HbA1c — what is it, and why does it matter? HbA1c, or glycated haemoglobin, is one of the most important blood tests used in UK clinical practice to assess average blood glucose levels over the preceding two to three months. Unlike a standard finger-prick glucose reading, it provides a broader picture of how well blood sugar has been controlled over time. Used by GPs and diabetes specialists across the NHS, HbA1c plays a central role in diagnosing prediabetes and type 2 diabetes, setting treatment targets, and monitoring long-term glycaemic control. This article explains what the test measures, how to interpret your results, and what to do if your reading is abnormal.

What Is HbA1c and What Does It Measure?

HbA1c measures the proportion of haemoglobin that has become glycated by circulating glucose, providing a weighted average of blood glucose control over the preceding 8–12 weeks. It is reported in mmol/mol in the UK and does not require fasting.

HbA1c — formally known as glycated haemoglobin — is a blood marker that reflects your average blood glucose (sugar) levels over the preceding two to three months. Understanding what it measures requires a brief look at the underlying biology. Haemoglobin is the protein found inside red blood cells that carries oxygen around the body. When glucose circulates in the bloodstream, it naturally attaches to haemoglobin in a process called glycation. The more glucose present over time, the greater the proportion of haemoglobin that becomes glycated, producing what is measured as HbA1c.

Red blood cells have a lifespan of approximately 120 days (around four months). Because newer red cells contribute more to the HbA1c reading than older ones, the test effectively provides a weighted average of blood glucose control over roughly the preceding 8–12 weeks. This makes it fundamentally different from a standard finger-prick glucose test, which only captures a single moment in time and can fluctuate significantly depending on recent meals, stress, or physical activity.

HbA1c is expressed as a percentage or, more commonly in the UK since 2011, in millimoles per mole (mmol/mol) — a standardised unit adopted in line with the International Federation of Clinical Chemistry (IFCC) reporting system. For example, an HbA1c of 48 mmol/mol corresponds to approximately 6.5% in the older NGSP percentage scale. Both units may appear on laboratory reports, so it is worth being familiar with each. The test is performed on a simple venous blood sample and does not require fasting beforehand, which makes it a practical and convenient tool in routine clinical practice. Where possible, diagnosis should be confirmed using a laboratory-standardised (IFCC-aligned) method; point-of-care results may require laboratory confirmation.

Important limitations: HbA1c is not appropriate for diagnosing or monitoring diabetes in certain groups, including people with suspected type 1 diabetes, children and young people, women who are pregnant, people with acute illness, or those with conditions affecting red blood cell lifespan or haemoglobin structure (such as haemoglobinopathies). In these situations, alternative tests — such as fasting plasma glucose or an oral glucose tolerance test (OGTT) — are used instead.

Understanding Your HbA1c Results and NHS Target Ranges

An HbA1c of 48 mmol/mol (6.5%) or above confirms type 2 diabetes on a repeat test; 42–47 mmol/mol indicates prediabetes, and below 42 mmol/mol is generally considered normal. NICE recommends individualised targets for people already diagnosed with diabetes.

Interpreting an HbA1c result depends on the clinical context in which it has been requested. The NHS and NICE provide clear reference ranges to guide both diagnosis and ongoing management:

• Below 42 mmol/mol (6.0%): Generally considered normal. However, a result below this threshold does not exclude diabetes in people who have symptoms or conditions that can lower HbA1c artificially (see the section on factors affecting accuracy). If diabetes is clinically suspected, a fasting plasma glucose or OGTT may still be appropriate.

• 42–47 mmol/mol (6.0–6.4%): Classified as prediabetes (also called non-diabetic hyperglycaemia). Blood glucose is higher than normal but not yet in the diabetic range. This is an important window for lifestyle intervention.

• 48 mmol/mol (6.5%) or above: Diagnostic of type 2 diabetes when confirmed on a repeat test in the absence of symptoms, or on a single test if symptoms of diabetes are present.

HbA1c should not be used to diagnose type 1 diabetes, diabetes in children, gestational diabetes, or diabetes in the context of acute illness or rapidly developing symptoms. In these situations, plasma glucose measurement is required.

