Oral Glucose Tolerance Test (OGTT): What It Involves and What the Results Mean

What the test measures

After a meal, glucose enters the blood and the body has to clear it: the pancreas releases insulin, and muscle and liver take the glucose up. In a healthy adult, glucose peaks within an hour and returns close to baseline within two. In insulin resistance and early diabetes, that clearance is slow, so glucose rises higher and stays up longer even while fasting glucose remains normal.

The oral glucose tolerance test (OGTT) measures this directly. You drink a fixed dose of glucose, and blood glucose is measured at set times to see how quickly the body deals with it. The 2-hour value is the one used for diagnosis.

Because after-meal glucose is usually the first thing to become abnormal, the OGTT detects prediabetes and diabetes in people whose fasting glucose and A1C are still normal.

What the test involves

1. Preparation. Eat normally, including adequate carbohydrate, for at least three days beforehand. Fast for 8–12 hours overnight; water is allowed. Avoid smoking, caffeine and strenuous exercise on the morning of the test. Tell the laboratory about medications, particularly corticosteroids, which affect the result.
2. Fasting sample. Blood is drawn on arrival.
3. The drink. A solution containing 75 grams of glucose in about 300 ml of water, drunk within five minutes. It is very sweet; some people feel nauseous.
4. Waiting. You stay seated and do not eat, drink (other than a little water), smoke or exercise for two hours. Activity would lower glucose and distort the result.
5. Second sample. Blood is drawn at two hours. Some protocols also sample at one hour.

The whole visit takes a little over two hours.

The ranges

Standard 75-gram test in non-pregnant adults:

The fasting sample is interpreted using the usual fasting glucose thresholds. A person can have a normal fasting value and an abnormal 2-hour value, or the other way round; each identifies a somewhat different problem (impaired fasting glucose reflects overnight liver glucose output; impaired glucose tolerance reflects slow clearance after a load).

Some research and guidelines now pay attention to the 1-hour value: a 1-hour glucose of 155 mg/dL (8.6 mmol/L) or above, even with a normal 2-hour value, predicts higher future risk of diabetes and cardiovascular disease.

Gestational diabetes

The OGTT is the standard test for gestational diabetes, usually performed at 24–28 weeks of pregnancy (earlier if risk is high). Protocols differ between countries:

• One-step (75 g, used in the UK, Australia and many other countries, and by some U.S. providers): fasting, 1-hour and 2-hour samples, with lower diagnostic thresholds than for non-pregnant adults — for example, fasting 92 mg/dL (5.1 mmol/L), 1-hour 180 mg/dL (10.0 mmol/L), 2-hour 153 mg/dL (8.5 mmol/L) under the widely used IADPSG criteria; one abnormal value is diagnostic. UK guidance uses slightly different cut-offs.
• Two-step (common in the U.S.): a 50-gram drink without fasting as a screen; if the 1-hour result is raised, a 100-gram, three-hour test follows.

Your maternity care team will explain which protocol they use and what your numbers mean.

Why the OGTT is used less than it could be

It is inconvenient, takes two hours, requires fasting, and its results vary more from day to day than A1C does. For routine screening, A1C or fasting glucose is used first. The OGTT is reserved for situations where its sensitivity matters: pregnancy, borderline or conflicting results on the other tests, suspected diabetes in someone with a condition that makes A1C unreliable (anemia, haemoglobin variants, kidney disease), and research.

What affects the result

• Carbohydrate restriction in the preceding days (falsely high)
• Illness, stress or recent surgery (falsely high)
• Corticosteroids, some diuretics, antipsychotics and other medications
• Physical activity during the test (falsely low)
• Vomiting the drink (invalid)
• Prolonged bed rest or inactivity beforehand (falsely high)

What to do with your result

A 2-hour value in the prediabetes range means the same as any prediabetes result: repeat testing, checks of blood pressure and lipids, and the lifestyle changes described in the prediabetes guide, which are particularly effective in people with impaired glucose tolerance — the Diabetes Prevention Program enrolled participants on the basis of this test. A result in the diabetes range is confirmed and followed by a conversation about treatment for type 2 diabetes. A gestational diabetes diagnosis is managed by your maternity team, usually with diet, monitoring and sometimes medication, and followed by re-testing after delivery because it raises the lifetime risk of type 2 diabetes.

Frequently asked questions

References