Diabetes and glucose testing are closely linked — accurate blood glucose measurement is essential for diagnosing diabetes, identifying those at risk, and guiding safe, effective management. Glucose tests measure the concentration of sugar in the blood, reflecting how well the body's insulin system is functioning. In the UK, the NHS and NICE recommend specific tests depending on the clinical situation, including HbA1c, fasting plasma glucose, and the oral glucose tolerance test. Whether you have recently been referred for testing, are newly diagnosed, or are managing an existing condition, understanding how these tests work and what your results mean is a vital part of your care.

What Is a Glucose Test and Why It Matters in Diabetes

A glucose test measures blood sugar concentration and is central to diagnosing diabetes and preventing complications from persistently high or low glucose levels.

A glucose test measures the concentration of glucose (sugar) in the blood. Glucose is the body's primary source of energy, derived from the carbohydrates we eat and regulated by the hormone insulin, which is produced by the pancreas. In people with diabetes, this regulatory system is impaired — either because the pancreas produces insufficient insulin (type 1 diabetes), because the body's cells become resistant to insulin's effects (type 2 diabetes), or a combination of both. Gestational diabetes — a form of diabetes that develops during pregnancy — is a further important category discussed later in this article.

When blood glucose levels remain persistently elevated — a condition known as hyperglycaemia — it can cause damage to blood vessels, nerves, kidneys, and the eyes over time. Conversely, blood glucose that drops too low (hypoglycaemia) can cause immediate symptoms such as dizziness, confusion, and, in severe cases, loss of consciousness. This is why accurate glucose testing is central to both diagnosing diabetes and managing it safely on an ongoing basis.

In the UK, glucose testing plays a critical role at several stages: identifying individuals at risk of diabetes, confirming a diagnosis, and guiding treatment decisions. The NHS and NICE (National Institute for Health and Care Excellence) both recommend specific glucose tests depending on the clinical context. It is important to note that HbA1c — one of the main diagnostic tests — is not appropriate in all situations. It should not be used to diagnose diabetes in pregnancy, in people with suspected type 1 diabetes, in children and young people, during acute illness, or in people with conditions that affect red blood cell turnover (such as haemolytic anaemia, haemoglobinopathies, or following a recent blood transfusion). In these circumstances, plasma glucose-based tests are used instead. Understanding what these tests involve — and what the results mean — helps patients engage more confidently with their care.

Types of Blood Glucose Tests Used in the UK

UK clinical practice uses HbA1c, fasting plasma glucose, OGTT, and random plasma glucose, each for distinct diagnostic purposes; all diagnostic tests require laboratory venous plasma samples.

Several different glucose tests are used in UK clinical practice, each serving a distinct diagnostic or monitoring purpose. It is important to note that for diagnostic purposes, all blood glucose measurements should be performed on venous plasma samples analysed in an accredited laboratory. Finger-prick capillary glucometers are not suitable for confirming a diagnosis of diabetes.

HbA1c (glycated haemoglobin): The HbA1c test does not require fasting and reflects average blood glucose levels over the preceding two to three months. It works by measuring the proportion of haemoglobin — the oxygen-carrying protein in red blood cells — that has glucose attached to it. NICE recommends HbA1c as the preferred initial diagnostic test for type 2 diabetes in most adults, as it provides a broader picture of glucose control than a single fasting measurement. However, HbA1c is not suitable for diagnosis in pregnancy, in suspected type 1 diabetes, in children and young people, during acute illness, or where red blood cell turnover is altered (for example, in haemolytic anaemia, haemoglobinopathies, or following a recent blood transfusion or erythropoiesis-stimulating therapy). In these situations, plasma glucose-based tests should be used.

Fasting plasma glucose (FPG): This test measures blood glucose after a period of fasting — typically eight to fourteen hours, usually overnight. It is used to diagnose diabetes and prediabetes when HbA1c is not appropriate, or as a supplementary test in certain clinical situations. It is not the preferred first-line diagnostic test for most adults in UK primary care.

