Glucose tolerance test: what it is, what it tells you and when it’s tested

The oral glucose tolerance test measures how your body handles sugar. You have a fasting blood test, drink a glucose solution, and have more blood tests over the next two hours. If your blood sugar stays high for too long, insulin is not working well.

When it’s tested

Doctors mostly order it when fasting glucose or HbA1c is borderline, to see whether there is prediabetes or diabetes. It is also used in people at high risk, such as those who had diabetes in pregnancy or have polycystic ovaries.

In pregnancy it is done differently, with its own thresholds. Your gynaecologist orders it and explains how to read it.

It is not a test for people who feel well and have normal results: it takes two hours at the lab and simpler tests often answer the same question. That is why Rosa does not run it. Instead it measures glucose, HbA1c and insulin, which together say a lot about how you handle sugar.

What it tells you, and what it does not

The two-hour value matters most. It shows whether your body brings sugar back down in time. Sometimes it is abnormal while fasting glucose is still normal: that is why it can find prediabetes before other tests.

It does not say why. The problem can be insulin that works poorly, insulin released late, or both.

Some labs also measure insulin during the test. It helps your doctor, but the diagnosis rests on glucose.

That is why it is read together with other tests:

Normal glucose tolerance test results

Fasting, glucose is normal below 100 mg/dL. From 100 to 125 it is impaired; 126 or more fits with diabetes.

After two hours it is normal below 140 mg/dL. From 140 to 199 there is impaired glucose tolerance, meaning prediabetes. 200 or more fits with diabetes.

Many labs also take samples in between, for example at half an hour, one hour and an hour and a half. It is normal for glucose to rise there: for the diagnosis, the fasting and two-hour values are what count.

What your result means

Find the row for your two-hour value, in mg/dL. Which values need a doctor? 140 or more; 200 or more, soon.

If your result is abnormal

Prediabetes is not a life sentence. It can often be turned around. Where to start:

With a result in the diabetes range, do not change medicines or supplements on your own: your doctor decides on confirmation and treatment.

How to prepare, and what can skew the result

The test is only reliable if it is done properly. What to know beforehand:

When to worry

Talk to your doctor if:

Frequently asked questions

What are normal glucose tolerance test results? Fasting below 100 mg/dL, after two hours below 140. After two hours, 140 to 199 is prediabetes and 200 or more fits with diabetes.

How is the glucose tolerance test done? You have a fasting blood test, then drink a glucose solution within a few minutes. Afterwards you stay seated at the lab and have more blood tests, up to two hours. It takes a little over two hours in all.

What should I eat before the test? In the three days before, eat normally, without cutting carbohydrates. The evening before, have your usual dinner, then fast until the test. Water is fine.

A test with 3 or 5 samples: what is the difference? It changes how many in-between points are measured. For the diagnosis, the fasting and two-hour values are what count. The extra samples help your doctor see the shape of the curve.

Does the glucose tolerance test have side effects? Usually not. Some people feel sick from the very sweet drink, or a little weak towards the end. If you feel unwell, tell the staff.

Does Rosa do the glucose tolerance test? No. Rosa measures fasting glucose, HbA1c and insulin, which together say a lot about how you handle sugar. If your doctor wants the test, they order it separately.

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