An Overview of Glucose Tolerance Test
The glucose tolerance test or GTT is a medical diagnostic procedure used to assess how effectively the body processes glucose, a type of sugar, and to evaluate the presence of diabetes or other glucose-related disorders. The test measures the body’s response to a specific amount of glucose consumed orally. The examination is commonly employed to assess for diabetes mellitus, insulin resistance, compromised function of pancreatic beta cells, and occasionally reactive hypoglycaemia or acromegaly, as well as infrequent conditions related to carbohydrate metabolism.
During the GTT, a person fasts overnight and then drinks a sugary solution containing a predetermined amount of glucose. Blood samples are taken before consuming the solution and at regular intervals afterward, typically every 30 minutes to two hours, depending on the specific protocol used. These blood samples are analyzed to measure the levels of glucose present, allowing healthcare professionals to assess how efficiently the body metabolizes the sugar.
The GTT helps identify abnormalities in glucose metabolism. A normal response shows a rise in blood glucose levels after the glucose load, followed by a gradual decline as the body effectively clears the sugar from the bloodstream. Abnormal results may indicate impaired glucose tolerance or diabetes, with elevated blood glucose levels persisting for an extended period.
The GTT is commonly used to diagnose gestational diabetes during pregnancy, as well as to evaluate prediabetes and diabetes in non-pregnant individuals. It is an essential tool in assessing glucose metabolism, aiding in the early detection and management of diabetes-related conditions. As with any medical test, the GTT should be performed and interpreted by qualified healthcare professionals to ensure accurate results and appropriate follow-up care.
Purpose of the test
The glucose tolerance test serves as a method to evaluate the body’s glucose metabolism. There are various reasons why your doctor might recommend this test:
- Diagnosis of diabetes and prediabetes: Diagnosis involves identifying the underlying cause of change in your health over time, which you may have noticed. If you exhibit symptoms of diabetes and other tests for diabetes are indecisive, the glucose tolerance test may be prescribed.
- Screening for diabetes and prediabetes: Screening involves testing for a disease before it shows any symptoms or signs. If you are aged 45 or above or have a higher-than-average risk of developing diabetes, your doctor may recommend screening for diabetes. Typically for screening, other types of diabetes tests are more commonly used, but the glucose tolerance test is often used to test for diabetes in pregnant women.
- Screening and diagnosis of gestational diabetes: Gestational diabetes is a type of diabetes that can occur in pregnant women. During pregnancy, your doctor may prescribe the glucose tolerance test as part of screening and diagnosis procedure.
Risks involved in the test
The potential hazards linked to acquiring a blood sample are minimal. Following the blood extraction, there is a chance of encountering bruising or bleeding, as well as experiencing dizziness or light-headedness. Additionally, there is a possibility of infection following the procedure in certain instances.
Finding a Glucose Tolerance Test
The glucose tolerance test evaluates the quantity of glucose present in your blood following a period of fasting, as well as after consuming a sweetened beverage at regular intervals. Blood glucose levels are commonly assessed using the unit of milligrams per decilitre (mg/dL).
How to undergo the test?
Typically, your doctor will be responsible for requesting a glucose tolerance test. The test can be conducted in various settings such as your doctor’s office, a hospital, clinic, or laboratory.
Can the test be done at home?
Currently, there is no option for an at-home glucose tolerance test.(2) This test involves multiple blood draws and laboratory analysis of the samples, so it must be carried out in a clinic or laboratory setting.
What is the cost of the test?
The cost of a glucose tolerance test can vary depending on your health insurance coverage and the location where the test is conducted. The overall cost may include the price of the medical consultation, the charges for obtaining blood samples by a technician, and the laboratory fees for analyzing the samples.
When your doctor prescribes a GTT test, your insurance provider usually covers the associated costs and fees. However, you may be responsible for certain expenses out of pocket, such as deductibles and copayments. For more information about the cost of your glucose tolerance test, it is recommended to consult your doctor, health insurance company, or the staff at the office or facility where the test will be performed.
Procedure of taking a glucose tolerant test
A phlebotomist will extract a blood sample from a vein in your arm using a needle after you have fasted for a minimum of eight hours, usually overnight while conducting the gtt blood test. Subsequently, you will consume a sugary drink, and the technician will collect additional blood samples at intervals of two to three hours.
- Prior to the test: Before taking the test, it is essential to fast for at least eight hours, which means refraining from consuming any food or drinks except water. Since the test is typically scheduled in the morning, most of the fasting period can occur while you are asleep the night before.
You can generally follow your regular diet until you commence fasting. However, your doctor might instruct you to consume a minimum of 150 grams of carbohydrates each day in the three days leading up to your glucose tolerance test.
If you are taking any prescription medications or over-the-counter drugs, it is important to discuss them with your doctor prior to the test. Your doctor can inform you whether any of these medications might affect the test results and advise whether you need to discontinue any medication while fasting.
To ensure the most accurate results, consult your doctor for any specific preparations before the test and diligently follow the provided instructions in the days leading up to your glucose tolerance test.
- During the test:
The glucose tolerance test is typically conducted in the morning, and you may need to remain at the testing site for up to three hours.
The phlebotomist administering the test will begin by extracting a small amount of blood from a vein, usually in your arm. They might use a band to make the vein more visible. After sterilizing the skin over the vein, a small needle will be inserted into the vein to collect the blood sample, which will be collected in a tube attached to the needle.
