Oral Glucose Tolerance Test: Results Explained

The oral glucose tolerance test (OGTT) is a key diagnostic tool used across the NHS to assess how well the body processes glucose. Most commonly offered during pregnancy to screen for gestational diabetes, it is also used when standard HbA1c testing is unreliable or inconclusive. The test involves a fasting blood draw, a measured glucose drink, and a second blood draw two hours later. Understanding what the OGTT involves, how to prepare, and what your results mean can help you approach the appointment with confidence and act on any findings promptly.

What is an oral glucose tolerance test and why might you need one?

An OGTT measures how effectively the body clears a controlled glucose load from the bloodstream over two hours, and is used primarily to diagnose gestational diabetes or when HbA1c testing is unreliable or inconclusive.

An oral glucose tolerance test (OGTT) is a blood test that measures how well your body processes glucose over a two-hour period. It is used primarily to diagnose gestational diabetes in pregnancy, and outside pregnancy it is used when the standard diagnostic test (HbA1c) is unreliable or gives inconclusive results, or to investigate specific conditions associated with disordered glucose regulation.

The test works by giving the body a controlled amount of glucose and then measuring how effectively insulin clears it from the bloodstream. A healthy pancreas responds to the glucose load by releasing enough insulin to bring blood sugar back to a normal range within two hours; if it cannot do so, this points to impaired glucose regulation.

Your GP or midwife may refer you for an OGTT for several reasons. These include a fasting blood glucose result that falls in an uncertain range, symptoms suggestive of diabetes (such as increased thirst, frequent urination, or unexplained fatigue), a previous diagnosis of gestational diabetes, or a high-risk pregnancy. Outside pregnancy, NICE guidance (NG28) recommends HbA1c as the first-line diagnostic test for type 2 diabetes in most adults.^[1][2] An OGTT is used instead when HbA1c is unreliable, for example in people with haemoglobinopathies, anaemia, chronic kidney disease (stage 4 or 5), or those taking corticosteroids or certain other medicines that affect red blood cell turnover.^[1][2]

In the UK, the term non-diabetic hyperglycaemia (NDH) is preferred over 'prediabetes' for results that fall between normal and diabetic thresholds. NDH may reflect impaired fasting glycaemia, impaired glucose tolerance, or both. Routine follow-up for NDH is typically carried out using HbA1c at agreed intervals rather than repeat OGTT.

For gestational diabetes, NICE guideline NG3 recommends the OGTT as the diagnostic method in pregnancy, as it captures the body's dynamic response to a glucose challenge rather than a single snapshot measurement.^[3][4]

Important: if you or someone you are with has symptoms suggesting type 1 diabetes (such as rapid onset of thirst, weight loss, and fatigue, particularly in a child or young person) or signs of a hyperglycaemic emergency (such as vomiting, confusion, or rapid breathing), seek same-day urgent medical assessment. An OGTT is not appropriate in these situations and must not delay urgent care.

How should you prepare for the test on the day?

You must fast for 8 to 14 hours before the test, drinking only plain water, and avoid smoking, caffeine, chewing gum, and strenuous exercise. Inform the clinical team of any regular medications, as some can affect blood glucose results.

To prepare for an oral glucose tolerance test, you must fast for between 8 and 14 hours beforehand, meaning no food or drink other than plain water from the evening before your appointment. This fasting period is essential because eating or drinking anything containing calories or sugar would alter your baseline blood glucose reading and make the results unreliable. You should book a morning appointment where possible, as this makes the fasting period easier to manage overnight.

There are several additional points to be aware of before your test:

• Medicines: Tell your GP or the testing team about any regular medications, as some (including corticosteroids, certain antipsychotics, and diuretics) can affect blood glucose levels. Do not stop any prescribed medicine without medical advice.

• Smoking: Avoid smoking on the morning of the test, as this can influence glucose metabolism.

• Caffeine and chewing gum: Avoid caffeinated drinks and chewing gum (including sugar-free gum) before and during the test, as these may affect results. Check with your clinic if you are unsure.

