3 Hour Glucose Test: Results and What to Expect

The 3 hour glucose test is a diagnostic oral glucose tolerance test that measures how the body processes a glucose load over three hours, most commonly used to investigate gestational diabetes. In the UK, NICE guideline NG3 recommends a standard 2 hour 75 g OGTT for diagnosing gestational diabetes, so the 3 hour version is not routine NHS practice but may be used in specific clinical or private settings. This article explains who needs the test, what to expect on the day, how results are interpreted, and what happens next if your levels fall outside the normal range.

What is the 3 hour glucose test and why might you need one?

The 3 hour glucose test is an extended oral glucose tolerance test used to confirm or rule out gestational diabetes when an earlier screening result is borderline. In the UK, the standard NHS test is a 2 hour 75 g OGTT under NICE guideline NG3.

The 3 hour glucose test is a diagnostic blood test used to confirm or rule out gestational diabetes mellitus (GDM) when an earlier screening result has been borderline or abnormal. It is a form of oral glucose tolerance test (OGTT) that measures how effectively the body processes a standardised glucose load over a three-hour period, providing a more detailed picture of glucose metabolism than a single fasting measurement.

In the UK, the standard diagnostic test for gestational diabetes is a 2 hour 75 g OGTT, as recommended by NICE guideline NG3 (Diabetes in pregnancy). A 3 hour version using a 100 g glucose load is not standard practice within the NHS; it is used only in specific clinical protocols, for example in some private or internationally aligned settings, and is not the pathway most women in England will follow. Women are usually referred for an OGTT between 24 and 28 weeks of pregnancy if they have risk factors identified by NICE NG3. These include a BMI above 30 kg/m², a previous pregnancy affected by GDM, a family history of type 2 diabetes, a baby from a previous pregnancy weighing 4.5 kg or more at birth, or a family origin associated with a higher prevalence of diabetes (for example South Asian, Black African, African Caribbean, or Middle Eastern background).^[1][7] Repeated glycosuria detected at antenatal appointments may also prompt referral for testing.

The test may occasionally be used outside pregnancy in certain clinical circumstances, for example when investigating impaired glucose regulation and a shorter OGTT has produced inconclusive results, though this is uncommon in routine UK practice. Your GP or specialist will explain the specific reason for your referral. If you have questions about whether a glucose tolerance test is appropriate for your situation, speaking with your GP or midwife is the right first step.

How is the test carried out and what should you expect on the day?

You fast for at least 8 hours, then have a fasting blood sample taken before drinking a glucose solution; further samples are collected at one, two, and three hours. You must remain seated and avoid food, non-water drinks, smoking, and exercise throughout.

The 3 hour glucose test requires you to fast for at least 8 hours beforehand, usually from midnight the night before, though you may drink plain water. Always follow the specific preparation instructions provided by your clinic or local laboratory, as local protocols may vary slightly. On arrival, a healthcare professional will take a fasting blood sample from a vein in your arm to establish your baseline blood glucose level.

You will then be asked to drink a glucose solution, typically containing 75 g or 100 g of glucose depending on the protocol being followed. The drink is sweet and some people find it causes mild nausea, particularly during pregnancy. After drinking the solution, you will need to remain at the clinic for the duration of the test. Further blood samples are taken at set intervals, commonly at one hour, two hours, and three hours after the glucose drink. Each sample is a straightforward venepuncture.

During the waiting period, you should remain seated and avoid eating, drinking anything other than water, smoking, or doing strenuous exercise, as any of these can affect your results. Bring something to read or a way to pass the time comfortably. If you vomit after drinking the glucose solution, tell the healthcare team straight away, as this may mean the test cannot continue and will need to be rearranged.

The test is generally well tolerated, though some people experience lightheadedness, sweating, or nausea after the glucose drink. If you feel unwell at any point, inform the healthcare team immediately. If you experience any unexpected symptoms after the test that concern you, contact your GP or call NHS 111.

Results are usually sent to your referring clinician within a few days, who will then contact you to discuss the findings and any next steps.

What do the results of a 3 hour glucose test actually mean?

