Glucose Tolerance Test

What is a glucose tolerance test?

The glucose tolerance test (GTT) is a timed series of blood glucose measurements taken after a glucose load is taken by the patient.

People with normal glucose metabolism will have a relatively modest rise in their blood sugar levels, whereas those with diabetes or impaired glucose tolerance will have a higher peak of their blood glucose levels.

When would you need a glucose tolerance test?

A GTT may be ordered by your doctor if they are suspicious of diabetes or impaired glucose tolerance, a milder form of disease which may occur before full-blown diabetes develops.

A single fasting or random blood glucose test may show a glucose level high enough to diagnose diabetes, but if there is doubt a glucose tolerance test may be needed.

Some women develop impaired glucose tolerance or gestational diabetes during pregnancy only, and a GTT is one way to detect this.

Symptoms of diabetes include:

• Excessive thirst leading to increased fluid intake (polydipsia)
• Frequent urination or increased urine output (polyuria)
• Weight loss or weight gain
• Frequent infections (skin infection, thrush etc.)

How is a glucose tolerance test performed?

The GTT is a blood test performed in the morning, after an 8 to 16 hour overnight fasting period. During this fasting period (at home), patients are not allowed to eat or drink, smoke, or exercise excessively.

The test takes 2 hours and is performed at the laboratory’s collection centre. Blood tests are taken at the beginning of the test and at regular intervals after a glucose-containing drink is taken.

Sometimes it is possible to take the blood samples from a small plastic tube left in the vein – an intravenous cannula.

Glucose tolerance test results explained

The GTT result will be forwarded to your doctor after several days. It may show:

• Normal test
• Impaired glucose tolerance – this is not diabetes, but may progress to diabetes; it usually requires a modification to the diet
• Diabetes mellitus – which may require diet control, oral medications, or insulin injections

Related specialists

• General Practitioner (GP)
• Diabetologist
• Endocrinologist
• General Physician
• Obstetrician
• Occupational Health Physician
• Paediatrician
• General Surgeon

Related procedures

• Blood Test (venesection)
• Intravenous cannulation
• Pre-operative anaesthetic assessment

Related tests

• Glycated Haemoglobin (HbA1C)
• Glucose (glu)
• Exercise Stress Test (EST)
• Lipid Profile
• Urinalysis (UA)

Also known as

• Oral glucose tolerance test
• OGTT
• GTT

Links

• Diabetes Australia
• Baker Institute (International Diabetes Institute)
• Wikipedia – Glucose Tolerance Test

This article is for informational purposes only and should not be taken as medical advice. If in doubt, HealthEngine recommends consulting with a registered health practitioner.