How Long Does a Glucose Test Take?

How long does a glucose test take? The answer depends on which type of test you are having. A simple fasting plasma glucose or HbA1c test requires only a single blood draw and takes just a few minutes, whereas the oral glucose tolerance test (OGTT) used on the NHS takes approximately two hours from first to final blood sample, with the total appointment often extending to two and a half hours or more. Understanding what to expect, how to prepare, and how to interpret your results can help you feel confident and ready for your appointment.

How long does a glucose test actually take from start to finish?

An NHS OGTT takes approximately two hours from first to final blood sample, with the total appointment often reaching two and a half hours. Simpler tests such as fasting plasma glucose or HbA1c require only a single blood draw and take just a few minutes.

A standard oral glucose tolerance test (OGTT) on the NHS takes approximately two hours from the first blood draw to the last, though the total appointment time including registration and waiting can extend to two and a half hours or more.^[3] The test itself follows a fixed structure: a fasting blood sample is taken first, you then drink a glucose solution, and a second blood sample is collected two hours later. You are asked to remain at the clinic or hospital throughout this period. Exact timings and on-site requirements follow local NHS trust protocols, which may vary slightly.

A simpler fasting plasma glucose test or a random blood glucose test is considerably quicker, typically taking only a few minutes for the blood draw itself, with results often available within the same day or the following working day depending on the laboratory. A HbA1c blood test, which measures average blood glucose over roughly three months, is also a brief procedure requiring only a single blood sample.^[12]

The OGTT is the most time-intensive of the common glucose tests because the two-hour wait is clinically essential. The body's response to the glucose drink over that period is what the test is measuring, so leaving early would produce an incomplete or unreliable result. Patients are generally advised to plan for a half-day commitment when attending for an OGTT, particularly if they are travelling by public transport or need to arrange childcare.

What happens during a glucose tolerance test on the NHS?

A healthcare professional takes a fasting blood sample, you drink 75 g of glucose solution within five minutes, then a second blood sample is taken two hours later. You must remain seated and avoid eating, smoking, or exercising throughout the wait.

During an NHS oral glucose tolerance test, a healthcare professional takes an initial blood sample from a vein in your arm after you have fasted overnight, usually for at least eight hours. This baseline sample measures your fasting blood glucose level. You are then given a glucose drink containing 75 grams of anhydrous glucose dissolved in water, which you are asked to consume within five minutes.^[1][3]

After drinking the solution, you wait in the clinic for two hours. During this time you should remain seated and avoid eating, drinking anything other than small sips of water, smoking, or doing strenuous exercise, as any of these can affect the accuracy of the result.^[3] At the end of the two-hour period, a second blood sample is taken from your arm. Some local trust protocols, particularly those used in pregnancy screening, may include an additional sample at one hour; however, NICE NG3 (Diabetes in pregnancy) specifies fasting and two-hour values as the standard approach, so any one-hour sample reflects local variation rather than national guidance.^[1][2]

It is worth noting that HbA1c is not used to diagnose gestational diabetes; the OGTT is the required method in pregnancy.^[1][2]

The glucose drink can occasionally cause mild nausea, particularly if consumed quickly. If you vomit shortly after drinking it, the test may need to be rescheduled, as the result would not be reliable. Staff will advise you on what to do in this situation. If you feel severely unwell at any point during or after the test, such as experiencing persistent vomiting, abdominal pain, drowsiness, or rapid breathing, seek urgent medical attention rather than waiting for routine follow-up. Results are usually sent to your referring GP or specialist within a few days, though turnaround times vary between NHS trusts. Your GP will then contact you to discuss what the findings mean and whether any further action is needed.

How should you prepare before your glucose test appointment?

Fast for at least eight hours before the test, drinking only water, and maintain your usual diet in the days beforehand to avoid affecting results. Inform your clinical team of all medications, and do not stop any prescribed medicine without explicit clinician guidance.

To prepare for a glucose tolerance test, you should fast for at least eight hours before your appointment, consuming nothing except water during that period. Most NHS clinics schedule OGTTs in the morning so that the overnight fast covers the majority of the waiting time. In the days before the test, you should maintain your usual diet and not restrict carbohydrates, unless your clinic advises otherwise, as a low-carbohydrate diet in the days preceding the test can affect results. You should also avoid strenuous exercise on the day of the test. Your clinic's own preparation leaflet takes precedence over general guidance, so read any instructions you are given carefully.

You should avoid alcohol for at least 24 hours beforehand, as it can interfere with glucose metabolism. If your clinic's instructions differ, follow those.

Certain medications can affect blood glucose levels, so it is important to inform your GP or the clinic about everything you are currently taking, including prescription medicines, over-the-counter remedies, and supplements. Your clinical team will advise whether any medication should be paused before the test. Do not stop any prescribed medicine without explicit guidance from a clinician.

On the day of the test, wear comfortable clothing with sleeves that can be rolled up easily to allow straightforward access to the vein. Bring something to occupy yourself during the two-hour wait, such as a book or a fully charged phone, as you will need to remain in the waiting area. You may also wish to bring a small snack to eat immediately after the test is complete, as you will have been fasting for a significant period by that point. Arriving on time is important because a late start can disrupt the two-hour window and may require the appointment to be rescheduled.

What do your glucose test results mean?