Results are compared against protocol-specific thresholds at each time point; under NICE NG3, a fasting plasma glucose of 5.6 mmol/l or above, or a 2 hour value of 7.8 mmol/l or above, indicates gestational diabetes. Borderline results may prompt dietary advice or closer monitoring rather than a formal diagnosis.

The results of a 3 hour glucose test are interpreted by comparing each blood glucose measurement against established diagnostic thresholds at each time point. In the UK, NICE guideline NG3 sets the diagnostic thresholds for gestational diabetes using the standard 2 hour 75 g OGTT: a fasting plasma glucose of 5.6 mmol/l or above, or a 2 hour value of 7.8 mmol/l or above, indicates gestational diabetes. Where a 3 hour protocol is used, the thresholds applied will depend on the specific clinical protocol in use; some 3 hour criteria also require more than one abnormal value before a diagnosis is made. Your clinician will explain which criteria apply to your results. These 3 hour protocols are not standard in the UK NHS, so the NICE NG3 thresholds remain the reference point for most women tested in England.

A result within the normal range at all time points suggests that glucose metabolism is functioning adequately at the time of testing. It does not entirely exclude the possibility of developing glucose intolerance later in pregnancy, so monitoring may continue if risk factors remain present.

If one or more values exceed the diagnostic threshold, a diagnosis of gestational diabetes may be made. This does not mean the pregnancy is high risk in an absolute sense, but it does mean that additional monitoring and management will be recommended. Your clinician will consider the full picture before making a diagnosis.

Results that fall just below the diagnostic threshold but are higher than expected may be described as borderline. In pregnancy, these are not the same as the formal category of impaired glucose tolerance (IGT), which is a prediabetes classification used outside pregnancy. If your results are borderline, your team may recommend dietary advice, repeat testing, or closer monitoring as a precaution.

What happens next if your results are outside the normal range?

A diagnosis of gestational diabetes triggers referral to a joint obstetric and diabetes team, with dietary changes, self-monitoring, and medication such as metformin or insulin if targets are not met. A fasting plasma glucose test at 6 to 13 weeks postpartum is recommended to confirm glucose levels have returned to normal.

If your 3 hour glucose test results indicate gestational diabetes, your care will be stepped up to include specialist input, typically from a joint obstetric and diabetes team. NICE guideline NG3 recommends that women diagnosed with gestational diabetes are offered dietary advice, blood glucose monitoring, and, where necessary, medication to keep glucose levels within a safe range throughout the remainder of the pregnancy.

Management usually begins with dietary changes, focusing on reducing refined carbohydrates and spreading meals evenly throughout the day. A diabetes specialist dietitian or diabetes nurse will often provide structured guidance. NICE NG3 sets specific self-monitoring targets for women with gestational diabetes: a fasting plasma glucose of 5.3 mmol/l or below, a 1 hour postprandial value of 7.8 mmol/l or below, or a 2 hour postprandial value of 6.4 mmol/l or below. Your team will explain what these targets mean for your individual monitoring plan. If blood glucose targets are not met through diet alone, metformin may be considered, and insulin therapy may be recommended if glucose levels remain above target or if metformin is not suitable.

After delivery, women who have had gestational diabetes should be offered a fasting plasma glucose test at 6 to 13 weeks postpartum to check whether glucose levels have returned to normal.^[4] NICE also recommends ongoing periodic diabetes screening for women with a history of GDM, as they carry a higher long-term risk of developing type 2 diabetes.^[1][4] Your midwife or GP will advise on the timing and type of follow-up test.

For people tested outside pregnancy, a result above the diagnostic threshold for type 2 diabetes (a fasting plasma glucose of 7.0 mmol/l or above, or a 2 hour OGTT value of 11.1 mmol/l or above) would prompt referral for further assessment. In people without symptoms, a second confirmatory test is usually required before a diagnosis of diabetes is made.^[1] Results in the intermediate range may indicate prediabetes, and lifestyle interventions are typically recommended at this stage.

If you are managing a diagnosis of gestational diabetes or impaired glucose regulation and are exploring further support, 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. Your midwife or GP remains your primary point of contact for ongoing care. Contact your GP or call NHS 111 if you have concerns about your symptoms between appointments. If you experience any unexpected reaction to medication prescribed as part of your diabetes management, you can report this to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk.

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