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Glucose Tolerance Test

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The Oral Glucose Tolerance Test (OGTT/GTT) is used to diagnose diabetes. The test is also helpful in monitoring glucose levels in persons diagnosed with diabetes. Measurement of blood sugar (also known as blood glucose) plays an important role in designing treatment; if you are diagnosed with a disease or a disorder. Though this test is not regularly ordered to confirm the diagnosis of diabetes in normal patients, it is still widely used to measure blood sugar levels in pregnant women. The test helps diagnose gestational diabetes. Here follows more information on OGTT.

Oral Glucose Tolerance Test
Generally, if a person shows symptoms of diabetes or prediabetes, like increased thirst/hunger, increased urination, increased fatigue, slow healing infections, blurred vision, confusion, anxiety, trembling, etc.; then blood glucose levels are checked. Screening of glucose is done in various ways, by testing fasting blood glucose (glucose level checked after 8 hours overnight fasting), postprandial blood glucose (glucose level checked 2 hours after lunch), random blood glucose (glucose level checked anytime during the day), Intravenous Glucose Tolerance Test (IGTT) and OGTT (which is performed in several ways), etc.

If you are asked to undergo an OGTT, then you should not eat or drink anything after midnight. When you approach the doctor, your blood will be taken and then you will be asked to drink a sweet liquid containing (usually) 75 gm of glucose. To diagnose gestational diabetes, pregnant women are given 100 gm of glucose. You are not supposed to eat anything during the test. The test may take up to 3 hours and your blood will be taken 4-5 times every 30 to 60 minutes after you drink the liquid.

You are supposed to eat normally for several days before the test and your diet should contain 150-200 gm carbohydrates per day for three days before the test. In the morning, before the test, smoking or even drinking a cup of coffee is not allowed. If you are suffering from any type of illness (even common cold), if you are taking any medicines, you should postpone the test as this could affect the test results. Test results for a normal, active, healthy person would be considered as reliable.

OGTT Results
If you don’t have diabetes, then blood glucose levels rise after taking the glucose solution and then fall quickly. But if you have diabetes, a higher than normal rise in glucose levels is seen and the high glucose levels fail to come down as fast as would have come down in a normal person. Those who don’t have high blood glucose levels like diabetics and not as normal levels as healthy people, are said to have impaired glucose tolerance (IGT). This condition is known as prediabetes. With proper diet, regular exercise and certain medicines, onset of diabetes can be delayed or prevented.

Fasting Blood Sugar Level Chart
The following fasting blood sugar level chart will help you know about normal and abnormal blood sugar levels.

Type of Test Normal Range Prediabetes Diabetes
Fasting Blood Sugar Levels 70-100 mg/dL 100-125 mg/dL Above 126 mg/dL
Postprandial Blood Sugar Levels Up to 140 mg/dL 140-200 mg/dL Above 200 mg/dL
Random Blood Sugar Levels Up to 140 mg/dL 140-200 mg/dL Above 200 mg/dL

Glucose Test: Normal Values
Normal fasting blood sugar level should be between 60-100 mg/dL. After taking the glucose drink the level may rise up to 200 mg/dL within one hour. The following chart shows normal values for OGTT sample drawn 2 hours after having a 75-gram glucose drink:

Glucose Levels Indication
Less than 140 mg/dL (7.8 mmol/L) Normal glucose tolerance
From 140 to 200 mg/dL (7.8 to 11.1 mmol/L) Impaired glucose tolerance (prediabetes)
Over 200 mg/dL (11.1 mmol/L) on more than one testing occasion Diabetes

OGTT During Pregnancy
The following chart shows normal values for OGTT (performed to diagnose gestational diabetes) sample after having a 100-gram glucose drink:

Time of Sample Collection Target Level
Fasting glucose level (before drinking glucose drink) 95 mg/dL (5.3 mmol/L)
One hour after having the drink 180 mg/dL (10.0 mmol/L)
Two hours after having the drink 155 mg/dL (8.6 mmol/L)
Three hours after having the drink 140 mg/dL (7.8 mmol/L)

Note: mg/dL = milligrams per deciliter and mmol/L = millimoles/liter.
The woman is said to have developed gestational diabetes when two or more values meet or exceed the target levels.

Side effects of this test are negligible. Sometimes, nurses find it difficult to obtain a blood sample; owing to the nature and size of the veins and arteries. Sometimes, there can be excessive bleeding or fainting. The patient may feel light-headed. Sometimes, blood may get accumulated under the skin or some patients may catch infection if the skin is broken. But, such cases are very rare. The test is helpful in diagnosing diabetes and prediabetes. I hope you found the information helpful.


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