Fasting Blood Glucose Test: Results Explained

A fasting blood glucose test is one of the most important tools used in UK clinical practice to screen for, diagnose, and monitor conditions such as type 2 diabetes and prediabetes. By measuring the concentration of glucose in your blood after a period without eating, the test gives clinicians a reliable snapshot of how well your body regulates blood sugar. Whether your GP has requested it as part of a routine health check, because of symptoms such as increased thirst or fatigue, or to investigate a specific concern, understanding what the test involves, how to prepare, and what your results mean can help you take an active role in your own care.

What is a fasting blood glucose test and why might you need one?

A fasting blood glucose test measures blood glucose after a period without eating to screen for or diagnose diabetes and prediabetes. Your GP may request it if you have symptoms such as increased thirst, frequent urination, or unexplained fatigue.

A fasting blood glucose test measures the concentration of glucose in your blood after a period without eating, and is used to screen for or diagnose diabetes and prediabetes, or to investigate symptoms that may suggest a problem with blood glucose regulation. Glucose is the body's primary energy source, and the hormone insulin regulates how cells absorb it. When insulin production or sensitivity is impaired, glucose accumulates in the bloodstream, which over time can damage blood vessels, nerves, and organs.

Your GP or practice nurse may request a fasting blood glucose test if you have symptoms such as increased thirst, frequent urination, unexplained fatigue, or blurred vision. It is also used during health checks and as part of the diagnostic pathway for gestational diabetes, though UK screening for gestational diabetes uses a 75 g oral glucose tolerance test (OGTT) at 24 to 28 weeks of pregnancy, in line with NICE guideline NG3; the fasting sample taken at the start of the OGTT is not a standalone screening test.^[1][2] In people already diagnosed with type 2 diabetes, HbA1c (glycated haemoglobin) is the standard test for monitoring glycaemic control; fasting glucose may be used in specific circumstances where HbA1c is unreliable, such as in haemoglobinopathies or during pregnancy.

NICE recommends identifying people at risk of type 2 diabetes using a validated risk assessment tool such as QDiabetes, and then offering a blood test to those found to be at high or very high risk.^[3] Risk factors that contribute to a higher score include a BMI above 25 kg/m² (or above 23 kg/m² in people of South Asian, Chinese, African-Caribbean, or Black African descent, in line with NICE PH46), a family history of type 2 diabetes, and a history of cardiovascular disease.^[4][5]

The test itself is straightforward: a healthcare professional takes a small blood sample, usually from a vein in your arm, which is then analysed in a laboratory. Results are expressed in millimoles of glucose per litre of blood (mmol/L). Some GP surgeries and pharmacies also offer finger-prick point-of-care testing, though venous laboratory samples remain the standard for diagnostic purposes under NHS guidance.

Understanding why the test has been requested can help you engage more confidently with your results and any follow-up care your clinician recommends.

How do you prepare for a fasting blood glucose test on the NHS?

Fast for eight to ten hours before the test, drinking only plain water. Discuss all regular medicines with your GP beforehand, as some can affect glucose levels.

To prepare for a fasting blood glucose test, you must not eat or drink anything other than plain water for at least eight to ten hours before the sample is taken. This fasting period allows blood glucose levels to stabilise, so that the result reflects your baseline metabolic state rather than a recent meal. Most people find it easiest to fast overnight and attend an early morning appointment.

During the fasting period, you may drink plain water freely, as staying hydrated makes it easier for the phlebotomist to locate a vein. You should avoid tea, coffee, fruit juice, and other drinks, even if unsweetened, as these can influence glucose metabolism. Chewing gum and mints should also be avoided.

If you take regular prescription medicines, speak to your GP or practice nurse before the test. Some medications, including corticosteroids, certain antipsychotics, and diuretics, can raise blood glucose, while others may lower it. Your clinician will advise whether to take your usual doses before the test or to delay them until afterwards. People taking insulin or other glucose-lowering medicines should follow specific guidance from their diabetes care team, as omitting doses without advice can be unsafe.

