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

1. Detects early glucose metabolism abnormalities
   Identifies prediabetes before overt diabetes develops.
2. More sensitive than fasting glucose alone
   Captures post-glucose response.
3. Essential for diagnosing gestational diabetes
   Widely used in pregnancy screening.
4. Assesses insulin effectiveness indirectly
   Reflects how well insulin manages glucose load.
5. Supports personalised prevention strategies
   Enables early lifestyle or medical intervention.
6. Useful when HbA1c results are inconclusive
   Provides additional diagnostic clarity.
7. Standardised and clinically validated test
   Recognised in UK and international guidelines.
8. 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.