Blood tests for diabetes — including HbA1c and full blood count (FBC) — are central to diagnosing, monitoring, and managing diabetes in the UK. HbA1c reflects average blood glucose levels over two to three months and is the preferred diagnostic tool for type 2 diabetes under NICE guidance, while an FBC provides important supporting information about overall health, anaemia, and nutritional status. Understanding what these tests measure, how to interpret your results, and when to seek further advice empowers you to take an active role in your diabetes care.

Which Blood Tests Are Used to Diagnose and Monitor Diabetes

HbA1c is the primary diagnostic test for type 2 diabetes in the UK, with fasting plasma glucose, random plasma glucose, and OGTT used when HbA1c is unreliable or inconclusive. A full blood count is routinely requested alongside metabolic investigations to assess overall health.

Diabetes mellitus is diagnosed and monitored through a combination of blood tests that assess how the body regulates glucose. In the UK, the National Institute for Health and Care Excellence (NICE) recommends several key investigations, each serving a distinct clinical purpose. The most commonly used tests include:

• HbA1c (glycated haemoglobin) – the primary test for diagnosing type 2 diabetes and monitoring long-term glucose control

• Fasting plasma glucose – used when HbA1c results are inconclusive or in specific clinical circumstances; a result of 7.0 mmol/L or above is diagnostic of diabetes

• Random plasma glucose – typically used when symptoms of hyperglycaemia are present; a result of 11.1 mmol/L or above alongside classic symptoms is diagnostic

• Oral glucose tolerance test (OGTT) – used when other results are borderline, when HbA1c is unreliable, or as the standard diagnostic test for gestational diabetes

• Full blood count (FBC) – not a diabetes-specific test, but routinely requested alongside metabolic investigations to assess overall health

For most adults, HbA1c is the preferred diagnostic tool because it reflects average blood glucose levels over the preceding two to three months, removing the variability associated with a single glucose reading. It does not require fasting, making it more convenient for patients.

Important: HbA1c should not be used for diagnosis in the following situations: pregnancy (including suspected gestational diabetes), children and young people, suspected type 1 diabetes, acute illness, recent steroid therapy, advanced chronic kidney disease (CKD), haemoglobinopathies or anaemias (including iron deficiency, haemolytic anaemia, sickle cell disease, and thalassaemia), or following a recent blood transfusion. In these circumstances, fasting or random plasma glucose, or an OGTT, should be used instead. Your GP or diabetes team will advise on the most appropriate test for your situation.

Confirming a diagnosis: Unless classic symptoms of diabetes are present, a diagnosis should be confirmed by repeating the abnormal test on a separate day before treatment is initiated.

In people already living with diabetes, regular blood testing forms the backbone of ongoing management. NICE guidelines recommend HbA1c monitoring every three to six months when treatment is being adjusted, and at least annually once a person is stable. These tests help clinicians identify whether blood glucose targets are being met and whether medication changes are needed. Understanding which tests are being requested — and why — helps patients engage more meaningfully with their care.

What the HbA1c Test Measures and What Your Result Means

HbA1c measures the proportion of haemoglobin that has become glycated; a result of 48 mmol/mol or above is diagnostic of type 2 diabetes, while 42–47 mmol/mol indicates non-diabetic hyperglycaemia (prediabetes). Treatment targets are individualised based on medication, age, and hypoglycaemia risk.

HbA1c measures the proportion of haemoglobin — the oxygen-carrying protein in red blood cells — that has become glycated, meaning glucose has attached to it. Because red blood cells have a lifespan of approximately 120 days, HbA1c provides a reliable picture of average blood glucose control over the preceding two to three months. It is expressed in millimoles per mole (mmol/mol) in the UK, following the IFCC (International Federation of Clinical Chemistry) standardisation.