Oral glucose tolerance test (OGTT): The OGTT involves fasting overnight, having a fasting blood sample taken, then drinking a standardised glucose solution (75 g of glucose dissolved in water). A second blood sample is taken two hours later. This test is more sensitive than the FPG alone and is the standard test used to diagnose gestational diabetes during pregnancy. NICE (NG3) specifies 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. The OGTT is also used in non-pregnant adults when diabetes is suspected but other test results are borderline, or when HbA1c is not appropriate.

Random plasma glucose: This test can be taken at any time, regardless of when the patient last ate. It is typically used when a person presents with classic symptoms of diabetes (such as excessive thirst, frequent urination, and unexplained weight loss) and a rapid result is needed to guide urgent management.

What Your Results Mean and NHS Diagnostic Thresholds

An HbA1c of 48 mmol/mol or above, fasting plasma glucose of 7.0 mmol/L or above, or a two-hour OGTT value of 11.1 mmol/L or above is diagnostic of diabetes in non-pregnant adults.

Understanding glucose test results requires reference to established NHS and NICE diagnostic thresholds. These thresholds are based on the level of blood glucose at which the risk of diabetes-related complications — particularly diabetic retinopathy — begins to increase significantly. All diagnostic tests should be performed using laboratory venous plasma glucose or IFCC-aligned HbA1c methods.

HbA1c thresholds (non-pregnant adults; not applicable in pregnancy, suspected type 1 diabetes, children/young people, acute illness, or altered red-cell states):

• Below 42 mmol/mol: Normal range

• 42–47 mmol/mol: Prediabetes (also called non-diabetic hyperglycaemia)

• 48 mmol/mol or above: Diagnostic of type 2 diabetes (in the absence of symptoms, a second confirmatory test on a separate day is required)

Fasting plasma glucose thresholds:

• Below 6.0 mmol/L: Normal

• 6.1–6.9 mmol/L: Impaired fasting glucose (prediabetes)

• 7.0 mmol/L or above: Diagnostic of diabetes

OGTT (two-hour post-load glucose) — non-pregnant adults:

• Below 7.8 mmol/L: Normal glucose tolerance

• 7.8–11.0 mmol/L: Impaired glucose tolerance (prediabetes)

• 11.1 mmol/L or above: Diagnostic of diabetes

OGTT thresholds in pregnancy (NICE NG3 — gestational diabetes):

• Fasting plasma glucose 5.6 mmol/L or above: Diagnostic of gestational diabetes

• Two-hour plasma glucose 7.8 mmol/L or above: Diagnostic of gestational diabetes

In an asymptomatic person, a single abnormal result is not sufficient to diagnose diabetes — NICE guidance recommends a repeat test on a separate day to confirm the diagnosis. However, if a person presents with clear symptoms of hyperglycaemia (such as thirst, polyuria, or unexplained weight loss), a single diagnostic result is sufficient: this applies to a random plasma glucose of 11.1 mmol/L or above, a fasting plasma glucose of 7.0 mmol/L or above, or a two-hour OGTT value of 11.1 mmol/L or above. If you receive an abnormal result, your GP will discuss what it means for you personally and outline the next steps in your care.

How to Prepare for a Glucose Test

Preparation varies by test type: HbA1c requires no fasting, while fasting plasma glucose and OGTT both require an eight-to-fourteen-hour fast with water only.

Preparation for a glucose test depends on which type of test has been requested. Failing to follow the correct preparation can lead to inaccurate results, which may delay diagnosis or result in unnecessary further testing.

For a fasting plasma glucose test:

• Fast for eight to fourteen hours before the test, taking water only. Tea, coffee, juice, and other drinks should be avoided.

• If you take insulin or a sulphonylurea (such as gliclazide or glipizide), seek specific advice from your GP or diabetes team before fasting, as these medicines carry a risk of hypoglycaemia when food is withheld.

For an HbA1c test:

• Fasting is not required; the test can be taken at any time of day.

• Certain conditions — including haemolytic anaemia, haemoglobin variants, and recent blood transfusions — can affect HbA1c accuracy. Your GP or practice nurse will advise if an alternative test is more appropriate.

For an oral glucose tolerance test (OGTT):

• Fast for eight to fourteen hours beforehand (water only); attending in the morning is usual practice, as the test takes approximately two hours.

• Avoid strenuous exercise the day before, as this can temporarily lower blood glucose.

• Do not smoke or consume caffeine before or during the test.

