Glucose Blood Test

• Plasma glucose (measured): the concentration of glucose in the liquid part of your blood, in mmol/L.

This test does not measure insulin or HbA1c, and it is not a glucose tolerance test.

Glucose is the body's main circulating fuel. After a meal, the pancreas releases insulin, helping muscle, fat and liver cells take glucose up. Overnight, the liver releases glucose to keep levels steady, under the influence of hormones including glucagon. Fasting glucose therefore reflects the balance between glucose output from the liver and how well the body responds to insulin. When insulin works less effectively, fasting glucose tends to drift upwards, usually gradually.

Fasting glucose is a snapshot of one moment, whereas HbA1c reflects an average over months, so the two tests answer slightly different questions.

Why sample handling matters

Blood cells keep using glucose after the sample is taken. A review in Clinical Chemistry reported that unprocessed samples lose roughly 5 to 7% of their glucose per hour, and that fluoride preservative has little effect for the first one to two hours. We therefore take a venous sample in a dedicated tube and send it promptly.

Laboratories set their own reference ranges; one NHS laboratory, for example, flags results outside 3.9 to 6.0 mmol/L. Interpretation depends on whether you had fasted, so please tell us accurately when you last ate.

Recognised thresholds for fasting plasma glucose

• 7.0 mmol/L or above: consistent with diabetes under WHO criteria. Without symptoms, diagnosis should not rest on one result; a repeat is needed and diagnosis is made by a clinician.
• 6.1 to 6.9 mmol/L: described by the WHO as impaired fasting glycaemia.
• 5.5 to 6.9 mmol/L: the range NICE uses in England to indicate high risk of type 2 diabetes (PH38). NICE's lower starting point reflects its aim of identifying people who may benefit from prevention support.

In a non-fasting sample, 11.1 mmol/L or above with symptoms is consistent with diabetes and needs prompt medical review.

Things that can affect results

• Eating or sugary drinks within the previous 8 hours raise the result.
• Delays in processing lower it, because glucose keeps falling in the tube.
• Infection, injury and acute stress can raise glucose temporarily, as can steroids and some other medicines.
• Fasting glucose varies from day to day, so one result is a single snapshot.

A low reading in a routine sample is often a handling artefact rather than true hypoglycaemia, which is only confirmed when symptoms, a low measured level and relief once glucose is raised occur together. Fasting results of 5.5 mmol/L or above are worth sharing with your GP; with your consent, we can write to your GP.

Fasting glucose is a simple, widely used measure. People sometimes choose it to add a current glucose value to an HbA1c result, or because of risk factors such as a family history of type 2 diabetes or an earlier borderline result. Where UK guidance prefers glucose-based testing over HbA1c, for example with some haemoglobin variants, anaemia or advanced kidney disease, your GP may recommend a particular route.

People who may find it useful

• Adults with risk factors for type 2 diabetes who want a laboratory glucose measurement
• People who want both a current glucose value and an HbA1c result

When this test is not the right route

• Symptoms of high glucose such as marked thirst, passing urine much more often, unexplained weight loss or new tiredness: contact your GP or NHS 111 the same day.
• Diagnosed diabetes: monitoring belongs with your diabetes team; do not change treatment based on this test.
• Suspected episodes of low glucose need a sample taken during an episode, arranged by your GP.
• Pregnancy: glucose testing is arranged through antenatal care.

Established clinical use

Fasting plasma glucose is part of the WHO criteria for diagnosing diabetes, and NICE uses it alongside HbA1c to identify adults at high risk who can be offered prevention support.

Population associations

A 2010 meta-analysis of 102 prospective studies by the Emerging Risk Factors Collaboration found that, in people without diabetes, fasting glucose was only modestly and non-linearly associated with vascular disease. There was no significant association between 3.90 and 5.59 mmol/L, and modestly higher coronary risk between 5.60 and 6.99 mmol/L. Adding fasting glucose to conventional risk factors did not significantly improve prediction of vascular disease.

How to read this evidence

A fasting glucose is useful for detecting raised glucose, but on its own it is a limited predictor of future cardiovascular health and says nothing about biological age.

