The HbA1c test is one of the most important blood tests used in UK diabetes care, yet many people are unsure exactly what it measures and why it matters. The HbA1c test checks the percentage of haemoglobin in your red blood cells that has bonded with glucose, providing a reliable picture of your average blood sugar levels over the preceding two to three months. Unlike a single blood glucose reading, which can fluctuate from hour to hour, HbA1c reflects longer-term glycaemic control. It is used by NHS GPs and specialists to diagnose type 2 diabetes, identify prediabetes, and monitor how well blood glucose is being managed in people already living with the condition.

What the HbA1c Test Measures and Why It Matters

The HbA1c test measures the proportion of haemoglobin chemically bonded to glucose, reflecting average blood sugar levels over the preceding two to three months rather than a single point in time.

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The HbA1c test — formally known as the glycated haemoglobin test — measures the percentage of haemoglobin in your red blood cells that has become chemically bonded to glucose. When blood glucose levels are elevated over time, glucose molecules attach to haemoglobin (the protein inside red blood cells that carries oxygen), forming glycated haemoglobin, or HbA1c. Because red blood cells have a lifespan of approximately 120 days (around three to four months), the HbA1c result reflects your average blood glucose levels over the preceding two to three months. Importantly, more recent weeks contribute more heavily to the result than earlier ones, so a recent improvement or deterioration in glucose control will begin to show in the HbA1c sooner than the full three-month window might suggest.

This makes the HbA1c test a particularly valuable clinical tool. Unlike a fasting glucose test, which can fluctuate depending on what you have eaten or your stress levels on a given day, HbA1c provides a broader, more stable picture of long-term glycaemic control. It is widely used by GPs and diabetes specialists across the NHS to both diagnose type 2 diabetes and monitor how well blood glucose is being managed in people already living with the condition.

For diagnostic purposes, HbA1c should be measured on a venous blood sample processed in a quality-assured laboratory using an IFCC-aligned method. Point-of-care or finger-prick devices may be used for monitoring in people already diagnosed with diabetes, but they are not recommended for making a new diagnosis. Results in the UK are reported in millimoles per mole (mmol/mol); some laboratory reports also include the older DCCT percentage figure alongside this. Understanding what the HbA1c test measures is the first step in appreciating why it plays such a central role in diabetes care and prevention.

How HbA1c Differs From a Standard Blood Glucose Test

Unlike a blood glucose test, which captures a single moment and is affected by recent food or stress, HbA1c provides a stable, longer-term measure of glycaemic control and does not require fasting.

A standard blood glucose test — whether fasting, random, or part of an oral glucose tolerance test (OGTT) — measures the concentration of glucose circulating in your blood at a specific point in time. Results can vary considerably depending on recent food intake, physical activity, illness, or emotional stress. This snapshot approach has its uses, particularly in diagnosing type 1 diabetes or assessing acute hypoglycaemia, but it does not capture the full picture of a person's glycaemic health.

The HbA1c test is far less susceptible to day-to-day variability than a single blood glucose measurement, though it is worth noting that more recent glycaemia does contribute more to the result. This makes it especially useful for:

• Diagnosing type 2 diabetes and prediabetes without requiring a fasting period

• Monitoring long-term glycaemic control in people with established diabetes

• Assessing treatment effectiveness following changes to medication, diet, or lifestyle

The two tests are not interchangeable. NICE guidance (NG28) confirms that HbA1c is the preferred diagnostic test for type 2 diabetes in most adults, but a blood glucose test (fasting plasma glucose or OGTT) is more appropriate — or should be used alongside HbA1c — in a number of situations, including:

• Children and young people (HbA1c is not recommended for diagnosis in this group)

• Pregnancy (gestational diabetes is diagnosed using an OGTT, not HbA1c)

• Suspected type 1 diabetes or rapid onset of symptoms (symptoms present for fewer than two months)

• Acute illness, which can transiently affect results

• Medicines that can cause hyperglycaemia, such as high-dose corticosteroids or certain antipsychotics

• Pancreatic disease or pancreatic surgery

• Haemoglobinopathies or conditions affecting red blood cell turnover (e.g., haemolytic anaemia, iron deficiency anaemia)

• HIV infection or antiretroviral therapy

Your GP will determine which test is most appropriate based on your individual clinical circumstances.

