Glucose Tolerance Test: What To Expect And How To Prepare

The

glucose tolerance test (GTT)

, also known as the oral glucose tolerance test (OGTT), is a diagnostic procedure used to assess how well your body processes glucose (sugar). It measures blood glucose levels before and after consuming a sugary drink, helping diagnose conditions like diabetes, pre-diabetes (impaired glucose tolerance or non-diabetic hyperglycaemia), and gestational diabetes.

This test is particularly valuable when routine blood tests like fasting glucose or HbA1c are inconclusive. Pre-diabetes occurs when blood glucose levels are higher than normal but not yet at diabetic levels, increasing the risk of progressing to type 2 diabetes (up to 6 in 10 people within 10 years) and cardiovascular disease.

What is the glucose tolerance test used for?

The OGTT evaluates your body’s ability to manage a large glucose load. After fasting, you drink a glucose solution, and blood samples track how levels rise and fall. Key uses include:

• Diagnosing

  gestational diabetes

  in pregnant women, typically between 24-28 weeks.
• Confirming

  pre-diabetes

  (impaired glucose tolerance, IGT, or impaired fasting glucose, IFG), where 2-hour post-glucose levels are elevated (140-199 mg/dL or 7.8-11.0 mmol/L) but below diabetes thresholds.
• Diagnosing

  type 2 diabetes

  when fasting glucose (≥126 mg/dL or 7.0 mmol/L) or 2-hour levels (≥200 mg/dL or 11.1 mmol/L) confirm it.
• Assessing

  insulin resistance

  or reactive hypoglycaemia in symptomatic patients.

Normal fasting glucose is 70-99 mg/dL (3.9-5.5 mmol/L); pre-diabetes ranges 100-125 mg/dL (5.6-6.9 mmol/L). The test is recommended for high-risk individuals: overweight, family history of diabetes, sedentary lifestyle, or conditions like PCOS.

Why would I have a glucose tolerance test?

Healthcare providers order an OGTT if initial screening suggests glucose issues. Common reasons:

• High-risk screening: Obesity (BMI ≥25), age ≥45, family history, hypertension, high cholesterol, or previous gestational diabetes.
• Pregnancy: Routine for gestational diabetes risk, as it affects 2-10% of pregnancies and raises maternal/fetal complications.
• Symptoms or inconclusive tests: Though pre-diabetes is often asymptomatic, subtle signs like fatigue, blurred vision, or slow-healing wounds may prompt testing.
• Follow-up: Monitoring lifestyle interventions in pre-diabetes to prevent progression.

Up to 1 in 3 people with pre-diabetes progress to diabetes without intervention, but early detection allows reversal through diet and exercise.

Preparing for the test

Proper preparation ensures accurate results. Follow these steps:

• Fast for 8-14 hours: Only water allowed; no food, drink, smoking, or exercise.
• Medication review: Inform your doctor about drugs affecting glucose (e.g., steroids, diuretics). May pause some.
• Avoid stress: Rest well; stress hormones elevate glucose.
• For gestational OGTT: Standard 75g glucose load; some use 100g stepwise.
• Timing: Morning test preferred for stable hormones.

Table of standard preparations:

Aspect	Instructions
Fasting	8-14 hours (water OK)
Glucose dose	75g adults; 100g pregnancy (some protocols)
Activity	Rest quietly; no vigorous exercise
Medications	Consult doctor

What happens during the test?

The OGTT takes 2-3 hours in a clinic:

1. Baseline sample: Fasting blood draw from vein or finger prick.
2. Glucose drink: 75g anhydrous glucose in 250-300mL water (flavored if needed). Drink within 5 minutes.
3. Timed samples: Blood at 1 and 2 hours (sometimes 30min/3hr). Remain seated, minimal activity.
4. Completion: Light snack provided; results discussed later.

In gestational tests, additional 1-hour and 3-hour samples may be taken for a 100g load. Discomfort is minimal: nausea from drink (5-10%), dizziness, or bruising.

Glucose tolerance test results

Results classify your status. Use WHO/ADA criteria (mmol/L and mg/dL):

Parameter	Normal	Pre-diabetes (IGT/IFG)	Diabetes
Fasting (0 hr)	<7.0 (<126)	6.1-6.9 (110-125)	≥7.0 (≥126)
2-hour post-glucose	<7.8 (<140)	7.8-11.0 (140-199)	≥11.1 (≥200)

Interpretation: Confirm with repeat test. Pre-diabetes signals lifestyle changes; diabetes requires treatment. Gestational thresholds differ (e.g., fasting <5.1 mmol/L normal). Factors like stress or illness can skew results—retest if borderline.

Are there any risks from the test?

Risks are low and rare:

• Common: Nausea/vomiting (from hyperosmolar drink), headache, dizziness.
• Rare: Hypoglycaemia in reactive cases; allergic reaction to drink.
• Pregnancy: Safe, no fetal risks documented.
• Venous draws: Bruising, phlebitis (<1%).

Compared to benefits, risks are negligible. Inform staff of allergies or eating disorders.

Frequently Asked Questions (FAQs)

Who needs a glucose tolerance test?

High-risk groups: overweight/obese, family history, gestational diabetes history, PCOS, or abnormal fasting glucose.

How long does the OGTT take?

2-3 hours total, with blood draws at intervals.

Can I drive after the test?

Yes, unless hypoglycaemia symptoms occur; eat soon after.

What if results show pre-diabetes?

Lifestyle changes (weight loss 5-7%, exercise 150min/week) can reverse it in 58% of cases.

Is the glucose drink safe in pregnancy?

Yes, standard for gestational screening; non-fasting alternatives emerging but OGTT gold standard.

What foods affect the test?

Avoid high-carb meals day before; fast properly.

This article provides an overview mirroring patient.info structure, expanded with evidence from credible sources. Consult a healthcare professional for personalized advice.