Once the needle is removed, the band on your arm will be released, and the injection site will be covered with a small bandage. Some individuals may experience slight pain or discomfort when the needle is inserted or removed, but this sensation is usually brief.
After the initial blood sample is taken, you will consume a beverage containing a specific amount of glucose. Typically, adults drink a solution containing 75 grams of glucose, while the amount for children is calculated based on body weight, with a maximum of 75 grams. The drink has a sweet taste resembling a sugary soda.
Following the consumption of the glucose solution, your blood will be drawn several more times at regular intervals over the next two to three hours. It is important not to eat anything during this period, and you will generally wait in the designated area.
- Post the test
Once the blood sampling process is finished, you might experience minor discomfort at the site where the blood was drawn. After the completion of your glucose tolerance test, you can typically resume your regular activities. At the end of the test, you will be able to eat again. It may be helpful to bring a snack with you to the testing site, so you have something to eat after the final blood draw.
Results of a Glucose Tolerance Test
After undergoing a glucose tolerance test, you can expect to receive your test results within a few business days. There are various ways you may receive your results, such as accessing them online, receiving them by email or postal mail. Additionally, your doctor may contact you via phone or email to discuss the results or schedule an appointment for an in-person discussion.
Understanding the Interpretation of Test Results
Skilled laboratory technicians will examine the glucose levels in each blood sample provided. Your lab report will include the values for each timed sample taken during the test.
The laboratory analysis aims to identify whether any of the blood samples display abnormally high glucose levels. Specifically, the measurement of your blood glucose level two hours after consuming the glucose solution can help in diagnosing prediabetes or diabetes.
Organizations such as the National Institute of Diabetes and Digestive and Kidney Disorders and the American Diabetes Association provide reference ranges for this two-hour value, as follows:
- Normal: Less than 140 mg/dL
- Prediabetes: 140 to 199 mg/dL
- Diabetes: 200 mg/dL or higher
Normal Results for Type 2 Diabetes or Type 1 Diabetes
Fasting glucose level 60 to 100 mg/dL
One-hour glucose level less than 200 mg/dL
Two-hour glucose level less than 140 mg/dL
Impaired Results for Type 1 Diabetes or Type 2 Diabetes
Fasting glucose level: 100 to 125 mg/dL
Two-hour glucose level 140 to 200 mg/dL
Abnormal (Diagnostic) Results for Type 1 Diabetes or Type 2 Diabetes
Fasting glucose level greater than 126 mg/dL
Two-hour glucose level greater than 200 mg/dL
Normal Results for Gestational Diabetes
Fasting glucose level less than 90 mg/dL
One-hour glucose level less than 130 to 140 mg/dL
Two-hour glucose level less than 120 mg/dL
Abnormal Results for Gestational Diabetes
Fasting glucose level greater than 95 mg/dL
One-hour glucose level greater than 140 mg/dL
Two-hour glucose level greater than 120 mg/dL
It’s important to note that while this test can aid in diagnosing both type 1 and type 2 diabetes, a single test is usually insufficient for a definitive diagnosis. Your doctor will typically consider other factors, including symptoms, additional blood tests, or repeat glucose tolerance tests, to reach a conclusive determination.
Therefore, the most effective way to understand the implications of your gtt blood test results is to consult your doctor. They can provide an explanation of how your results compare to the reference ranges, whether further testing or follow-up is necessary, and guide you on the necessary steps for your overall health.
Individuals diagnosed with prediabetes have an increased risk of developing diabetes and other health conditions in the years following their diagnosis. If your test results indicate prediabetes, your doctor may recommend dietary and lifestyle changes to reduce the likelihood of progression to diabetes or to delay its onset.
In the case of a diabetes diagnosis, your doctor will explain the necessary measures for managing your condition. These may involve self-monitoring of blood glucose, modifications to your diet and exercise regimen, medication prescriptions, and regular follow-up appointments.
Clinical significance of Glucose Tolerance Test
The purpose of the glucose tolerance test is to assess the speed at which glucose is eliminated from the bloodstream. This test is employed for the detection of various conditions related to carbohydrate metabolism, including diabetes, insulin resistance, impaired beta-cell function, reactive hypoglycaemia, acromegaly, and similar disorders.
Potential risks and adverse effects
A blood test generally carries minimal risks. A healthcare professional will use a needle to draw blood, which may cause mild to moderate discomfort for some individuals. There may be slight bruising at the site where the needle was inserted.
Glucose screening or tolerance tests, which involve fasting and blood testing, may lead to light-headedness. It is advisable for individuals undergoing these tests to have someone accompany them to the clinic who can assist with transportation.
Although less common, there are more serious risks associated with these tests, including:
- Fainting
- Excessive bleeding
- Infection
- Blood accumulation under the skin
In addition, some individuals may experience low blood sugar levels during the test or post the test, defined as having at least one glucose value below 60 mg/dL according to a 2021 study. The study found that approximately 10.4% of participants developed hypoglycaemia during the test. Symptoms of hypoglycaemia may include hunger, sweating, nervousness, and restlessness.
It is worth noting that adverse effects, such as hypoglycaemia and unstable blood sugar levels, can also occur in individuals who have undergone bariatric surgery and undergo a glucose tolerance test. Researchers advise against conducting this test in pregnant individuals who have undergone such surgery.