An OGTT fasting result of 7.0 mmol/L or above, or a two-hour result of 11.1 mmol/L or above, indicates diabetes and requires confirmation on a second test if asymptomatic. Results between normal and diabetic thresholds indicate impaired fasting glucose or impaired glucose tolerance, sometimes called prediabetes.

Glucose test results are interpreted against established reference ranges, and the figures used depend on which test was performed. For an oral glucose tolerance test, NHS and NICE guidance uses the following thresholds for venous plasma glucose:

• Normal: fasting below 6.1 mmol/L and two-hour reading below 7.8 mmol/L

• Impaired fasting glucose (IFG): fasting between 6.1 and 6.9 mmol/L

• Impaired glucose tolerance (IGT): two-hour reading between 7.8 and 11.0 mmol/L

• Diabetes: fasting of 7.0 mmol/L or above, or two-hour reading of 11.1 mmol/L or above (confirmed on a second test if the person is asymptomatic, in line with NICE guidance)

Impaired fasting glucose and impaired glucose tolerance are sometimes called prediabetes. They indicate that blood glucose regulation is not functioning optimally but has not yet reached the threshold for a diabetes diagnosis. NICE guidance recommends that people in this category are offered lifestyle advice and regular monitoring, as there is good evidence that dietary changes and increased physical activity can reduce the risk of progression to type 2 diabetes.

For most non-pregnant adults, HbA1c is usually the preferred first-line test for diagnosing type 2 diabetes. The OGTT is typically used when HbA1c is unreliable or gives equivocal results, for example in people with certain haemoglobin variants, haemolytic anaemia, or during pregnancy. HbA1c is not used to diagnose gestational diabetes.

For gestational diabetes, different thresholds apply. NICE NG3 recommends that gestational diabetes is diagnosed if the fasting plasma glucose is 5.6 mmol/L or above, or the two-hour value is 7.8 mmol/L or above. If you receive an abnormal result, your GP or midwife will explain the next steps, which may include referral to a specialist diabetes team.

Who is usually offered a glucose test in the UK?

NICE recommends glucose testing for people with risk factors such as a BMI above 25 kg/m2, a first-degree relative with type 2 diabetes, or a history of impaired glucose tolerance. Pregnant women with recognised risk factors are offered a 75 g OGTT between 24 and 28 weeks.

In the UK, glucose testing is offered to people who have risk factors for type 2 diabetes or gestational diabetes, or who present with symptoms that may suggest abnormal blood glucose regulation. NICE guidance identifies several groups who should be considered for testing, including people with a body mass index (BMI) above 25 kg/m2 (or above 23 kg/m2 in people of South Asian, Chinese, African-Caribbean or Black African descent), those with a first-degree relative with type 2 diabetes, people with a history of cardiovascular disease, and those who have previously been identified as having impaired glucose tolerance or impaired fasting glucose.

For gestational diabetes, NICE NG3 recommends a risk-based approach rather than universal screening. Women with one or more recognised risk factors are offered a 75 g two-hour OGTT, typically between 24 and 28 weeks of pregnancy. Recognised risk factors include a BMI above 30 kg/m2, a previous baby weighing 4.5 kg or more, previous gestational diabetes, a first-degree relative with diabetes, and family origin associated with a higher prevalence of diabetes (South Asian, Black Caribbean, and Middle Eastern). Women who have had gestational diabetes in a previous pregnancy may be offered testing earlier in the pregnancy or self-monitoring of blood glucose from the outset.

People experiencing symptoms such as increased thirst, frequent urination, unexplained weight loss, or persistent fatigue should speak to their GP, who may arrange a blood glucose test as part of the initial assessment. If you experience symptoms suggesting severe hyperglycaemia, such as significant drowsiness, rapid breathing, persistent vomiting, or abdominal pain, seek urgent same-day medical attention rather than waiting for a routine appointment. The NHS Health Check programme, offered to adults in England aged 40 to 74 every five years, also includes an assessment of diabetes risk and may lead to a referral for formal glucose testing.^[4][5]

If you are unsure whether you should be tested, your GP is the right first point of contact. For those exploring weight management support alongside diabetes risk, Bolt Pharmacy is a GPhC-registered UK online pharmacy offering online consultations, with prescriber review and pharmacist support, where clinically appropriate options can be discussed.

Scientific References

1. Diabetes in pregnancy: management from preconception to the postnatal period (NG3) – Recommendations.
2. Diabetes in pregnancy: management from preconception to the postnatal period (NG3).
3. Gestational diabetes.
4. NHS Health Check.
5. First national evaluation of NHS Health Check programme published.
6. Type 2 diabetes: prevention in people at high risk (PH38) – Recommendations.
7. Type 2 diabetes: prevention in people at high risk (PH38).
8. Appendix A1: Summary of evidence from surveillance (NG246).
9. Preventing the progression of pre-diabetes to type 2 diabetes: identification and risk assessment of adults with prediabetes (PH38 evidence review).
10. Evidence-based ranking of diabetes prevention in prediabetes: A comprehensive network meta-analysis of 378 randomised controlled trials.
11. Effectiveness of Structured Education Programmes on Diet and Exercise for Preventing Type 2 Diabetes in Adults With Prediabetes: A Systematic Review.
12. HbA1c | Information for the public | Type 1 diabetes in adults (NG17).

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