On the day of the test, wear a short-sleeved or loose-fitting top to allow easy access to your arm. Arrive at your appointment on time, as a prolonged wait after the fasting period ends could affect your result. If you feel unwell, dizzy, or hypoglycaemic before the test, inform the healthcare professional immediately rather than proceeding without telling them.

What do your fasting blood glucose results actually mean?

A fasting venous plasma glucose below 6.1 mmol/L is normal; 6.1 to 6.9 mmol/L indicates impaired fasting glucose; 7.0 mmol/L or above is indicative of diabetes using WHO criteria.

Fasting blood glucose results are interpreted against established reference ranges, and a single reading should always be considered alongside your symptoms, medical history, and other investigations. The following thresholds for venous plasma glucose are based on WHO diagnostic criteria, as adopted in UK practice:

• Normal: below 6.1 mmol/L

• Impaired fasting glucose (a form of prediabetes): 6.1 to 6.9 mmol/L

• Indicative of diabetes: 7.0 mmol/L or above

It is also worth knowing that the NHS Diabetes Prevention Programme (NHS DPP) uses a slightly broader range for eligibility, defined as non-diabetic hyperglycaemia: a fasting plasma glucose of 5.5 to 6.9 mmol/L, or an HbA1c of 42 to 47 mmol/mol. If your result falls in this range, your GP may refer you to the programme even if it does not meet the WHO threshold for impaired fasting glucose.

A result in the impaired fasting glucose range does not mean you have diabetes, but it does indicate that your blood glucose regulation is not optimal and that your risk of developing type 2 diabetes is elevated. NICE recommends that people in this range are offered structured lifestyle advice and regular monitoring.

A result of 7.0 mmol/L or above on a single test, in the absence of symptoms, requires confirmation with a repeat fasting test or an alternative diagnostic test such as an HbA1c measurement or an OGTT. If you have classic symptoms of diabetes alongside a result of 7.0 mmol/L or above, a diagnosis may be made on that single result alone, in line with WHO diagnostic criteria.

HbA1c provides a picture of average blood glucose over the preceding two to three months and is often used alongside fasting glucose to build a fuller picture.^[7][13] However, HbA1c is not appropriate for diagnosis in all situations: it should not be used in pregnancy, in suspected type 1 diabetes, in children, in people with haemoglobinopathies, after a recent blood transfusion, or in advanced chronic kidney disease.^[7][13] In these circumstances, glucose-based tests are preferred. Your GP will explain which combination of tests is most appropriate for your individual circumstances.

What happens if your results are outside the normal range?

Results in the prediabetes range may lead to NHS Diabetes Prevention Programme referral and annual monitoring; a confirmed diabetes diagnosis triggers a management plan including lifestyle changes and, if needed, metformin.

If your fasting blood glucose result is outside the normal range, your GP will discuss the findings with you and agree on the most appropriate next steps, which depend on whether the result suggests impaired fasting glucose or diabetes. A result does not automatically lead to a diagnosis without clinical review and, where necessary, confirmatory testing.

For people with non-diabetic hyperglycaemia (fasting plasma glucose 5.5 to 6.9 mmol/L, or HbA1c 42 to 47 mmol/mol), NICE recommends referral to the NHS Diabetes Prevention Programme (NHS DPP), a structured, evidence-based programme offering dietary advice, physical activity support, and behaviour change strategies. Research shows that lifestyle interventions can significantly reduce the risk of progression to type 2 diabetes. Your GP may also arrange repeat testing, typically annually, to monitor your glucose levels over time.

If results confirm a diagnosis of type 2 diabetes, your care team will discuss a management plan that may include:

• Lifestyle modification: dietary changes, increased physical activity, and weight management

• Glucose-lowering medicines: metformin is usually the first-line pharmacological option recommended by NICE (NG28), unless contraindicated; NICE also recommends considering an SGLT2 inhibitor for people with established cardiovascular disease, heart failure, or chronic kidney disease, where additional cardio-renal protection is indicated^[8][9]

• Regular monitoring: HbA1c checks, blood pressure measurement, kidney function tests, and annual diabetic eye screening

• Referral: to a diabetes specialist nurse, dietitian, or consultant diabetologist where appropriate

For people with very high glucose levels or symptoms of severe hyperglycaemia, more urgent assessment may be needed. Symptoms that may indicate a serious hyperglycaemic emergency, such as diabetic ketoacidosis (DKA) or hyperosmolar hyperglycaemic state (HHS), include severe thirst, abdominal pain or vomiting, drowsiness or confusion, rapid or laboured breathing, and marked dehydration. If you or someone else experiences these symptoms, call 999 or go to your nearest A&E department immediately. For less urgent concerns, contact your GP promptly if you receive an unexpectedly high result, or call NHS 111 if you feel unwell while awaiting follow-up. Early intervention significantly improves long-term outcomes.