• Remain seated and avoid physical activity during the two-hour interval between blood samples.

• Continue taking any prescribed medications unless specifically advised otherwise by your healthcare team.

General advice for all glucose tests:

• Inform your GP or practice nurse of any recent illness, significant changes in diet, or new or changed medications before the test, as these factors can influence results.

• Consider deferring testing if you are acutely unwell; discuss this with the clinician who requested the test, as acute illness can transiently raise blood glucose.

• Some medicines are known to raise blood glucose levels and can, in some cases, precipitate diabetes. These include systemic corticosteroids (such as prednisolone), certain atypical antipsychotics (such as olanzapine and clozapine), and some diuretics. This is recognised in UK prescribing information and MHRA guidance. Do not stop any prescribed medicine without medical advice; instead, inform your clinician so that results can be interpreted in context.

• If you suspect that a medicine is causing an unwanted effect, you can report this via the MHRA Yellow Card scheme (yellowcard.mhra.gov.uk).

• Always attend your appointment as scheduled, and contact your GP surgery if you have any concerns about preparation.

Monitoring Blood Sugar Levels If You Have Diabetes

Ongoing monitoring using self-testing, CGM, and regular HbA1c checks is essential for people with diabetes, with NICE recommending type 1 diabetes patients test at least four times daily.

For people already living with diabetes, ongoing blood glucose monitoring is an essential part of self-management. The frequency and method of monitoring will depend on the type of diabetes, the treatment regimen, and individual clinical circumstances.

Self-monitoring of blood glucose (SMBG): This involves using a finger-prick device to obtain a small drop of blood, which is then applied to a test strip read by a glucometer. SMBG is particularly important for people with type 1 diabetes and those with type 2 diabetes who are taking insulin or certain other medications (such as sulphonylureas) that carry a risk of hypoglycaemia. NICE guidance recommends that people with type 1 diabetes test at least four times daily, including before each meal and before bed.

Continuous glucose monitoring (CGM) and flash glucose monitoring: Technologies such as real-time CGM (rtCGM) and intermittently scanned CGM (isCGM, for example the FreeStyle Libre system) use a small sensor worn on the skin to measure glucose levels in the interstitial fluid continuously or on demand. NICE (NG17, updated 2022) recommends that all adults with type 1 diabetes are offered a choice of real-time CGM or intermittently scanned CGM. CGM is also available on the NHS for some adults with type 2 diabetes on insulin who meet specific criteria. These technologies can significantly reduce the burden of finger-prick testing while improving glucose awareness.

HbA1c monitoring: In addition to day-to-day self-monitoring, people with diabetes will typically have their HbA1c measured every three to six months in primary or secondary care. NICE recommends an HbA1c target of 48 mmol/mol (6.5%) or below for most people with type 2 diabetes managed with lifestyle changes or metformin alone. For people taking medicines that carry a risk of hypoglycaemia — such as sulphonylureas or insulin — a target of 53 mmol/mol (7.0%) or below is generally recommended to reduce the risk of hypoglycaemic episodes. Individual targets should always be agreed with your healthcare team.

Ketone testing: If your blood glucose is persistently elevated (above 13–15 mmol/L), you should also check for ketones using a blood or urine ketone test. Seek urgent medical advice if blood ketones are 1.5 mmol/L or above, or if you feel unwell with high glucose readings.

When to seek urgent advice:

• Contact your GP, diabetes team, or NHS 111 promptly if you experience persistent blood glucose readings above 15 mmol/L, frequent or severe hypoglycaemic episodes, or blood ketones of 1.5 mmol/L or above.

• Call 999 or go to A&E immediately if you or someone else has symptoms suggesting diabetic ketoacidosis (DKA) or hyperosmolar hyperglycaemic state (HHS), such as persistent vomiting, severe abdominal pain, deep or rapid breathing, fruity-smelling breath, confusion, severe drowsiness, or signs of significant dehydration.

Driving safety: If you experience recurrent hypoglycaemia, you have a legal obligation to inform the DVLA and your motor insurer. Your GP or diabetes team can advise you further.

Regular monitoring, combined with structured diabetes education and support from your healthcare team, is key to reducing the long-term risks associated with poorly controlled blood glucose.

Frequently Asked Questions