⚠️ General information only. Diagnostic thresholds shown are from ADIPS 2025 consensus recommendations (MJA, June 2025). GTT results are interpreted by your GP or obstetrician in the context of your full clinical history. This page does not constitute medical advice.
Glucose Tolerance Test (GTT)
How to prepare, what happens on the day, and what your results mean. Updated for ADIPS 2025 diagnostic criteria.
📋 Sources:
Australasian Diabetes in Pregnancy Society (ADIPS) 2025 Consensus Recommendations for the Screening, Diagnosis and Classification of Gestational Diabetes. Medical Journal of Australia, June 2025. mja.com.au —
ADIPS 2014 Consensus Guidelines —
World Health Organization diagnostic criteria for diabetes mellitus and impaired glucose tolerance —
RCPA Pathology Manual (fasting duration).
This page provides general information only. Results are interpreted by your GP or obstetrician based on your full clinical picture. This is not medical advice.
Also called: OGTT — Oral Glucose Tolerance Test — POGTT (in pregnancy)
📋 Sources
Glucose tolerance test guidance compiled from the Royal College of Pathologists of Australasia (RCPA); Pathology Tests Explained; RANZCOG and ADIPS (for pregnancy GDM testing); and Healthdirect Australia. Your pathology referral always takes precedence.
⏱ Quick prep summary
🚫
Fast for
8–12 hours
💧
Water is
Allowed
🍞
Eat normally for
3 days before
⏰
Test takes
~2 hours
How to prepare
✅ Do
• Fast for 8–12 hours overnight (labs vary — some allow up to 14–16 hours; check your referral if unsure)
• Drink plain water freely during the fast
• Eat your usual diet for 3 days before including carbohydrates
• Take your usual medications unless told otherwise
• Book a morning appointment — easiest to fast overnight
• Bring your referral, Medicare card, and something to read — you will be there 2 hours
❌ Do not
• Eat, chew gum, or have any drink except water
• Smoke before or during the test
• Exercise strenuously the morning of the test
• Low-carb diet in the days before — this falsely elevates results
• Leave the pathology centre between blood draws
• Walk around excessively during the 2-hour wait
⚠️ No special high-carbohydrate diet is needed beforehand. ADIPS 2025 guidelines state there is no need to follow a 3-day high-carbohydrate preparatory diet — just eat your usual diet. Previous guidelines sometimes recommended this. Check with your pathology lab if unsure.
What happens on the day
Fasting blood draw A blood sample is taken from your arm to measure your fasting blood glucose level. This is the baseline reading.
Drink the glucose solution You will be given a sweetened drink containing 75g of glucose dissolved in water. Drink it within 5 minutes. It is very sweet — some people find it nausea-inducing. Remaining seated and calm reduces nausea; tell the nurse if you feel sick.
Wait 1 hour — blood draw Stay seated at the pathology centre — don't leave, eat, or drink anything except water, and let the nurse know before going to the toilet. A second blood draw is taken at 1 hour for the one-hour glucose reading.
Wait another hour — final blood draw A third blood draw at 2 hours completes the test. Eat something straight away once it's done. Results are sent to your referring doctor, usually within 24–48 hours.
💡 Tip: Bring a book, podcast, or work to do. Eating normally after the test is fine once the final blood draw is done.
When is a GTT used?
• Gestational diabetes screening — the most common reason. All pregnant women are offered a GTT at 24–28 weeks gestation (ADIPS 2025)
• Early pregnancy testing — women with a previous history of gestational diabetes or an early HbA1c of 6.0–6.4% (42–47 mmol/mol) are offered a GTT before 20 weeks, ideally 10–14 weeks
• Pre-diabetes diagnosis — to confirm impaired glucose tolerance when fasting glucose is borderline
• Type 2 diabetes diagnosis — when other results are inconclusive
• Reactive hypoglycaemia investigation — episodes of low blood sugar after meals
Diagnostic criteria — what the results mean
Results are interpreted by your GP or obstetrician in the context of your clinical situation. These numbers alone do not constitute a diagnosis.
Any single result meeting a threshold is sufficient for diagnosis
1 hour after glucose drink
≥10.6 mmol/L
2 hours after glucose drink
≥9.0 mmol/L
Overt Diabetes in Pregnancy (DIP) ADIPS 2025
Criterion
Threshold
Fasting plasma glucose
≥7.0 mmol/L
2-hour plasma glucose (GTT)
≥11.1 mmol/L
HbA1c
≥6.5% (≥48 mmol/mol)
Non-pregnancy (impaired glucose tolerance / type 2 diabetes)
Category
Fasting
2-hour
Normal
<6.1 mmol/L
<7.8 mmol/L
Impaired fasting glucose (IFG)
6.1–6.9 mmol/L
<7.8 mmol/L
Impaired glucose tolerance (IGT)
<7.0 mmol/L
7.8–11.0 mmol/L
Diabetes mellitus
≥7.0 mmol/L
≥11.1 mmol/L
These criteria are for venous plasma glucose. Capillary (finger-prick) readings use slightly different thresholds and are not used for formal diagnosis.
📝 Note — ADIPS 2025 update: The gestational diabetes diagnostic thresholds were raised in June 2025. The fasting threshold increased from ≥5.1 to ≥5.3 mmol/L, and the 2-hour threshold from ≥8.5 to ≥9.0 mmol/L. The 1-hour threshold (new addition) is ≥10.6 mmol/L. Women already diagnosed under the previous criteria are managed based on current blood glucose monitoring and clinical context — not re-diagnosed under the new criteria. Always discuss results with your GP or obstetrician.
Generally, usual medications are taken as normal. However, some medications affect glucose metabolism (e.g. corticosteroids, some antidepressants). Your referring doctor or the pathology lab can advise for your specific situation. Medications for diabetes (if already diagnosed) may need special timing guidance from your GP.
If you vomit the glucose drink within a short time of consuming it, the test result will not be valid. Inform the phlebotomist immediately. The test may need to be rebooked. Remaining seated and calm during the test, and having the drink slowly but within 5 minutes, reduces the chance of nausea.
No — ADIPS guidelines (2014 and 2025) both state that no special high-carbohydrate preparatory diet is required before a pregnancy GTT. Simply eat your usual diet for the 3 days before the test. Some older patient information sheets still recommend a 3-day high-carb diet, but this is no longer current practice. Check with your pathology provider if you receive conflicting instructions.
For most people, driving after a GTT is not an issue — the test uses a fixed glucose load and does not typically cause hypoglycaemia in people without diabetes. If you have diabetes or are prone to low blood sugar, discuss this with your GP beforehand. Having a snack ready for immediately after the test is a common practice.
GTT results are usually available within 24–48 hours of the test. They are sent directly to your referring GP or obstetrician, who will contact you to discuss them. Results are not typically communicated by the pathology lab directly.