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

The OGTT is the gold-standard test for diagnosing gestational diabetes and pre-diabetes. It measures how your body handles a standardised glucose load over 2 hours.

Written and clinically reviewed by Suman Konda, PharmD, Clinical Pharmacist · Based on peer-reviewed sources · Editorial policy · Not medical advice

Last reviewed and updated: · How we check our content

Glucose dose
75g oral glucose
Fasting required
8–10 hours overnight
Duration
2 hours
Used for
T2DM, pre-diabetes, GDM screening

OGTT Interpretation

Time PointNormalPre-Diabetes (IFG/IGT)Diabetes
Fasting (0 min)<6.1 mmol/L6.1–6.9 mmol/L (IFG)≥7.0 mmol/L
2-hour post-glucose<7.8 mmol/L7.8–11.0 mmol/L (IGT)≥11.1 mmol/L

Gestational Diabetes Criteria (WHO 2013)

In pregnancy, GDM is diagnosed if ANY of: fasting ≥5.1 mmol/L, 1-hour ≥10.0 mmol/L, or 2-hour ≥8.5 mmol/L. Lower thresholds than general population because of fetal risk.

Who Needs an OGTT?

  • Suspected gestational diabetes (GDM) at 24–28 weeks
  • Pre-diabetes (HbA1c 42–47 mmol/mol): to stage more precisely
  • Post-gestational diabetes follow-up
  • Acromegaly (growth hormone suppression OGTT used for diagnosis)
  • Suspected reactive hypoglycaemia
How to PrepareFast for 8–10 hours (water only). Avoid unusual exercise, smoking, or illness on the test day: all can affect results. Do not test during acute illness, as glucose results will be unreliable.
Do I need to fast for an OGTT?
Yes. 8–10 hours overnight fast is required. Only water is permitted. Fasting glucose is the starting point of the test.
What happens during the test?
Fasting blood sample → drink 75g glucose solution → rest quietly (no eating, exercise, or smoking) → blood sample at 1 hour and 2 hours.
What is impaired glucose tolerance (IGT)?
A 2-hour OGTT glucose of 7.8–11.0 mmol/L with normal fasting glucose. It represents pre-diabetes with higher risk of progression to type 2 diabetes and cardiovascular disease.
If I have GDM, will I develop type 2 diabetes?
Up to 50% of women with GDM develop type 2 diabetes within 10 years. Annual HbA1c monitoring after delivery is recommended, along with lifestyle measures to reduce progression.

Frequently asked questions

Why does my lab's Oral Glucose Tolerance range differ from the one shown here?
Every laboratory sets its own reference interval from the analyser, reagents and reference population it uses, so two labs can publish different Oral Glucose Tolerance intervals and both be correct, even reporting in the same mmol/L. Read your result against the range printed on your own report.
What is worth asking a doctor about an abnormal Oral Glucose Tolerance?
Whether it needs repeating, whether a medicine or recent illness explains it, how far outside the range it actually falls, and whether it changes management or simply needs watching. Those four questions cover most of what matters.
Does age or sex affect the Oral Glucose Tolerance reference range?
For several analytes it does, which is why some reports print separate intervals. Where your laboratory gives an age or sex-specific range for Oral Glucose Tolerance, that range takes precedence over any general figure.

References

The clinical information on this page is drawn from peer-reviewed sources indexed by the US National Library of Medicine. Links go to the source so you can read it yourself.

  1. Glucose Tolerance Test. In: StatPearls. Treasure Island (FL): StatPearls Publishing. NCBI Bookshelf NBK532915

Related reading

Medical Disclaimer: This page is for general education only and does not replace professional medical advice. Always consult a qualified healthcare provider.

Preparing properly, or the result is meaningless

The oral glucose tolerance test is unusually sensitive to preparation, and poor preparation produces falsely abnormal results that lead to unnecessary diagnoses. For at least three days beforehand, carbohydrate intake should be unrestricted and normal; deliberately cutting carbohydrates in the days before, which people often do hoping to help, impairs insulin release and makes the curve look worse. An overnight fast of 8 to 14 hours is required, water only. During the test itself you must remain seated, not smoke, and not eat or drink anything else, because both activity and smoking alter glucose handling.

Why the two-hour value is the one that counts

Blood is taken fasting, the glucose drink is taken, and a second sample follows two hours later. Fasting glucose and the two-hour value measure different things: fasting reflects hepatic glucose output overnight, while the two-hour value reflects how efficiently tissues clear a load. It is entirely possible to have a normal fasting glucose and an abnormal two-hour value, which is why the test detects impaired glucose tolerance that a fasting sample alone misses. Intermediate samples add little for routine diagnosis and are mainly used in research or when investigating reactive hypoglycaemia.

Situations where the test is preferred

  • Pregnancy, where it remains the standard test for gestational diabetes and uses pregnancy-specific thresholds.
  • Where HbA1c is unreliable: haemoglobin variants, recent transfusion, iron or B12 deficiency, haemolysis and advanced kidney disease all distort it.
  • Suspected impaired glucose tolerance with normal fasting glucose, common in polycystic ovary syndrome and after gestational diabetes.
  • Investigating hypoglycaemia after meals, using an extended protocol.

What the test feels like

Most people find the drink very sweet and mildly unpleasant, and some feel nauseated, sweaty or lightheaded partway through. This usually passes. Allow around two and a half hours at the clinic, bring something to occupy the wait, and arrange to eat immediately afterwards. If you vomit after the drink the test cannot be interpreted and will need rescheduling, so mention it rather than continuing.