Diabetes is one of the most common chronic health conditions in the United Kingdom, affecting millions of people. Early detection is crucial for preventing serious complications such as heart disease, kidney damage, and nerve problems. The glucose tolerance test (GTT) is a valuable diagnostic tool that helps healthcare providers identify diabetes and prediabetes, even when other tests may appear normal.
This comprehensive guide explains how the glucose tolerance test works, what to expect during the procedure, and how to interpret your results.
What Is a Glucose Tolerance Test?
A glucose tolerance test, also known as an oral glucose tolerance test (OGTT), measures how well your body processes glucose (sugar). Unlike a fasting blood glucose test that provides a single snapshot of your blood sugar levels, the GTT evaluates how your body responds to a standardised glucose load over a period of time.
The test reveals how efficiently your pancreas produces insulin and how effectively your cells use that insulin to absorb glucose from the bloodstream. This dynamic assessment can detect problems with glucose metabolism that might not be apparent from fasting tests alone.
Why Is the Glucose Tolerance Test Important?
The glucose tolerance test serves several important purposes in diabetes diagnosis and management:
Detecting prediabetes: Many people have impaired glucose tolerance – a condition where blood sugar levels are higher than normal but not yet high enough to be classified as diabetes. The GTT is particularly sensitive at identifying this intermediate stage, giving you the opportunity to make lifestyle changes before diabetes develops.
Diagnosing type 2 diabetes: While fasting glucose and HbA1c tests are commonly used for diabetes screening, some individuals with diabetes may have normal fasting levels but abnormal post-meal glucose responses. The GTT can identify these cases.
Gestational diabetes screening: The glucose tolerance test is routinely offered to pregnant women between 24 and 28 weeks of pregnancy to screen for gestational diabetes, which can affect both mother and baby if left untreated.
Monitoring and research: The test is also used in research settings and for monitoring certain conditions affecting glucose metabolism.
How Does the Test Work?
The glucose tolerance test works by challenging your body with a known amount of glucose and measuring how your blood sugar levels respond over time. Here is what happens physiologically:
1. When you consume the glucose drink, sugar enters your bloodstream
2. Your pancreas detects the rising blood sugar and releases insulin
3. Insulin signals your cells to absorb glucose from the blood
4. Blood sugar levels should return to normal within a specific timeframe
In healthy individuals, blood glucose rises after consuming the drink but returns to near-normal levels within two hours. In people with diabetes or prediabetes, the body either does not produce enough insulin or the cells do not respond properly to insulin, causing blood sugar to remain elevated for longer.
Preparing for Your Glucose Tolerance Test
Proper preparation is essential for accurate results. Your healthcare provider will give you specific instructions, but general guidelines include:
Before the test:
The night before:
On the day:
For more detailed guidance on preparing for blood tests, read our article on the procedure, results, and after-effects of blood testing.
The Glucose Tolerance Test Procedure
The standard oral glucose tolerance test follows a specific protocol:
Step 1: Fasting blood sample
When you arrive at the clinic, a healthcare professional will take an initial blood sample to measure your fasting glucose level. This provides a baseline for comparison.
Step 2: Glucose drink
You will be given a sweet drink containing a precise amount of glucose – typically 75 grams for adults (or 100 grams for gestational diabetes screening in some protocols). The drink must be consumed within a few minutes.
Step 3: Waiting period
After drinking the glucose solution, you will need to remain at the clinic. During this time, you should sit quietly and avoid eating, drinking (except water), smoking, or strenuous activity, as these can affect results.
Step 4: Follow-up blood samples
Additional blood samples are taken at specific intervals – usually at one hour and two hours after consuming the glucose drink. For gestational diabetes screening, samples may be taken at one, two, and three hours.
The entire process typically takes two to three hours. While the glucose drink is very sweet, most people tolerate it well, though some may experience mild nausea.
Understanding Your Results
Results are typically reported in millimoles per litre (mmol/L). The diagnostic criteria for the two-hour glucose tolerance test are:
Fasting glucose:
Two-hour glucose (after 75g glucose):
A diagnosis of diabetes typically requires confirmation with a repeat test on a different day, unless symptoms are clearly present.
Glucose Tolerance Test vs Other Diabetes Tests
Several tests are used to diagnose and monitor diabetes. Understanding the differences helps you appreciate when a GTT might be recommended:
Fasting plasma glucose (FPG):
Measures blood sugar after an overnight fast. Simple and convenient but may miss some cases of diabetes or prediabetes.
HbA1c (glycated haemoglobin):
Reflects average blood sugar over the past 2-3 months. Does not require fasting and provides a longer-term picture. Learn more in our comprehensive guide to understanding the A1C blood test.
Random plasma glucose:
Measures blood sugar at any time, regardless of when you last ate. Used when diabetes symptoms are present.
Glucose tolerance test:
Evaluates how your body handles glucose over time. More time-consuming but can detect impaired glucose tolerance that other tests might miss.
Who Should Have a Glucose Tolerance Test?
Your healthcare provider may recommend a glucose tolerance test if you:
What If Your Results Are Abnormal?
If your glucose tolerance test indicates prediabetes or diabetes, your healthcare provider will discuss next steps:
For prediabetes:
Lifestyle modifications including dietary changes, increased physical activity, and weight management can often prevent or delay progression to type 2 diabetes. Regular monitoring with tests such as fasting glucose, HbA1c, and insulin levels helps track your progress.
For diabetes:
A comprehensive management plan will be developed, which may include lifestyle changes, medication, and regular monitoring. Early intervention significantly reduces the risk of complications.
Additional Tests to Consider
Alongside glucose testing, other blood tests provide valuable information about metabolic health:
Glucose Tolerance Testing in London
At Private Blood Tests London, we offer comprehensive diabetes screening and monitoring services at our CQC-registered clinic in South Kensington. Our experienced healthcare team provides professional, efficient service with results delivered quickly and securely.
Explore our [diabetes testing services](/diabetes-test), [health screening packages](/health-screening), and learn about [HOMA-IR testing for insulin resistance](/blog/homa-ir-explained-the-blood-test-that-detects-insulin-resistance-early).
Whether you need a glucose tolerance test, HbA1c monitoring, or a full metabolic health assessment, we are here to support your health journey.
Take control of your metabolic health – book your glucose tolerance test today.