What Your HbA1c Result Means: NHS Reference Ranges

In the UK, an HbA1c below 42 mmol/mol is normal, 42–47 mmol/mol indicates prediabetes, and 48 mmol/mol or above is diagnostic of type 2 diabetes when confirmed on a second test.

In the UK, HbA1c results are expressed in mmol/mol and interpreted according to ranges established by NHS England and aligned with NICE guidance. Understanding where your result falls within these ranges is essential for making sense of what your doctor may discuss with you.

NHS HbA1c reference ranges:

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

• 42–47 mmol/mol — Prediabetes (also called non-diabetic hyperglycaemia); blood glucose is higher than normal but not yet in the diabetic range

• 48 mmol/mol or above — Indicative of type 2 diabetes (when confirmed on a second test, or on a single test if classic symptoms of hyperglycaemia are present)

For people already diagnosed with diabetes, HbA1c targets are individualised. NICE (NG28) generally recommends:

• 48 mmol/mol for people managing type 2 diabetes with lifestyle changes alone or with a single non-hypoglycaemic drug (such as metformin)

• 53 mmol/mol for those taking medicines associated with hypoglycaemia, such as insulin or a sulfonylurea

Targets should be adjusted for individual circumstances. For people who are frail, have significant comorbidities, or are at higher risk of hypoglycaemia, a less stringent target may be more appropriate. Conversely, some people may benefit from tighter control. These decisions are made in discussion with your healthcare team.

It is important to understand that in the absence of symptoms, a diagnosis of type 2 diabetes requires two separate abnormal results. If your result falls in the prediabetes range, your GP will typically offer lifestyle advice and arrange a repeat test within 12 months. Results should always be interpreted in the context of your overall health, symptoms, and medical history.

Conditions Diagnosed and Monitored Using the HbA1c Test

HbA1c is primarily used to diagnose and monitor type 2 diabetes and prediabetes, and is also used to monitor long-term control in type 1 diabetes and assess glycaemic status before elective surgery.

The HbA1c test is most closely associated with type 2 diabetes, but its clinical applications extend across several conditions and risk scenarios.

Type 2 diabetes is the primary condition for which HbA1c is used diagnostically. It is also the cornerstone of ongoing monitoring — people with well-controlled type 2 diabetes typically have their HbA1c checked every six months, while those with poorly controlled glucose or recent treatment changes may be tested every three months.

Prediabetes (non-diabetic hyperglycaemia) is identified when HbA1c falls between 42 and 47 mmol/mol. This is a clinically significant finding because, without intervention, a proportion of individuals will progress to type 2 diabetes. The NHS Diabetes Prevention Programme (NHS DPP) offers structured, evidence-based lifestyle support to people identified in this range.

Type 1 diabetes is not typically diagnosed using HbA1c, as the onset is often acute and requires immediate blood glucose testing. However, once diagnosed, HbA1c is used routinely to monitor long-term control in people with type 1 diabetes. NICE (NG17) recommends a target of 48 mmol/mol where achievable without problematic hypoglycaemia, with targets individualised according to circumstances.

Beyond diabetes, HbA1c may also be used to:

• Assess glycaemic status as part of cardiometabolic risk assessment in people with conditions such as metabolic syndrome — though it is one component of a broader cardiovascular risk evaluation rather than a direct measure of cardiovascular risk

• Screen for diabetes risk in people with polycystic ovary syndrome (PCOS), where NICE (QS170) recommends offering diabetes risk assessment; an OGTT may be preferred in higher-risk individuals

• Optimise glycaemic control prior to elective surgery in people with known diabetes — UK peri-operative guidance (CPOC, 2021) advises considering deferral of non-urgent surgery if HbA1c is 69 mmol/mol or above, as elevated levels are associated with increased surgical risk

HbA1c is not appropriate for diagnosing gestational diabetes, where an OGTT remains the standard approach as recommended by NICE guideline NG3.

Factors That Can Affect HbA1c Accuracy

Conditions such as haemolytic anaemia, iron deficiency, haemoglobin variants, and chronic kidney disease can cause falsely low or high HbA1c results, making alternative tests necessary in some cases.

