Understanding Glucose Tolerance Test: Normal Range Results and Interpretation

Blood glucose is the primary sugar in your body and serves as its main energy source. It comes from the food you eat. When you consume food, your body converts it into glucose and releases it into the bloodstream. The increase in blood glucose triggers the pancreas to produce insulin, a hormone that facilitates glucose entry into cells for energy generation. As cells take in glucose, blood glucose levels decrease.

When insulin production or utilisation is inadequate, it can lead to elevated blood glucose levels, which can ultimately result in diabetes. The buildup of glucose in the blood can cause significant health problems.

The glucose tolerance test (GTT) is a vital diagnostic method used to evaluate how the body metabolises sugar and is a cornerstone in diabetes management. This test is instrumental in detecting prediabetes, gestational diabetes, and type 2 diabetes, enabling healthcare providers to devise personalised treatment strategies, monitor blood sugar levels, and reduce the risk of diabetes-related complications. Let's delve deeper into the significance of blood glucose testing.
 

Glucose tolerance test: Definition

The primary function of the glucose tolerance test is to find if the body can adequately store and utilise blood sugar. 

There are two types of this test: oral and intravenous. 

The oral glucose tolerance test (OGTT) is more commonly conducted and assesses the body's ability to metabolise sugar. High blood sugar levels may suggest insufficient cell sugar uptake, indicating potential diabetes. In cases of gestational diabetes, pregnancy hormones can lead to elevated blood sugar levels, which generally return to normal after delivery.

Conversely, the intravenous (IV) glucose tolerance test (IGTT) is rarely used and not recommended for diagnosing diabetes. 

In this test, glucose is administered directly into a vein over 3 minutes. 

Blood insulin levels are measured before the injection and then at 1 and 3 minutes afterwards. The timing for this procedure may differ. Researchers use this type of testing for blood glucose.
 

When might a glucose tolerance test be necessary?

Your doctor may suggest a glucose tolerance test to assist in screening for or identifying any of the following issues:

• Diabetes (including prediabetes, Type 2 diabetes, or gestational diabetes)
• Insulin resistance
• Reactive hypoglycemia (low blood sugar)
• Acromegaly (a condition caused by the pituitary gland's excessive synthesis of growth hormone, characterised by the gradual expansion of the hands, feet, and face)
• Uncommon carbohydrate metabolism disorders

A one-hour glucose challenge test is a standard procedure during pregnancy to check for gestational diabetes.

If the results indicate a problem, your physician will recommend a three-hour OGTT to definitively diagnose or exclude the condition.
 

How should I prepare for a glucose tolerance test?

Since there are various types of glucose tolerance tests, the specific preparation steps may differ. Generally, you will likely need to:

1. Inform your doctor about all medications and supplements you are currently taking. Some drugs can influence the test results. Your doctor may advise you to discontinue certain medications before the test.

2. Maintain your diet for at least three days before the test. You should consume a minimum of 150 grams of carbohydrates each day.

3. Maintain your normal activity level for at least three days before the test, and avoid excessive exercise in the days before it.

4. Fast (refrain from eating or drinking anything besides water) for at least eight hours immediately before the test.

5. Avoid eating or smoking during the test.

Your healthcare provider will offer specific guidance based on your circumstances and the type of GTT you are undergoing. 

Follow their instructions, and feel free to ask questions. Your active participation is key to the test's success.
 

What occurs during a glucose tolerance test?

The exact procedure for a glucose tolerance test can vary depending on the type. We will describe the two-hour oral glucose tolerance test (OGTT), the most commonly conducted type. 

Typically, the test starts in the morning, between 7:00 and 9:00 a.m. You will remain at the hospital or medical facility until the test is complete, so it's a good idea to bring something to entertain you while you wait.

The test typically involves the following steps:

1. A healthcare professional (usually a phlebotomist) will collect a blood sample from your vein to determine your fasting blood glucose levels.

