Glucose Tolerance Test/OGTT
The Glucose Tolerance Test, also known as the Oral Glucose Tolerance Test (OGTT), evaluates how efficiently your body processes glucose, helping diagnose prediabetes, diabetes, and insulin resistance.
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Results available 1 day after receipt in laboratory
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Details about Glucose Tolerance Test/OGTT
Understanding the Glucose Tolerance Test / Oral Glucose Tolerance Test (OGTT)
The Glucose Tolerance Test / Oral Glucose Tolerance Test (OGTT) is a structured blood test that measures the body’s response to a controlled glucose load over time. It provides a dynamic assessment of glucose metabolism, rather than a single snapshot.
After a fasting blood sample is taken, you drink a glucose solution containing a standard amount of sugar. Blood glucose levels are then measured at specific intervals, most commonly at 2 hours, to assess how effectively insulin moves glucose from the bloodstream into cells.
The OGTT is considered one of the most sensitive tests for identifying early abnormalities in glucose handling, including prediabetes, insulin resistance, and diabetes, even when fasting glucose or HbA1c results are normal.
When a Glucose Tolerance Test / OGTT is recommended
This test is recommended when there is suspicion of impaired glucose regulation that may not be fully captured by fasting glucose or HbA1c testing alone.
It is commonly used in both general medical assessment and pregnancy-related screening.
Testing may be appropriate if you:
- Have borderline or abnormal fasting glucose results
- Have symptoms suggestive of diabetes
- Have a family history of diabetes
- Have polycystic ovary syndrome (PCOS)
- Are overweight or have metabolic risk factors
- Have unexplained fatigue or increased thirst
- Are being screened for gestational diabetes
- Want early detection of insulin resistance
Benefits of the Glucose Tolerance Test / OGTT
- Detects early glucose metabolism abnormalities
Identifies prediabetes before overt diabetes develops. - More sensitive than fasting glucose alone
Captures post-glucose response. - Essential for diagnosing gestational diabetes
Widely used in pregnancy screening. - Assesses insulin effectiveness indirectly
Reflects how well insulin manages glucose load. - Supports personalised prevention strategies
Enables early lifestyle or medical intervention. - Useful when HbA1c results are inconclusive
Provides additional diagnostic clarity. - Standardised and clinically validated test
Recognised in UK and international guidelines. - Provides actionable, time-based insights
Shows glucose handling over time, not just one value.
How the test is carried out
This test is performed in-clinic only, as timed blood samples are required.
Clinic-based Glucose Tolerance Test
- You attend the clinic after an overnight fast (8–12 hours)
- A fasting blood glucose sample is taken
- You drink a glucose solution
- Further blood samples are taken at set intervals (usually at 2 hours)
- You remain in the clinic for the duration of the test
This test is not suitable for home phlebotomy due to timing and supervision requirements.
Understanding your Glucose Tolerance Test / OGTT results
Normal glucose tolerance
Blood glucose levels rise after the glucose drink and return to normal within the expected time frame.
Typical UK 2-hour value:
- Less than 7.8 mmol/L
This indicates normal glucose metabolism.
Impaired glucose tolerance (Prediabetes)
Blood glucose levels remain higher than normal at the 2-hour measurement.
Typical UK 2-hour value:
- 7.8 – 11.0 mmol/L
This indicates increased risk of developing type 2 diabetes.
Diabetes range result
Blood glucose levels remain significantly elevated after the glucose load.
Typical UK 2-hour value:
- 11.1 mmol/L or higher
This is consistent with diabetes and requires clinical follow-up.
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Follow these guidelines for accurate results
Sample Timing
Where possible, attend your blood test between 7am and 12pm. Please wait until any short-term illness or infection has fully resolved before testing. Avoid intense exercise for 24–48 hours beforehand, as this can affect certain markers.
Fasting & Hydration
Fasting is not always required, but some tests may recommend it. If fasting is advised, avoid food for 8–12 hours before your appointment and drink water only. Stay well hydrated, as this helps with sample collection and accuracy.
Medications & Supplements
Continue prescribed medications unless advised otherwise by your clinician. Avoid vitamin, mineral, or biotin supplements for at least 24–48 hours before your test, as these can interfere with results. If you are unsure about any medication or supplement, please let us know before your appointment.
Hormones & Menstrual Cycle
For hormone-related tests, timing within your menstrual cycle may be important. If relevant, follow any specific guidance provided on your test page.
If you have any questions or special circumstances, our team is happy to advise before your appointment.