A Glucose tolerance test is one of the lab tests to diagnose diabetes. It measures the body’s response to glucose. It is usually done to diagnose type 2 diabetes. It is modified in the case of testing for gestational diabetes.
It is one of the gold standard tests i.e. it is accurate than other diabetes tests.
The most common type of glucose tolerance test is the oral glucose tolerance test (OGTT).
Why a glucose tolerance test?
In most cases, a simple blood glucose test isn’t enough to detect diabetes. There are chances of people getting borderline results on normal blood tests, to overcome this a glucose tolerance test may be useful. Unlike other tests, glucose tolerance tests can detect prediabetes too. It can show when the body can’t manage blood sugar levels even before it reaches the stage of diabetes. In the case of normal people, blood glucose levels increase after a meal and go back to normal after a period of time. A glucose tolerance test is able to distinguish this from diabetes or prediabetes.
Fasting is required prior to the test and a liquid containing glucose is given during the test. Normally, the body moves the glucose from the blood into the cells. Hence, the glucose levels in the blood would be reduced. When this process doesn’t work properly (diabetes or prediabetes) the glucose remains in the bloodstream and hence higher levels of glucose are found in the blood samples.
Preparation for a Glucose tolerance test:
- In the days prior to the test, one should eat and drink normally.
- One should not smoke on the morning of the test.
- One should keep track of any illnesses or injuries prior to the test.
- This test requires one to fast for 8 to 12 hours before the test.
Glucose tolerance test procedure:
The glucose tolerance test procedure involves several steps:
- Prior to the test, the individual is required to fast for 8 to 12 hours.
- Before the test begins, a sample of blood is taken from the individual. This will be the fasting blood glucose level of the individual.
- The individual is then asked to drink a liquid containing 75 grams of glucose. This should be drunk within 5 minutes.
- Blood is drawn after intervals, usually 2 hours. The intervals and number of samples depend on the purpose of the test. Usually, the 0 and 2 hours sample will be enough.
- This test may usually take up to 3 hours.
Reliability of the glucose tolerance test:
For the test to be reliable, the individual must be in good health i.e. not even affected by a single illness. The reliability of the glucose test increases if the person is active normally. To have reliable results, the individual shouldn’t be taking any medication that may affect the blood glucose level.
Side effects of the test:
There aren’t significant side effects of the glucose tolerance tests. However, there can be few side effects due to the needle or the sugary drink at times. These side effects are usually not very common.
Due to the needle/blood test:
- Excess Bleeding when not done properly.
- It can hurt a little during an attempt to find a vein.
- Can cause infection in very rare cases.
Due to the sugary drink:
Glucose tolerance test results:
The glucose tolerance test results may give one of these diagnoses:
Normal: A person is said to have a normal blood sugar level if the glucose tolerance test gives a value of less than 140 mg/dL.
Prediabetes or Impaired glucose tolerance (IGT): A person is said to be diagnosed with prediabetes if the blood glucose level is between 140 and 199 mg/dL. Prediabetes increases the risk of developing diabetes. It also increases the risk of heart diseases.
Diabetes: A blood glucose level above 200 mg/dL means that the person is diagnosed with diabetes.
Diabetes during pregnancy: A pregnant woman has diabetes if the blood glucose level is above 153 mg/dL
Glucose tolerance test for gestational diabetes:
Glucose tolerance test also proves effective to diagnose gestational diabetes. Gestational diabetes occurs in pregnant women. There are two types of tests that are used to diagnose it. These are:
Initial glucose challenge test: One has to drink a glucose syrup solution. One hour later, the blood is drawn to check the blood sugar level. A normal result is lower or equal to 140 mg/dL. The doctor usually recommends a second test, the glucose tolerance test.
Glucose tolerance test: This involves fasting overnight and then testing the fasting blood sugar level. Then, one is asked to drink a glucose syrup solution and the blood sugar is checked every hour for a span of 3 hours. If at least two values are more than the normal blood sugar value, then the person is diagnosed with gestational diabetes.
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