• Exercise: Avoid strenuous physical activity in the 24 hours before the test, as this may affect results.

• Illness: If you are unwell on the day, contact the clinic, as acute illness can temporarily raise blood glucose and may lead to a misleading result.

Wear comfortable clothing with easy access to the inside of your elbow, as blood will be taken from a vein in your arm. Plan to stay at the clinic or hospital for approximately two hours, so bring something to read or listen to. You will not be able to eat or drink anything other than water during the test itself, so it is sensible to bring a small snack to have immediately afterwards.

If you vomit after drinking the glucose solution, tell a member of staff immediately. The test result may be invalid and the test may need to be rescheduled. If you feel faint or unwell at any point during the waiting period, inform staff straight away.

What happens during the procedure and how long does it take?

The OGTT takes approximately two hours: a fasting venous blood sample is taken, you drink 75 g of glucose solution within five minutes, then a second venous sample is taken at two hours while you remain seated and at rest.

The oral glucose tolerance test takes approximately two hours from start to finish, and you will need to remain at the clinic for the full duration. The procedure follows a straightforward sequence that a healthcare professional will guide you through at each stage.

The steps are as follows:

1. Baseline blood sample: A healthcare professional takes a blood sample from a vein in your arm to measure your fasting blood glucose level. Venous plasma samples are used for diagnosis; finger-prick capillary readings are not used for OGTT diagnosis.^[3][5]
2. Glucose drink: You are given a glucose solution to drink, typically containing 75 grams of glucose dissolved in water. The drink is sweet and should be consumed within five minutes. For children, the dose may be lower (commonly 1.75 g per kg of body weight, up to a maximum of 75 g); your clinical team will confirm the appropriate dose.
3. Waiting period: You remain seated and at rest for two hours. You must not eat, drink (other than plain water), smoke, or exercise during this time, as any of these can affect the result.
4. Second blood sample: At the two-hour mark, a second blood sample is taken from your arm to measure how your blood glucose level has changed in response to the glucose load.

Some clinics may take an additional blood sample at one hour. For gestational diabetes, NICE guideline NG3 bases the diagnosis on the fasting and two-hour venous plasma glucose values; a one-hour sample, if taken locally, is not part of the NICE diagnostic criteria.^[3][4]

The blood samples are sent to a laboratory for analysis, and results are not usually available on the same day. The test itself is not painful beyond the brief discomfort of the blood draws, and most people tolerate the glucose drink without difficulty, though some find it very sweet. Occasionally, mild nausea is reported after drinking the solution.

What do your results mean and what happens next?

Results are based on fasting and two-hour venous plasma glucose values; a two-hour level of 11.1 mmol/L or above indicates type 2 diabetes, while 7.8 to 11.0 mmol/L indicates impaired glucose tolerance (non-diabetic hyperglycaemia).

Your oral glucose tolerance test results are interpreted using two blood glucose readings: the fasting level taken before the glucose drink and the two-hour level taken afterwards, both measured in millimoles per litre (mmol/L).

For non-pregnant adults, the following diagnostic thresholds are based on World Health Organization (WHO) criteria:

• Normal: Fasting below 6.1 mmol/L and two-hour level below 7.8 mmol/L

• Non-diabetic hyperglycaemia (NDH): This includes impaired fasting glycaemia (fasting 6.1 to 6.9 mmol/L) and/or impaired glucose tolerance (two-hour level 7.8 to 11.0 mmol/L)

• Type 2 diabetes: Fasting 7.0 mmol/L or above, or two-hour level 11.1 mmol/L or above

For gestational diabetes, NICE guideline NG3 uses different thresholds: a fasting glucose of 5.6 mmol/L or above, or a two-hour glucose of 7.8 mmol/L or above, indicates gestational diabetes.

If your results are normal, no further action is usually needed unless you remain at high risk, in which case your GP may recommend repeat testing after a set interval using HbA1c.