Interpreting your HbA1c result:

• Below 42 mmol/mol – Normal range; diabetes is unlikely

• 42–47 mmol/mol – Non-diabetic hyperglycaemia (sometimes called prediabetes); indicates increased risk of developing type 2 diabetes

• 48 mmol/mol or above – Diagnostic of type 2 diabetes (when confirmed on a second test on a separate day, unless classic symptoms are present)

Treatment targets for people already diagnosed with diabetes:

NICE recommends individualised HbA1c targets based on treatment, age, and risk of hypoglycaemia:

• 48 mmol/mol – Recommended target for most people with type 2 diabetes managed with lifestyle changes or metformin alone (low hypoglycaemia risk)

• 53 mmol/mol – Recommended target if you are taking medications that carry a risk of hypoglycaemia, such as insulin or a sulfonylurea

• 58 mmol/mol or above – NICE recommends considering treatment review or intensification when HbA1c reaches or exceeds this level, though targets should always be agreed individually with your care team

Targets may be further adjusted based on age, frailty, comorbidities, and personal preferences.

When HbA1c may be unreliable:

HbA1c can give misleading results in conditions that affect red blood cell turnover or haemoglobin structure, including:

• Haemolytic anaemia, iron deficiency anaemia, or B12/folate deficiency anaemia

• Haemoglobinopathies (including sickle cell disease and thalassaemia)

• Advanced chronic kidney disease

• Pregnancy

• Recent blood transfusion

• Acute illness or recent high-dose steroid therapy

In these situations, alternative tests such as fasting or random plasma glucose, or an OGTT, may be used. HbA1c should also not be used to diagnose diabetes in children and young people, or in those with suspected type 1 diabetes. If any of these circumstances apply to you, your GP or diabetes team will advise on the most appropriate monitoring approach.

How a Full Blood Count Relates to Diabetes Care

A full blood count supports diabetes care by detecting anaemia — which can affect HbA1c reliability — and identifying nutritional deficiencies such as vitamin B12 deficiency linked to long-term metformin use. It also flags infection risk and monitors platelet and white blood cell counts.

A full blood count (FBC) is a broad screening test that measures the different components of blood, including red blood cells, white blood cells, and platelets. While it is not used to diagnose or monitor diabetes directly, it plays an important supporting role in diabetes care for several reasons.

Key components of an FBC and their relevance to diabetes:

• Haemoglobin and haematocrit – Anaemia, detected through low haemoglobin levels, can affect the reliability of HbA1c results (both iron deficiency and B12 or folate deficiency anaemia can alter HbA1c readings). Anaemia may also be a complication of chronic kidney disease, which is more prevalent in people with diabetes. Interpreting HbA1c alongside FBC and B12 results can therefore be helpful.

• White blood cell count – Elevated levels may indicate infection or inflammation; people with diabetes are at higher risk of infections and may have impaired immune responses

• Platelet count – Provides information about blood clotting function; an abnormal platelet count may prompt further investigation but does not in itself determine cardiovascular risk

• Mean corpuscular volume (MCV) – Can indicate nutritional deficiencies such as vitamin B12 or folate deficiency, which are associated with long-term metformin use

Metformin and vitamin B12:

Metformin, the most commonly prescribed first-line medication for type 2 diabetes in the UK, can reduce the absorption of vitamin B12 over time. The Medicines and Healthcare products Regulatory Agency (MHRA) advises that vitamin B12 levels should be checked if you develop symptoms or signs of deficiency, or if you have risk factors (such as a poor diet, older age, or a long duration of metformin use). Periodic monitoring of B12 may be considered in people at higher risk or on long-term metformin therapy. An FBC — alongside a serum B12 measurement — may therefore be checked periodically in these circumstances.

If you are taking metformin and experience symptoms such as fatigue, tingling or numbness in the hands or feet, a sore tongue, or memory difficulties, discuss B12 monitoring with your GP. Do not stop taking metformin unless advised to do so by your healthcare team.

How These Tests Are Carried Out on the NHS

HbA1c and FBC are simple blood tests requiring no fasting, performed by a phlebotomist or practice nurse, usually as part of an annual diabetes review. Results are typically available within a few days via your GP surgery, NHS app, or patient portal.

Both HbA1c and full blood count tests are straightforward, minimally invasive procedures carried out by a trained healthcare professional, typically a phlebotomist, practice nurse, or GP. In most cases, a small sample of blood is drawn from a vein in the arm — usually at the antecubital fossa (the inner elbow) — using a fine needle and a collection tube. The procedure takes only a few minutes and is generally well tolerated.