For people already diagnosed with type 2 diabetes, NICE guidelines (NG28) recommend an individualised HbA1c target, typically 48 mmol/mol (6.5%) for those managed by lifestyle or a single non-hypoglycaemic drug, or 53 mmol/mol (7.0%) for those on medications that carry a risk of hypoglycaemia, such as sulfonylureas or insulin.

For people with type 1 diabetes, NICE (NG17) recommends aiming for an HbA1c of 48 mmol/mol (6.5%) or below if this can be achieved safely without problematic hypoglycaemia. Targets are always balanced against individual circumstances, including age, comorbidities, and quality of life. It is important to remember that a lower HbA1c is not always better — excessively tight control can increase the risk of hypoglycaemia, particularly in older adults or those on insulin therapy.

When and Why an HbA1c Test Is Requested

HbA1c is requested for screening and diagnosing type 2 diabetes and prediabetes, and for monitoring glycaemic control in people with established diabetes every three to twelve months. It is not used to diagnose gestational diabetes, for which an OGTT is required.

An HbA1c test may be requested in a variety of clinical situations, and understanding the reason behind the test can help patients engage more meaningfully with their results.

Screening and diagnosis is one of the most common reasons. The NHS Health Check programme, offered to adults aged 40–74 in England, assesses overall cardiovascular and metabolic risk. For those identified as being at increased risk of type 2 diabetes — based on a prior risk assessment — HbA1c or blood glucose testing is offered to check for prediabetes or undiagnosed diabetes. GPs may also request the test if a patient presents with symptoms suggestive of diabetes — such as increased thirst, frequent urination, unexplained weight loss, or persistent fatigue — or if routine blood tests reveal elevated fasting glucose. HbA1c should not be used for diagnosis during acute illness, as results may be unreliable; fasting plasma glucose or OGTT is preferred in this setting.

Monitoring established diabetes is the other primary use. For people already diagnosed with type 1 or type 2 diabetes, HbA1c is typically measured every three to six months until blood glucose is stable and well-controlled, after which it may be checked every six to twelve months. The result helps clinicians and patients assess whether current treatment — whether dietary, pharmacological, or both — is achieving adequate glucose control and reducing the long-term risk of complications such as retinopathy, nephropathy, neuropathy, and cardiovascular disease.

HbA1c may also be requested when initiating or adjusting medications known to affect blood glucose, such as corticosteroids or antipsychotics. In women with pre-existing diabetes, HbA1c is used during preconception planning and in early pregnancy to assess glycaemic control and guide management; however, it is not recommended for diagnosing gestational diabetes, for which an oral glucose tolerance test (OGTT) is used instead, as per NICE guideline NG3. HbA1c is also less reliable in the immediate postpartum period and should be interpreted with caution.

Factors That Can Affect HbA1c Accuracy

Conditions affecting red blood cell lifespan — such as haemolytic anaemia, haemoglobinopathies, iron deficiency, and advanced CKD — can cause falsely low or falsely high HbA1c results. Alternative tests such as fructosamine or continuous glucose monitoring may be used when accuracy is in doubt.

Whilst HbA1c is a robust and widely validated test, several physiological and medical factors can affect its accuracy, potentially leading to falsely high or falsely low results. Clinicians are trained to interpret results in context, but patients should be aware of these variables.

Conditions that can cause falsely low HbA1c:

• Haemolytic anaemia — increased red blood cell turnover means cells are replaced more quickly, reducing the time available for glycation.

• Iron deficiency anaemia treated with iron supplementation — can transiently lower HbA1c as new red cells are produced.

• Haemoglobin variants (e.g., sickle cell trait, haemoglobin C or E) — certain variants interfere with the laboratory assay used to measure HbA1c, though modern IFCC-aligned methods reduce some of these interferences; laboratories will often flag known assay limitations.

• Pregnancy — haemodilution and increased red cell turnover can lower HbA1c values.

• Recent blood transfusion or significant blood loss — introduces donor red cells or stimulates new cell production, altering the result.

