The Oral Glucose Tolerance Test (oGTT): Procedure and Values Explained

The oral glucose tolerance test, or oGTT for short, is the test that reliably confirms or rules out a GDM diagnosis. For many pregnant women it is the least pleasant appointment in the entire screening process — mostly because of the waiting time and the sweet test drink. What happens during the test and how to interpret the values is often less clear than the procedure itself.

When the Test Is Carried Out

The regular oGTT takes place between weeks 24 and 28 of pregnancy. With certain risk factors (for example, a previous GDM, pronounced overweight, or a family history of diabetes) earlier investigation in the first trimester may be appropriate, usually via a fasting blood glucose measurement rather than a full oGTT.

How the Test Works

The test is performed fasting, typically in the morning after at least 8 hours without food. First your fasting value is measured; you then drink a solution containing 75 grams of glucose. Blood is drawn again after one hour and after two hours. The appointment therefore lasts about two hours in total, during which you must remain in the waiting room without eating or engaging in strenuous activity.

Which Values Are Considered Abnormal

According to the current IADPSG criteria on which the DDG practice recommendation is based, the following venous plasma thresholds apply:

• Fasting: from 92 mg/dl (5.1 mmol/l)
• After 1 hour: from 180 mg/dl (10.0 mmol/l)
• After 2 hours: from 153 mg/dl (8.5 mmol/l)

Even a single value above the threshold is sufficient for a GDM diagnosis. If the fasting value is at or above 126 mg/dl or the two-hour value is at or above 200 mg/dl, this may indicate a pre-existing, previously undetected diabetes rather than classic gestational diabetes. In that case, further investigation follows.

What Happens After an Abnormal Result

Following the diagnosis, a dietary counselling appointment usually takes place, often combined with an introduction to self-monitoring of blood glucose. How this self-monitoring looks in everyday life and which target values apply is covered in a dedicated article.

Frequently Asked Questions About the oGTT

Can I decline the test?

Screening is voluntary and based on informed consent. Your practice must inform you beforehand and hand you the official information leaflet.

Is a single elevated value already a problem?

A single abnormal value is sufficient for the diagnosis. For the question of how significant the metabolic change actually is, the overall picture from self-measurements over several days plays the decisive role afterwards — not a single test value.

Do I have to be tested again from scratch in a subsequent pregnancy?

In a subsequent pregnancy, screening is generally repeated regardless of the result from the previous pregnancy.

This article does not replace medical advice. Questions about your specific test result are best discussed with your treating practice.

Sources

Sources (secondary citations from the S3 guideline / DDG practice recommendation):

• The thresholds cited (92/180/153 mg/dl) are not from a single study but represent the IADPSG consensus, as presented in the DDG practice recommendation 2025 (Fig. 2/3).
• Sacks DA, Hadden DR, Maresh M, et al. Frequency of gestational diabetes mellitus at collaborating centers based on IADPSG consensus panel-recommended criteria: The Hyperglycemia and Adverse Pregnancy Outcome (HAPO) Study. Diabetes Care 2012;35(3):526–8. — Foundational study on which the IADPSG consensus is based.