What Is Glucose in a Blood Test? UK Guide

A blood glucose test measures the concentration of glucose circulating in your bloodstream, expressed in millimoles per litre (mmol/L). Understanding what glucose in a blood test means is important for identifying diabetes, prediabetes and other conditions affecting how your body regulates sugar. Whether you have received a fasting plasma glucose result, an HbA1c reading or a random glucose measurement, knowing how to interpret the figures and when to act on them can help you take the right steps for your health.

What does glucose in a blood test actually measure?

A blood glucose test measures the concentration of glucose in your bloodstream in mmol/L, using fasting, random or oral glucose tolerance tests to assess how well your body processes sugar.

A blood glucose test measures the concentration of glucose (a simple sugar) circulating in your bloodstream at the time the sample is taken, expressed in millimoles per litre (mmol/L) in the UK.^[2][11] Glucose is the body's primary energy source, derived mainly from carbohydrates in food and drink. After a meal, glucose is absorbed from the gut into the blood, prompting the pancreas to release insulin, a hormone that helps cells take up glucose for energy or store it as glycogen in the liver and muscles.

Blood glucose tests are used in several clinical contexts. A fasting plasma glucose test requires you to avoid food and drink (other than water) for at least eight hours beforehand, giving a baseline reading unaffected by recent meals.^[5][8] A random plasma glucose test can be taken at any time of day, regardless of when you last ate; a result of 11.1 mmol/L or above alongside classic symptoms of hyperglycaemia (such as increased thirst, frequent urination and unexplained fatigue) is consistent with a diagnosis of diabetes. A two-hour oral glucose tolerance test (OGTT) involves drinking a standardised solution containing 75 g of anhydrous glucose dissolved in water, then measuring blood glucose before and two hours after, using venous plasma samples, to assess how efficiently your body processes sugar.^[8][2]

A separate but related test, glycated haemoglobin (HbA1c), reflects average blood glucose over the preceding two to three months by measuring how much glucose has attached to haemoglobin in red blood cells.^[2][11] Unlike a single glucose reading, HbA1c is not affected by short-term dietary changes, making it particularly useful for diagnosing and monitoring type 2 diabetes. However, HbA1c is unreliable or not recommended in certain situations, including pregnancy, haemoglobin variants (such as sickle cell trait or thalassaemia), anaemia, recent blood transfusion, chronic kidney disease and other conditions that alter red blood cell turnover. In these circumstances, venous plasma glucose-based tests are preferred. Your GP or practice nurse will advise which test or combination of tests is most appropriate for your circumstances.

What do your blood glucose results mean?

Results are interpreted across three categories: normal, impaired (prediabetes) and diabetic range. NICE (NG28) recommends a repeat test on a separate day to confirm type 2 diabetes in people without symptoms.

Your blood glucose result tells your clinician whether your body is regulating sugar effectively, and whether further investigation or action is needed. A single result is rarely interpreted in isolation; your GP will consider it alongside your symptoms, medical history, weight, family history and other test findings before drawing any conclusions.

Results are generally interpreted against three clinical categories. A normal result suggests your body is managing glucose well. An impaired result, sometimes called prediabetes or non-diabetic hyperglycaemia, indicates that blood glucose is higher than ideal but not yet at the threshold for a diabetes diagnosis. This is an important window for lifestyle intervention. A result in the diabetic range will prompt further assessment.

Where symptoms of hyperglycaemia are clearly present, a single abnormal venous plasma glucose result may be sufficient to confirm a diagnosis. In people without symptoms, NICE guidance (NG28) recommends that a diagnosis of type 2 diabetes should be confirmed by a repeat test on a separate day, using either fasting plasma glucose or HbA1c, before a diagnosis is made.^[3][4]

Low blood glucose (hypoglycaemia, generally below 4.0 mmol/L) is less commonly identified on routine testing but can occur in people taking certain diabetes medications, those with specific hormonal conditions, or occasionally after prolonged fasting. Symptoms such as shakiness, sweating, confusion or palpitations alongside a low reading warrant prompt clinical review.

What blood glucose levels are considered normal in the UK?

Normal fasting glucose is below 6.1 mmol/L; 7.0 mmol/L or above indicates diabetes. An HbA1c of 48 mmol/mol or above on two occasions is diagnostic of type 2 diabetes.

In the UK, normal fasting blood glucose is generally considered to be below 6.1 mmol/L, in line with WHO and UK convention. A fasting result of 6.1 to 6.9 mmol/L indicates impaired fasting glucose (a form of prediabetes), and a fasting result of 7.0 mmol/L or above on two separate occasions (or once with symptoms) is consistent with a diagnosis of diabetes. For a two-hour OGTT, a result below 7.8 mmol/L is considered normal, 7.8 to 11.0 mmol/L indicates impaired glucose tolerance, and 11.1 mmol/L or above suggests diabetes. A random plasma glucose of 11.1 mmol/L or above in the presence of classic hyperglycaemic symptoms is also consistent with a diabetes diagnosis.

For HbA1c, NICE defines the following thresholds: below 42 mmol/mol is considered normal, 42 to 47 mmol/mol indicates non-diabetic hyperglycaemia (prediabetes), and 48 mmol/mol or above on two occasions is diagnostic of type 2 diabetes in people without symptoms.^[3][4]

For people already diagnosed with type 2 diabetes, NICE (NG28) recommends an individualised HbA1c target, commonly 48 mmol/mol (6.5%) for those managed by lifestyle alone or with metformin. Where treatment includes medicines that carry a higher risk of hypoglycaemia, such as sulphonylureas or insulin, a target of 53 mmol/mol (7.0%) may be more appropriate. Targets are adjusted based on treatment, age, risk of hypoglycaemia and other individual factors. People with type 1 diabetes are typically advised to aim for an HbA1c below 48 mmol/mol where this can be achieved safely, in line with NICE guidance (NG17).^[9][10]

Pregnancy uses different diagnostic thresholds, and HbA1c is not used to diagnose gestational diabetes. If you are pregnant and concerned about blood glucose, your midwife or maternity team will advise on the appropriate screening pathway.