• Erythropoietin (EPO) therapy — used in anaemia management, increases red cell turnover and can lower HbA1c.

• Advanced chronic kidney disease (CKD) — reduced red cell survival in advanced CKD commonly lowers HbA1c, meaning results may underestimate true glycaemic control. Assay interference can also vary depending on the laboratory method used, making HbA1c unreliable in this group.

Conditions that can cause falsely high HbA1c:

• Iron deficiency anaemia (untreated) — reduced red cell production prolongs the lifespan of existing cells, allowing more glycation to occur.

• Vitamin B12 or folate deficiency — similarly reduces red cell turnover.

• Splenectomy — removal of the spleen prolongs red cell survival.

• Severe liver disease, hyperbilirubinaemia, or markedly elevated triglycerides — can interfere with certain assay methods.

• High-dose vitamin C or vitamin E supplementation, chronic heavy alcohol use, and certain opioid medicines — may affect results in some assay systems.

In situations where HbA1c reliability is in doubt — including advanced CKD, haemoglobinopathies, or other conditions affecting red cell lifespan — alternative markers such as fructosamine or continuous glucose monitoring (CGM) may be used to assess glycaemic control. If you have any of the above conditions, your GP or diabetes team will take this into account when interpreting your results and may use additional tests to build a fuller picture.

Next Steps If Your HbA1c Result Is Abnormal

A prediabetic result (42–47 mmol/mol) warrants referral to the NHS Diabetes Prevention Programme and lifestyle intervention; a result of 48 mmol/mol or above requires confirmatory testing and a structured care plan including lifestyle advice, medication, and regular monitoring.

Receiving an abnormal HbA1c result can feel unsettling, but it is important to understand that it is a starting point for action rather than a definitive verdict. The appropriate next steps depend on where your result falls and your individual clinical circumstances.

If your result indicates prediabetes (42–47 mmol/mol): This is a significant but potentially reversible stage. NICE recommends referral to the NHS Diabetes Prevention Programme (NHS DPP), a free, evidence-based behavioural intervention available across England that supports people in making sustainable lifestyle changes. Clinical trials of intensive lifestyle programmes have shown risk reductions of around 30–60% in progression to type 2 diabetes; real-world evaluations of the NHS DPP demonstrate meaningful reductions in practice. Modest weight loss (5–10% of body weight), increased physical activity, and dietary improvements are the key components. Your GP may also arrange a repeat HbA1c in 6–12 months to monitor trends.

If your result is diagnostic of type 2 diabetes (48 mmol/mol or above): A confirmatory repeat test is usually required unless you have clear symptoms of diabetes. Once confirmed, your GP will initiate a structured care plan, which typically includes:

• Lifestyle advice — dietary changes, physical activity, and weight management.

• Medication — metformin is usually the first-line pharmacological treatment recommended by NICE (NG28), unless contraindicated.

• Referral — to a diabetes structured education programme such as DESMOND, and to relevant specialists if complications are suspected.

• Monitoring — regular HbA1c checks, blood pressure, kidney function, cholesterol, and annual eye and foot examinations.

Urgent red flags — seek immediate medical attention:

• If you develop symptoms suggesting type 1 diabetes — such as rapid and significant weight loss, excessive thirst, frequent urination, and ketones in your urine — contact your GP or NHS 111 the same day, as this requires urgent assessment.

• If you or someone you know develops symptoms of diabetic ketoacidosis (DKA) or hyperosmolar hyperglycaemic state (HHS) — including vomiting, severe abdominal pain, drowsiness, deep or laboured breathing, confusion, or signs of severe dehydration — call 999 immediately.

• If you are already on treatment and experience symptoms of hypoglycaemia (shakiness, sweating, confusion, or loss of consciousness), follow your agreed sick-day or hypoglycaemia management plan and seek medical advice promptly if symptoms do not resolve.

Reporting side effects: If you experience a suspected side effect from any medicine used to treat diabetes, you can report this to the MHRA via the Yellow Card Scheme at yellowcard.mhra.gov.uk or through the Yellow Card app. Early engagement with your healthcare team is the most effective way to protect your long-term health.

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