• Fasting: nothing to eat or drink except water for at least 8 hours. An early morning appointment makes this easiest. Avoid milky or sweet drinks, juice and chewing gum.
• Diabetes medicines: if you take insulin or tablets that can cause low glucose, ask your diabetes team before fasting. Do not stop or change any medicine on your own.
• Other medicines: keep taking them and tell us about them, particularly steroids.
• Illness and exercise: rebook if you have an infection, and avoid an intense workout on the morning of the test.

Choose how your sample is taken when you book. We confirm the collection option, any collection fee and the expected turnaround time before you book.

• Juvenology Clinic, Maidstone (82 King Street, Maidstone ME14 1BH): Available.
• Partner clinic near you, anywhere in the UK: Available on request. We confirm availability for your postcode before you book.
• Postal home finger-prick kit: Not available for this test.

Sample: Venous blood sample (plasma), in a glucose tube type confirmed by the laboratory (usually fluoride)

Collection notes: Fast for at least 8 hours beforehand (water only); an early morning appointment is easiest. Not offered as a postal finger-prick kit: glucose keeps falling in the sample until it is processed and fluoride preservative takes one to two hours to act, so postal delays could give falsely low results. Anyone taking insulin or other medicines that can cause hypoglycaemia should ask their diabetes team before fasting.

The price of this test covers laboratory analysis and the laboratory's results report. The report lists each result beside the laboratory's reference range.

Beyond the blood test: optional support, priced separately

• Personalised written interpretation report: £50
• General consultation: £50
• Results consultation (results discussion and follow-up advice): £100
• Personalised programme: from £150, after we assess whether a programme suits you

Consultations are with Marina, an NMC-registered nurse. She explains what your results show and how they relate to lifestyle. No prescription service is offered with these tests.

If a result suggests you need medical follow-up, we will recommend that you see your GP and, with your consent, can provide a letter to take with you. A blood test cannot rule out every condition. If you have symptoms, speak to your GP or NHS 111, and call 999 in an emergency.

Do I have to fast?

We recommend at least 8 hours, as recognised thresholds apply to fasting samples. If you could not fast, tell us at your appointment so the result is read correctly.

How is this different from an HbA1c test?

Glucose shows your level at the moment of sampling; HbA1c reflects an average over roughly two to three months and needs no fasting. Glucose is preferred where HbA1c is unreliable, and the two can be used together.

Why is there no home finger-prick kit for this test?

Glucose keeps falling in a sample until it is processed, and fluoride preservative takes one to two hours to act. Postal delays could give a falsely low result, so we use a venous sample that reaches the laboratory promptly.

Is this a glucose tolerance test?

No. A glucose tolerance test involves a sugary drink and timed samples over two hours, and is not offered here.

Is interpretation included in the price?

The price covers laboratory analysis and the laboratory's report. You can add an optional personalised written interpretation report (£50) or book a results discussion consultation (£100).

1. Definition and diagnosis of diabetes mellitus and intermediate hyperglycaemia: report of a WHO/IDF consultation. World Health Organization, 2006. https://www.who.int/publications/i/item/definition-and-diagnosis-of-diabetes-mellitus-and-intermediate-hyperglycaemia
2. Type 2 diabetes: prevention in people at high risk. NICE public health guideline PH38, 2012 (updated 2017). https://www.nice.org.uk/guidance/ph38
3. Diagnostic criteria for diabetes. Diabetes UK. https://www.diabetes.org.uk/About_us/What-we-say/Diagnosis-ongoing-management-monitoring/New_diagnostic_criteria_for_diabetes/
4. Glucose (plasma): test information. Black Country Pathology Services. https://www.bcpathology.org.uk/test-information/glucose-plasma/
5. Bruns DE, Knowler WC. Stabilization of glucose in blood samples: why it matters. Clinical Chemistry, 2009;55(5):850-852. https://doi.org/10.1373/clinchem.2009.126037
6. The Emerging Risk Factors Collaboration (Sarwar N, Gao P, Seshasai SR, et al.). Diabetes mellitus, fasting blood glucose concentration, and risk of vascular disease: a collaborative meta-analysis of 102 prospective studies. The Lancet, 2010;375(9733):2215-2222. https://doi.org/10.1016/S0140-6736(10)60484-9