Can anything affect the accuracy of your fasting glucose reading?

Not fasting for long enough is the most common cause of a falsely elevated result; acute illness, stress, and medicines such as corticosteroids or antipsychotics can also distort readings.

Several factors can affect the accuracy of a fasting blood glucose reading, and being aware of them helps you and your clinician interpret results correctly. The most common cause of a falsely elevated result is not fasting for long enough: even a small snack or a sugary drink within the fasting window can raise glucose levels substantially. Conversely, an unusually prolonged fast may cause glucose to fall below your typical baseline, so it is best to follow the eight to ten hour window advised by your GP or the requesting service.

Acute illness, infection, or significant physical or emotional stress can temporarily raise blood glucose through the release of counter-regulatory hormones such as cortisol and adrenaline. If you are unwell on the day of your test, it is worth informing your GP, as the result may not reflect your usual metabolic state and a repeat test may be more informative.

Medicines are another important variable. The following are among those known to influence glucose readings:

• Corticosteroids (such as prednisolone): can raise blood glucose significantly^[11][12]

• Thiazide diuretics and beta-blockers: may impair glucose tolerance; note also that beta-blockers can mask the warning signs of hypoglycaemia (such as tremor and palpitations) in people taking insulin or sulfonylureas, which is an important safety consideration^[14]

• Antipsychotics (particularly olanzapine and clozapine): associated with raised glucose^[17][18]

• Fluoroquinolone antibiotics (such as ciprofloxacin): associated with disturbances in blood glucose, including both hypoglycaemia and hyperglycaemia, as noted in BNF and MHRA guidance^[15][16]

Always tell your GP or the requesting clinician about all medicines, supplements, and herbal remedies you take before the test.

Sample handling also matters: delays in processing, incorrect tube type, or haemolysis (breakdown of red blood cells) can alter results. Laboratory venous samples are more reliable than finger-prick readings for diagnostic purposes, in line with NHS and Royal College of Pathologists guidance. If you have concerns about a result, discuss them with your GP, who can arrange a repeat test or an alternative investigation such as HbA1c to provide a more complete picture.

Scientific References

1. Diabetes in pregnancy: management from preconception to the postnatal period (NG3).
2. Diabetes in pregnancy: management from preconception to the postnatal period (NG3) – Recommendations.
3. Type 2 diabetes: prevention in people at high risk (PH38).
4. Appendix A1: Summary of evidence from surveillance (PH46).
5. PH46 Expert report 2: Analyses from the SABRE cohort study.
6. Gestational diabetes.
7. Diabetes | Treatment summaries – BNF – NICE.
8. Type 2 diabetes in adults: management (NG28).
9. Type 2 diabetes in adults: management (NG28) – Initial medicines.
10. Type 2 diabetes in adults: choosing, reviewing and stopping medicines – visual summary (NG28).
11. Glucocorticoid therapy | Treatment summaries – BNF – NICE.
12. Prednisolone | Drugs – BNF – NICE.
13. Type 2 diabetes | Treatment summaries – BNF – NICE.
14. Gliclazide | Drugs – BNF – NICE.
15. Ciprofloxacin | Drugs – BNF – NICE.
16. Fluoroquinolone antibiotics: must now only be prescribed when other commonly recommended antibiotics are inappropriate.
17. Olanzapine | Drugs – BNF – NICE.
18. Clozapine | Drugs – BNF – NICE.
19. Pharmacological Management of Glucose Dysregulation in Patients Treated with Second-Generation Antipsychotics. Drugs..

Frequently Asked Questions