Glucose Tolerance Test – Procedure, Values & Diagnosis
The glucose tolerance test measures how well the body processes sugar. It is used to diagnose diabetes mellitus and gestational diabetes.
Things worth knowing about "Glucose Tolerance Test"
The glucose tolerance test measures how well the body processes sugar. It is used to diagnose diabetes mellitus and gestational diabetes.
What is the Glucose Tolerance Test?
The glucose tolerance test (GTT), most commonly performed as the oral glucose tolerance test (OGTT), is a diagnostic procedure used to evaluate how efficiently the body absorbs and processes glucose (sugar) from the bloodstream. It is primarily used to diagnose type 2 diabetes mellitus and gestational diabetes (diabetes during pregnancy).
In healthy individuals, the hormone insulin regulates blood sugar levels by transporting glucose from the blood into body cells. When this regulation is impaired, the condition is referred to as impaired glucose tolerance or diabetes.
How is the Test Performed?
The glucose tolerance test is typically conducted as follows:
- The patient must fast for at least 8 hours beforehand.
- A fasting blood glucose level is measured first.
- The patient then drinks a glucose solution containing 75 g of glucose dissolved in water (in pregnancy screening, 50 g or 75 g may be used depending on the protocol).
- Blood glucose is measured again after 1 hour and 2 hours (sometimes also after 3 hours).
During the test, the patient should remain seated and avoid eating, drinking (except water), or smoking.
Interpreting the Results
The measured blood glucose values are compared against established thresholds:
- Normal fasting value: below 100 mg/dL (5.6 mmol/L)
- Normal 2-hour value: below 140 mg/dL (7.8 mmol/L)
- Impaired glucose tolerance (prediabetes): 2-hour value between 140 and 199 mg/dL
- Diabetes mellitus: 2-hour value of 200 mg/dL (11.1 mmol/L) or above, or fasting value of 126 mg/dL or above
These thresholds are based on recommendations from the World Health Organization (WHO) and major diabetes associations worldwide.
When is the Test Used?
The glucose tolerance test is recommended in the following situations:
- Suspected type 2 diabetes when fasting blood glucose is elevated
- Screening for gestational diabetes during pregnancy (typically between weeks 24 and 28)
- Assessment in patients with risk factors such as obesity, family history of diabetes, or metabolic syndrome
- Monitoring patients with known impaired glucose tolerance
Risks and Side Effects
The test is generally well tolerated. Some individuals may experience mild nausea or a feeling of fullness after drinking the concentrated sugar solution. Dizziness occurs rarely. In patients with known diabetes, the test should only be performed under medical supervision due to the risk of very high blood glucose levels.
Importance in Pregnancy
For gestational diabetes screening, a 50 g glucose challenge test (non-fasting) is often performed first. If the result is abnormal, a diagnostic 75 g OGTT under fasting conditions follows. Undetected or untreated gestational diabetes can lead to complications for both mother and child, which is why screening is recommended as part of standard prenatal care in many countries.
References
- World Health Organization (WHO): Definition and diagnosis of diabetes mellitus and intermediate hyperglycaemia. WHO Press, Geneva, 2006.
- American Diabetes Association: Standards of Medical Care in Diabetes. Diabetes Care, 2024.
- International Association of Diabetes and Pregnancy Study Groups (IADPSG): Recommendations on the diagnosis and classification of hyperglycemia in pregnancy. Diabetes Care, 2010.
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