Although HbA1c is a reliable and well-validated test, several physiological and medical factors can affect its accuracy, potentially leading to falsely high or falsely low results. Clinicians are trained to recognise these variables, but it is helpful for patients to be aware of them too.

Conditions that may 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 temporarily lower HbA1c readings

• Haemoglobin variants (e.g., sickle cell trait, haemoglobin C or E) — certain variants interfere with the laboratory assay used to measure HbA1c

• Recent blood transfusion or acute blood loss — introduces donor red blood cells that have not been exposed to the patient's glucose levels

• Erythropoietin therapy — stimulates production of new red blood cells, shortening average cell age

Conditions that may cause falsely high HbA1c:

• Iron deficiency anaemia (untreated) — reduced red blood cell turnover prolongs exposure to glucose

• Vitamin B12 or folate deficiency

• Chronic kidney disease — can affect red blood cell lifespan and may make HbA1c unreliable depending on the assay method used

• Splenectomy or splenomegaly — altered red blood cell survival affects glycation

• Severe liver disease

• HIV infection or antiretroviral therapy

Pregnancy also affects HbA1c reliability due to physiological changes in red blood cell turnover, which is one reason why it is not used to diagnose gestational diabetes.

If your clinician suspects that any of these factors may be influencing your result, they may request alternative tests — such as fructosamine, a fasting plasma glucose, or an OGTT — to assess glycaemic control more accurately. Your local laboratory can advise on assay-specific interferences where relevant.

What Happens After an Abnormal HbA1c Result

A prediabetes result prompts lifestyle advice and NHS Diabetes Prevention Programme referral, while a result of 48 mmol/mol or above leads to confirmatory testing and, if confirmed, a structured diabetes management plan.

Receiving an abnormal HbA1c result can feel unsettling, but it is important to understand that it is the beginning of a clinical conversation, not a definitive verdict. The steps that follow will depend on where your result falls and your broader clinical picture.

If your result is in the prediabetes range (42–47 mmol/mol): Your GP will typically provide lifestyle advice focused on diet, physical activity, and weight management. You may be referred to the NHS Diabetes Prevention Programme, a free, evidence-based programme shown to reduce the risk of progression to type 2 diabetes. A repeat HbA1c will usually be arranged within 12 months to monitor for any change.

If your result suggests type 2 diabetes (48 mmol/mol or above): In the absence of symptoms, a second confirmatory test will be arranged. If classic symptoms of hyperglycaemia are present (such as excessive thirst, frequent urination, or unexplained weight loss), a single result of 48 mmol/mol or above may be sufficient to confirm the diagnosis. Once confirmed, your GP will discuss a management plan that may include:

• Structured diabetes education (e.g., the DESMOND programme)

• Dietary and lifestyle modifications

• Initiation of medication, most commonly metformin as first-line therapy per NICE guideline NG28

• Regular monitoring of HbA1c (every three months after a treatment change or if targets are not being met; every six months once stable), blood pressure, kidney function, and cholesterol

At the point of diagnosis, your GP should also arrange:

• Referral to the NHS Diabetic Eye Screening Programme

• Foot risk assessment

• Urine albumin:creatinine ratio (ACR) and estimated glomerular filtration rate (eGFR) to assess kidney health

When to seek urgent medical attention: If you experience symptoms such as excessive thirst, frequent urination, unexplained weight loss, or extreme fatigue, contact your GP promptly, as these may indicate significantly elevated blood glucose. If you or someone you know develops symptoms that may suggest diabetic ketoacidosis (DKA) — including abdominal pain, vomiting, rapid or laboured breathing, drowsiness, or a fruity smell on the breath — seek same-day emergency medical assessment (call 999 or go to A&E). Children and young people with suspected new-onset type 1 diabetes require immediate same-day referral to a specialist paediatric diabetes team.

Living with an elevated HbA1c does not mean diabetes is inevitable or unmanageable. With appropriate support, many people successfully reduce their HbA1c through lifestyle changes alone. Regular monitoring and open communication with your healthcare team are the most effective tools for protecting your long-term health.

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