Oral glucose tolerance test

What is it?

According to MedlinePlus, the glucose tolerance test is a laboratory test that checks how your body moves sugar from the blood into tissues such as muscle and fat. The most common type is the oral one (OGTT). There is an intravenous type that, according to the page, is rarely used clinically.

The NIDDK describes it as a fasting blood draw, then drinking a high-sugar liquid, then measuring again after two hours.

Why is it done?

According to MedlinePlus, the test diagnoses diabetes by measuring how the body handles sugar, and is used when fasting glucose or A1C results are borderline. On the MedlinePlus glucose page: it is used to diagnose gestational diabetes, and in non-pregnant people sometimes to confirm prediabetes or type 2.

Preparation

According to MedlinePlus: eat your usual food for several days before the test, then fast for at least 8 hours before it (no food or drink), and no food during the test. Tell the provider about any medicine that may affect the result. The NIDDK also states fasting for at least 8 hours.

How is it done?

According to MedlinePlus: a blood sample is taken while fasting, then you are asked to drink a liquid containing a set amount of glucose (usually 75 grams), and samples are taken every 30 to 60 minutes afterwards; the test takes up to 3 hours.

According to the NIDDK and MedlinePlus, in pregnancy: samples are taken every hour for 2 to 3 hours, and it may be preceded by a glucose challenge test one hour after drinking a sugary solution without fasting. If its result is high, an OGTT is usually needed to confirm or rule out. The NIDDK notes that the abnormal threshold for the challenge test starts at 135 to 140 mg/dL.

Understanding the result

According to the NIDDK, for non-pregnant adults, the two-hour value after the drink in mg/dL: 139 or lower is normal, 140 to 199 is prediabetes, and 200 or higher is diabetes.

According to MedlinePlus, for the 75-gram test: fasting 60 to 99 mg/dL (3.3 to 5.5 mmol/L), after one hour below 200 mg/dL (11.1), and after two hours below 140 mg/dL (7.8) is considered normal; 140 to 199 mg/dL (7.8 to 11.1) after two hours is described as impaired glucose tolerance or prediabetes; and 200 mg/dL (11.1) or higher indicates diabetes.

Laboratories and guidelines differ, so what counts is your laboratory's report and your doctor's interpretation. The NIDDK notes that the doctor usually confirms the diagnosis with a second test.

Risks and side effects

According to MedlinePlus: the solution tastes like a very sweet fizzy drink, and uncommon symptoms such as nausea, sweating and dizziness may occur. Risks include excessive bleeding, multiple needle pricks, fainting or dizziness, blood collecting under the skin, and infection to a small degree.

Limits of the test

According to MedlinePlus, drugs may raise or lower the glucose level, severe stress (injury, stroke, heart attack, surgery) raises it, and strenuous exercise may raise it temporarily. Tell the doctor about this.

After the test

According to the NIDDK, the result is reviewed with the doctor, who decides whether a confirmatory test is needed and the later testing schedule.

Common questions