2. Afterwards, a healthcare provider will give you a glucose solution to drink. This solution usually contains 75 grams of glucose mixed in 250 to 300 mL water. The provider will instruct you to consume the entire drink within five minutes. The dosage differs for children and pregnant individuals. 

Note: The glucose solution has a flavour similar to that of overly sweet flat soda, which some individuals may find unappealing.

3. Two hours after you consume the glucose solution, a healthcare professional will take another blood sample from your vein to assess your blood glucose levels. 

4. Depending on the rationale behind the test, they may draw your blood multiple times at 30-to 60-minute intervals after you consume it.
 

Different variations of the glucose tolerance test

The testing procedure varies for specific types of GTTs. For instance:

• One-hour glucose tolerance test: This is generally a glucose challenge test for screening gestational diabetes during pregnancy. You drink a solution containing 50 grams of glucose, and then a provider takes a blood sample one hour later to evaluate your blood sugar. Fasting is typically not required for this test.
• Three-hour glucose tolerance test: Blood samples will be collected before consuming the glucose solution (fasting) and at one, two, and three hours following ingestion.  
• Glucose tolerance assessment for acromegaly: In this procedure, your healthcare provider will obtain blood samples to evaluate your human growth hormone (HGH) level (not your blood glucose). After you ingest the glucose solution, blood draws will likely occur every 30 minutes for two hours at the 30-, 60-, 90-, and 120-minute marks.
    

Interpreting results and next steps  

What do the glucose tolerance test outcomes indicate?  

Your glucose tolerance test results will display the blood glucose level recorded at each blood draw.  

Laboratory and healthcare providers may use slightly different reference ranges to identify blood sugar-related issues (such as diabetes). Always consult the reference range provided in your report.  

Typically, healthcare providers apply these ranges for the two-hour glucose tolerance test:  

• Normal: Below 140 mg/dL
• Prediabetes: 140 to 199 mg/dL
• Diabetes: 200 mg/dL or higher

For conditions like gestational diabetes, ranges and timing intervals might vary. For instance, the GTT monitors alterations in growth hormone levels to diagnose acromegaly. Your provider will discuss the outcomes with you.  

Encountering an unusual test result can be overwhelming. Remember that an abnormal glucose tolerance test result does not automatically indicate you have diabetes.

If your results reveal elevated blood glucose levels, your healthcare provider will likely recommend further testing to establish a diagnosis. Don't hesitate to ask your provider any questions; they are there to assist you.  
 

Conclusion

Regular blood sugar testing is essential for early detection and prevention of diabetes and related complications. The glucose tolerance test provides critical insights into your body’s ability to process sugar. Take charge of your health with TruePal diabetes management monitor your blood sugar levels and stay healthy.
 

FAQs

1. What should I do next if the results are outside the normal range?  

If your glucose tolerance test results are outside the normal range, your healthcare provider will guide you on the subsequent steps. While it can be distressing to receive an abnormal result, understand that a diagnosis of prediabetes or diabetes typically isn't made based on a single abnormal OGTT.  

Your healthcare provider may choose another diagnostic approach, like an A1C test, or you will likely be asked to retake the test on a different day soon after. Regardless, your provider will support you during the diagnostic process. Don't hesitate to express your inquiries or concerns.  

2. What can I expect following a glucose tolerance test?  

After the glucose tolerance test, you should be able to return to your usual activities and start eating again. Bring a snack with you to enjoy afterwards. Your healthcare provider will provide any further instructions if needed and inform you when you can anticipate the results.  

3. When can I expect to receive the results of my glucose tolerance test?  

In general, you should receive the results of a glucose tolerance test within a few business days. The time frame can vary depending on the laboratory.  

4. What potential side effects might arise from a glucose tolerance test?  

Glucose tolerance tests are usually considered safe. Most individuals experience no adverse effects from the drink. Some may report feeling nauseous or dizzy, but severe side effects are uncommon.  

Blood tests (draws) are a routine and crucial component of medical evaluations. The risk associated with having blood tests is minimal. You may experience slight tenderness or bruising at the site of the blood draws, but this typically resolves quickly.