Glucose Tolerance Test

Introduction:

The glucose tolerance test (GTT), also known as the oral glucose tolerance test (OGTT), is a laboratory test to screen for type 2 diabetes. A modified version of the test is done between the 24th and 28th weeks of pregnancy to diagnose a type of diabetes that affects pregnant women, called gestational diabetes.

What Is Diabetes?

Diabetes is a condition that affects the body’s ability to use glucose. A person gets glucose from the foods they eat. The body’s cells require glucose to carry out their activities. Insulin, a hormone released by the pancreatic cells, acts as a key that enables glucose to enter the cells, thereby reducing blood sugars. An impaired insulin function can cause fluctuations in blood sugars leading to both high and low levels of blood sugars, which, when left untreated, can lead to severe complications. Diabetes is of three types, namely:

• Type 1 Diabetes: Type 1 diabetes is when the beta cells of the pancreas that secrete insulin get destroyed by the body’s immune system (an autoimmune condition), causing an insulin deficiency in the body. The condition can be present since childhood. The patients need to take daily insulin shots with regular monitoring of blood sugar levels.

• Type 2 Diabetes: Type2 diabetes occurs due to the body’s inability to use insulin, leading to a buildup of sugars in the blood. The condition can be transmitted from parents to children and can also occur in people who follow an unhealthy lifestyle and diet. Uncontrolled blood sugars that last for more extended periods can lead to complications of the eyes, nerves, kidneys, and heart.

• Gestational Diabetes: Gestational diabetes is a type of diabetes that occurs in pregnant women. It is a transient type that occurs for a few months during pregnancy and usually subsides after delivery. However, prompt treatment is necessary after detection, as uncontrolled sugar levels can impact the developing fetus. Also, women affected with gestational diabetes have higher chances of developing diabetes early in life.

What Are the Other Names of the Glucose Tolerance Test?

The other names of the test include:

1. Oral Glucose Tolerance Test - non-pregnant.

2. OGTT - non-pregnant.

3. Diabetic - glucose tolerance test.

4. Diabetes - glucose tolerance test.

Who Needs to Take the Glucose Tolerance Test?

A glucose tolerance test is recommended in people who:

• If a person is obese.

• If a person has a family history of diabetes.

• If a person has had gestational diabetes in previous pregnancies.

• If a person is having high blood pressure (BP) and triglycerides.

• Polycystic ovary syndrome (PCOS), is a common hormonal disorder that affects women of childbearing age, causing an imbalance in the menstrual cycle.

• If a person has delivered a baby with a weight of more than nine pounds.

Why Is the Glucose Tolerance Test Performed?

Blood sugar levels need to be checked to help take measures to maintain them within normal ranges. The commonly used tests to diagnose diabetes include:

• Fasting blood glucose showing blood glucose levels greater than 126 mg/dL (milligrams per deciliter) on two different tests is diagnostic of diabetes.

• A glycated hemoglobin test or HbA1c is a test that measures the average values of blood glucose for three months. A test result of 6.5 % or greater is considered diagnostic of diabetes.

• A glucose tolerance test is an oral test used to screen for or diagnose diabetes in people who have fasting levels closer to 125 mg/dL but not high enough to confirm the diagnosis. Abnormal test results are an indicator that confirms diabetes earlier than the fasting blood sugar test.

How to Prepare for the Glucose Tolerance Test?

The person taking the test has to eat normally for several days before the test. However, starting from eight hours before the test, the person should not eat or drink anything (except water) till the test gets completed, which can take about three hours. The patient must inform the health care provider about the medications taken as they can affect the test results. Accordingly, the doctor may advise stopping the drugs or providing an alternative to them.

How Is the Glucose Tolerance Test Performed?

The commonly performed glucose tolerance test is the oral glucose tolerance test (OGTT). Before starting the test, a sample of blood is collected from the patient’s vein to determine the level of fasting blood glucose. Then, the patient is given a solution containing a certain amount of glucose (about 75 grams) to drink, following which the blood samples are collected every 30 to 60 minutes for three hours. Another variation of the glucose tolerance test is the intravenous (IV) glucose tolerance test (IGTT). The test is used mainly for research purposes rather than a diagnosis of diabetes. A blood sample is collected before the test. During the test, glucose is injected into a vein for three minutes, following which blood insulin levels are measured after one and three minutes. A similar test diagnoses growth hormone excess (acromegaly) as well. The blood levels of both glucose and growth hormone are measured following the consumption of a glucose drink.

How Does It Feel During the Glucose Tolerance Test?

Severe side effects are uncommon during the test. The person may feel like drinking very sweet soda while drinking the glucose solution. Also, the injection may feel like an ant bite or a small prick. However, some people may experience nausea, sweating, and dizziness during the test. The patient must inform the health care provider about the same (if already experienced during other tests) before undergoing the test. The other side effects during the test include multiple punctures to locate the veins, bleeding, infection, or hematoma (buildup of blood under the skin).

What Do the Test Results Mean?

Normal blood sugar levels that are used to rule out type 2 diagnosis based on glucose tolerance tests include:

• Fasting blood glucose levels - 60 to 100 mg/dL (milligrams per deciliter), or 3.3 to 5.5 mmol/L (millimoles per liter).

• One-hour values - less than 200 mg/dL or 11.1 mmol/L.

• Two-hour values - These values help diagnose diabetes.

- Less than 140 mg/dL or 7.8 mmol/L.

- Between 141 mg/dL to 200 mg/dL or 7.8 mmol/L to 11.1 mmol/L.

- Above 200 mg/dL or 11.1 mmol/L.

The above results are the standard ranges in most of the laboratories. However, there may be a slight variation in the normal ranges of different laboratories. Therefore, healthcare providers can best guide what the test results mean.

What Do Abnormal Glucose Tolerance Test Results Mean?

Two-hour test results between 141 mg/dL and 200 mg/dL indicate pre-diabetes or impaired glucose tolerance, which means that the person’s chances of developing diabetes earlier are high. A value above 200 mg/dL is confirmatory of diabetes. However, there may be a rise in the blood glucose levels in people who had a trauma, surgery, or a heart attack. In addition, vigorous activity can lower blood sugar levels. Also, the intake of certain medicines can alter blood glucose levels.

Conclusion:

Maintaining normal blood glucose levels is essential for the smooth functioning of the body. The body has its regulatory system that helps keep the levels within normal limits. Untreated cases of high or low blood sugar can lead to complications. Therefore, screening for the detection of diabetes is pivotal, especially in people with risk factors of developing the condition and receiving prompt medical attention. Also, dietary and lifestyle changes are recommended along with medicines to lead a healthy life.