A result indicating NDH typically leads to referral to the NHS Diabetes Prevention Programme (also known as Healthier You), which offers structured lifestyle support to people at high risk of developing type 2 diabetes.^[1][2] Eligibility and referral are arranged through your GP. Routine monitoring for NDH is carried out using HbA1c at agreed intervals rather than repeat OGTT.

A diagnosis of type 2 diabetes or gestational diabetes will prompt a more detailed management plan, which may include dietary advice, monitoring, medication, and specialist referral depending on your circumstances.

Results are usually communicated by your GP or midwife within a few days. If you have not heard back within a week, contact your GP surgery. Do not attempt to interpret laboratory printouts without clinical guidance, as reference comments can vary between laboratories, though the diagnostic thresholds themselves are standardised by national and international guidance.

Who is offered an oral glucose tolerance test on the NHS?

The NHS offers an OGTT to pregnant women with specific risk factors (typically at 24 to 28 weeks), and to adults outside pregnancy when HbA1c is unreliable, equivocal, or when investigating conditions associated with secondary diabetes.

The NHS offers an oral glucose tolerance test to specific groups of people based on clinical risk, and it is not a routine screening test for the general population.

In pregnancy (NICE NG3):

The following groups are offered an OGTT, typically between 24 and 28 weeks of pregnancy, or earlier if gestational diabetes is suspected:

• Women with a BMI above 30 kg/m²

• Women who have had gestational diabetes in a previous pregnancy

• Women with a first-degree relative with type 2 diabetes

• Women from South Asian, Black African, African-Caribbean, or Middle Eastern family backgrounds

• Women who have previously had a baby weighing 4.5 kg or more

Outside pregnancy (NICE NG28):

For most adults outside pregnancy, HbA1c is the standard first-line diagnostic test for type 2 diabetes. An OGTT is used when HbA1c is unreliable or results are equivocal, for example in people with haemoglobinopathies, anaemia, chronic kidney disease (stage 4 or 5), or those taking medicines that affect HbA1c reliability.^[1][2] It may also be used for:

• People with symptoms of diabetes but a non-diagnostic HbA1c

• People being investigated for conditions associated with secondary diabetes, such as acromegaly or Cushing's syndrome

• Children or young people where type 1 diabetes is not suspected but glucose regulation needs assessment

Cystic fibrosis-related diabetes:

People with cystic fibrosis are offered annual OGTT screening for cystic fibrosis-related diabetes from the age of 10 years, in line with UK cystic fibrosis clinical guidelines.^[9]

Important: Children and adults with symptoms suggesting type 1 diabetes, or anyone showing signs of a hyperglycaemic emergency, require same-day urgent medical assessment. An OGTT is not appropriate in these situations.

Eligibility is determined by your GP, midwife, or specialist based on your individual clinical picture. If you believe you may be at risk of diabetes or gestational diabetes and have not been offered testing, it is appropriate to raise this with your GP.

Bolt Pharmacy is a GPhC-registered UK online pharmacy offering online consultations with prescriber review and pharmacist support, which may be relevant if you are exploring broader metabolic health management alongside NHS care. Your GP remains the right first point of contact for arranging an OGTT on the NHS.

If you experience any suspected side effects from a medicine you are taking, you can report these to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk.

Scientific References

1. Type 2 diabetes in adults: management (NG28).
2. Blood glucose management | Type 2 diabetes in adults (NG28).
3. Diabetes in pregnancy: management from preconception to the postnatal period (NG3) – Recommendations.
4. Diabetes in pregnancy: management from preconception to the postnatal period (NG3).
5. Diabetes in pregnancy: management from preconception to the postnatal period (NG3) – Terms explained.
6. Gestational diabetes.
7. Diabetes | Treatment summaries.
8. Glycemic assessment when hemoglobin A1c is unreliable: a clinical-laboratory framework for kidney disease, anemia, and hemoglobinopathies.
9. Quality Standard QS168 – Briefing paper (cystic fibrosis-related diabetes).

Frequently Asked Questions