of glucose. A nutritionist may be referred. Insulin is prescribed in some situations. In case you have gestation diabetes (pregnancy with diabetes) your blood glucose levels must be monitored strictly to avoid any problems.

*Disclaimer: This is not medical advice. The content is for educational purposes only. Please contact your doctor for any health care issues.