Glucose tolerance test (two points)

The two-point glucose tolerance test (OGTT, oral glucose tolerance test) shows how the body handles a larger amount of sugar. Blood glucose is measured fasting and two hours after drinking a solution containing 75 g of glucose. This detects diabetes and prediabetes, which are not always apparent from fasting glucose alone.

Who the test is for

• people whose fasting glucose is borderline or whose HbA1c is raised but the diagnosis is unclear,
• people with glucose found in urine (glucosuria) whose blood glucose is not clearly raised,
• people at increased risk of type 2 diabetes: excess body weight, a family history of diabetes, high blood pressure or blood lipids, or polycystic ovary syndrome,
• women who had gestational diabetes during pregnancy, 4 to 12 weeks after delivery.

What the glucose tolerance test shows

After a carbohydrate-rich meal, blood glucose rises and the pancreas secretes insulin to bring it back towards baseline. If cells respond less well to insulin or the pancreas does not secrete enough, glucose remains raised two hours after the glucose load.

In a healthy person, glucose after two hours can be slightly higher or lower than fasting glucose, but is below 7,8 mmol/L. OGTT can therefore detect impaired glucose tolerance and diabetes even when fasting glucose is still normal.

A separate test with three blood collections and different thresholds is used in pregnancy: glucose tolerance testing in pregnancy.

If you know your fasting glucose is 7,0 mmol/L or higher, or you already have a diabetes diagnosis, mention this on arrival. A glucose tolerance test is then usually unnecessary; a doctor decides whether to perform it.

Preparing for the test

• For three days before the test, eat as usual with enough carbohydrates (at least 150 g per day, for example bread, pasta, rice, potatoes and fruit) and do not change your usual physical activity. Do not start a diet just before the test.
• Fast for 8 to 14 hours before testing. You may drink plain water.
• Avoid alcohol and exhausting training the day before, and do not smoke in the morning.
• Do not stop medicines on your own. Tell your doctor which you take because some, such as corticosteroids, raise glucose.
• If you have an acute illness, such as an infection with fever, postpone the test until you recover.

What happens during the test

1. The first venous blood sample is taken in the morning while fasting.
2. Within five minutes, you drink a solution of 75 g glucose in about 3 dL of water.
3. You spend the next two hours in the laboratory: sit and rest without eating, drinking or smoking, because all of these would change the result.
4. The second blood sample is taken two hours after you started drinking the solution.

The solution is very sweet and makes some people nauseous. If you vomit, the test is stopped and repeated on another day. Bring something to eat afterwards.

How to interpret the result

For adults other than pregnant women, the World Health Organization (WHO) and American Diabetes Association (ADA) thresholds for venous blood glucose apply:

Result	Fasting	After 2 hours
normal	below 6,1 mmol/L	below 7,8 mmol/L
impaired fasting glucose	6,1–6,9 mmol/L	below 7,8 mmol/L
impaired glucose tolerance	below 7,0 mmol/L	7,8–11,0 mmol/L
diabetes	7,0 mmol/L or higher	11,1 mmol/L or higher

One of the two raised values is sufficient for diabetes. The ADA defines impaired fasting glucose from 5,6 mmol/L, so that threshold can be found on some reports and in the literature.

Impaired fasting glucose and impaired glucose tolerance are called prediabetes. They increase the risk of type 2 diabetes and cardiovascular disease, but this risk can be reduced through dietary changes, regular activity and losing excess weight.

A single raised result without pronounced symptoms such as thirst, frequent urination and weight loss is usually confirmed by repeat measurement or another test, such as HbA1c. The referring doctor interprets the result alongside other results and risk factors.

Two-point OGTT or OGTT with insulin?

A two-point test is sufficient to diagnose prediabetes and diabetes. When a doctor also wants to see the pancreatic insulin response, for example when insulin resistance is suspected, they may request a 4-point OGTT with insulin, in which glucose and insulin are measured in four blood samples. There are no universal thresholds for insulin during a glucose tolerance test, so a doctor interprets the result; read more in the article on C-peptide and insulin.

How to arrange testing

You can have blood taken without an appointment or referral at one of our branches across Croatia (listed under Contact). Come in the morning, fasting. Prices for all tests and panels are in the service price list.

Frequently asked questions

How long does an OGTT take?

Exactly two hours pass between the first and second blood collections, and allow a little longer for arrival and preparation. You remain in the laboratory throughout.

Can I drink water or coffee before the test?

While fasting, you may drink plain water. Avoid coffee, tea, juices and chewing gum. After drinking the glucose solution, drink nothing until the second blood collection.

What does glucose between 7,8 and 11,0 mmol/L after two hours mean?

This is impaired glucose tolerance, a form of prediabetes. It is not diabetes, but indicates an increased risk of developing it. A doctor will suggest lifestyle changes and arrange follow-up.

Is the glucose tolerance test dangerous?

No. The most common discomfort is nausea from the sweet solution. Less often, sweating, trembling or weakness occur near the end of the test; tell staff immediately if this happens.