Practical points for patients:

• HbA1c does not require fasting, so you can eat and drink normally beforehand

• FBC also does not routinely require fasting, unless it is being taken alongside other tests such as a fasting glucose or lipid profile

• Results are typically available within a few days and will be communicated via your GP surgery, NHS app, or patient portal

• Some GP surgeries and NHS community clinics offer walk-in phlebotomy services; others require a booked appointment

For people with diabetes, these blood tests are usually arranged as part of an annual diabetes review — a structured appointment offered by GP surgeries and diabetes clinics across England, Scotland, Wales, and Northern Ireland. This review typically includes up to nine care processes: HbA1c measurement, blood pressure, kidney function (eGFR and urine albumin-to-creatinine ratio), cholesterol, body mass index (BMI), foot examination, smoking status, and a review of any diabetes-related complications. Eye screening is also a routine part of diabetes care in the UK, delivered separately through the NHS Diabetic Eye Screening Programme, which invites eligible people for an annual retinal photograph. Attending your annual review and eye screening appointment are among the most effective ways to reduce the risk of long-term diabetes complications, including cardiovascular disease, kidney disease, neuropathy, and sight loss.

Understanding Your Results and Next Steps

An HbA1c of 48 mmol/mol or above warrants GP review; a result of 58 mmol/mol or above usually prompts treatment adjustment. Symptoms of DKA or HHS require immediate emergency care — call 999 or go to A&E.

Receiving blood test results can feel daunting, particularly if the numbers are outside the normal range. It is important to interpret results in the context of your overall health, medical history, and any symptoms you may be experiencing. A single abnormal result does not always indicate a serious problem, and your GP or diabetes care team is best placed to explain what your results mean for you individually.

When to contact your GP or diabetes team:

• Your HbA1c has risen above your agreed target, or is 48 mmol/mol or above for the first time

• Your HbA1c is 58 mmol/mol or above — your care team will usually consider reviewing or adjusting your treatment at this level

• You are experiencing symptoms of high blood glucose, such as increased thirst, frequent urination, fatigue, or blurred vision

• Your FBC shows anaemia or other abnormalities that have not previously been identified

• You are taking metformin and have symptoms that may suggest vitamin B12 deficiency (fatigue, tingling, numbness, or memory difficulties) and have not had a recent B12 check

• You feel unwell and are unsure whether your symptoms are related to your diabetes or medication

Urgent and emergency situations — seek help immediately:

If you or someone you know experiences symptoms that may suggest diabetic ketoacidosis (DKA) or hyperosmolar hyperglycaemic state (HHS) — including abdominal pain, vomiting, drowsiness or confusion, rapid or deep breathing, or signs of severe dehydration — call 999 or go to your nearest A&E immediately. These are medical emergencies.

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If you have unexplained weight loss, excessive thirst, or frequent urination and have not yet been diagnosed with diabetes, or if you are producing ketones, contact your GP the same day or call NHS 111 for urgent advice. These symptoms may indicate type 1 diabetes, which requires prompt assessment and specialist referral.

Prediabetes and prevention:

If your HbA1c falls in the non-diabetic hyperglycaemia range (42–47 mmol/mol), or your fasting plasma glucose is between 5.5 and 6.9 mmol/L, this is an important opportunity to make lifestyle changes that can delay or prevent the onset of type 2 diabetes. In England, the NHS Diabetes Prevention Programme (Healthier You) offers a free, evidence-based behavioural intervention for eligible adults, focusing on dietary changes, physical activity, and weight management. Equivalent programmes are available in Scotland, Wales, and Northern Ireland — your GP can advise on local referral pathways.

For those already managing diabetes, understanding your HbA1c trend over time — rather than focusing on a single result — is more clinically meaningful. A gradual rise may prompt a review of medication, diet, or lifestyle factors, while a stable or improving result is a positive indicator of effective self-management. Always keep a record of your results and bring them to appointments so that your care team can support you effectively.

Reporting side effects:

If you suspect that a medication is causing side effects — including symptoms that may be related to metformin or any other diabetes medicine — you can report this directly to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk or through the Yellow Card app. Your report helps improve the safety of medicines for everyone.

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