These thresholds apply to venous plasma samples analysed in an accredited laboratory. Home finger-prick glucose monitors use capillary blood and may give slightly different readings; they are useful for self-monitoring but are not used to make a formal diagnosis.

What can cause abnormal glucose readings?

The most common cause of persistently raised blood glucose is type 2 diabetes, but corticosteroids, antipsychotics, hormonal conditions such as PCOS, acute illness and excess body weight can also cause abnormal readings.

Abnormal blood glucose readings can result from a wide range of medical, lifestyle and medication-related factors, and identifying the underlying cause is essential before any management plan is put in place. The most common cause of persistently elevated blood glucose is type 2 diabetes, a condition in which the body either does not produce enough insulin or does not respond to it effectively. Type 1 diabetes, an autoimmune condition in which the pancreas produces little or no insulin, is another important cause, as are rarer forms such as maturity-onset diabetes of the young (MODY) and secondary diabetes arising from conditions affecting the pancreas, including chronic pancreatitis, pancreatic surgery, cystic fibrosis and haemochromatosis.

Several other factors can temporarily or persistently raise blood glucose:

• Diet: a high intake of refined carbohydrates and sugary drinks raises postprandial (after-meal) glucose.

• Physical inactivity: regular exercise improves insulin sensitivity; a sedentary lifestyle can impair it.

• Excess body weight: particularly central (abdominal) adiposity is strongly associated with insulin resistance.

• Acute illness or surgery: physiological stress raises counter-regulatory hormones such as cortisol and adrenaline, which increase blood glucose.

• Medications: corticosteroids, some antipsychotics, thiazide diuretics and certain immunosuppressants can raise blood glucose, sometimes substantially.^[16]

• Hormonal conditions: polycystic ovary syndrome (PCOS), Cushing's syndrome and acromegaly are among the conditions associated with impaired glucose regulation.

• Gestational diabetes: pregnancy can unmask impaired glucose tolerance. Screening and diagnosis use pregnancy-specific criteria via maternity services; HbA1c is not used to diagnose gestational diabetes. If you are pregnant and have concerns, speak to your midwife or maternity team.

Low blood glucose is most commonly seen in people with diabetes who are taking insulin or sulphonylurea medications, but can also occur with adrenal insufficiency, liver disease or insulinoma (a rare insulin-secreting tumour).

When should you speak to your GP about your glucose levels?

Speak to your GP if any result falls in the impaired or diabetic range, or if you develop symptoms of hyperglycaemia. Call 999 immediately if someone shows signs of diabetic ketoacidosis or loses consciousness.

You should speak to your GP if you receive a blood glucose or HbA1c result that falls in the impaired or diabetic range, even if you feel well, as early intervention can significantly reduce the risk of long-term complications. You should also contact your GP if you experience symptoms that may suggest high blood glucose (hyperglycaemia), including increased thirst, frequent urination, unexplained fatigue, blurred vision or slow wound healing. These symptoms, particularly if they develop quickly or are severe, warrant prompt assessment.

If you are already diagnosed with diabetes and notice that your home glucose readings are consistently outside your agreed target range, or if you experience frequent episodes of hypoglycaemia (blood glucose below 4.0 mmol/L), contact your GP or diabetes care team for a medication or management review. Do not adjust insulin doses or stop diabetes medication without clinical guidance.

Seek urgent help via NHS 111 if you feel very unwell alongside high glucose readings, or if you are unable to keep fluids down. Call 999 or go to A&E immediately if someone with diabetes loses consciousness, has a seizure, or shows signs of diabetic ketoacidosis (DKA), which may include rapid breathing, vomiting, abdominal pain and a fruity smell on the breath.^[15] If you are pregnant and experience symptoms that may suggest hyperglycaemia or DKA, seek urgent same-day assessment via your maternity unit or A&E without delay.

If you are concerned about your risk of diabetes and would like a structured assessment, a range of services are available. Your GP can refer you to the NHS Diabetes Prevention Programme if you are identified as being at high risk of developing type 2 diabetes. 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. Any suspected side effects from medicines affecting glucose should be reported via the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk.

Scientific References

1. High blood sugar (hyperglycaemia).
2. Blood and plasma glucose - Diabetes (NG18).
3. Type 2 diabetes in adults: management (NG28).
4. Blood glucose management | Type 2 diabetes in adults (NG28).
5. Type 2 diabetes: prevention in people at high risk (PH38).
6. Hypoglycaemia | Treatment summaries - BNF.
7. Low blood sugar (hypoglycaemia).
8. Gestational diabetes.
9. Type 1 diabetes in adults: diagnosis and management (NG17).
10. Type 1 diabetes | Treatment summaries - BNF.
11. Diabetes mellitus, diagnostic and monitoring devices - BNF.
12. The threshold for diagnosing impaired fasting glucose.
13. Understanding haemoglobinopathies - Sickle cell and thalassaemia screening handbook.
14. Glycated albumin beyond HbA1c: an equity-ready laboratory pathway for emergency diabetes and cardiorenal risk screening. Clinica Chimica Acta.
15. Diabetic ketoacidosis.
16. Drug-induced abnormal glucose tolerance. Nihon Rinsho.

Frequently Asked Questions