HbA1c results and their normal range are central to diagnosing and managing diabetes in the UK. The HbA1c test measures average blood glucose levels over the preceding two to three months, offering clinicians a reliable picture of long-term glycaemic control. Whether you have received a result and want to understand what it means, or you are being monitored for prediabetes or type 2 diabetes, knowing the NHS and NICE-defined thresholds is essential. This article explains what a normal HbA1c result looks like, what raised levels indicate, and what steps to take if your result falls outside the expected range.

What Is an HbA1c Test and Why Is It Used?

The HbA1c test measures the proportion of haemoglobin bonded to glucose, reflecting average blood glucose over 2–3 months. It is used by NHS clinicians to diagnose type 2 diabetes, identify prediabetes, and monitor long-term glycaemic control.

The HbA1c test — formally known as the glycated haemoglobin test — is a blood test that measures the average level of blood glucose (sugar) over the preceding two to three months. Unlike a standard fasting glucose test, which provides a snapshot of blood sugar at a single point in time, the HbA1c offers a broader picture of long-term glucose control, making it a particularly valuable tool in clinical practice.

The test works by measuring the proportion of haemoglobin — the protein in red blood cells that carries oxygen — that has become chemically bonded to glucose. Because red blood cells have a lifespan of approximately 90 to 120 days, the HbA1c reflects average glucose exposure over that period. The result is expressed as a percentage or, increasingly in the UK, in millimoles per mole (mmol/mol) in line with IFCC (International Federation of Clinical Chemistry) standardisation.

NHS clinicians use the HbA1c test for several specific purposes:

• Diagnosing type 2 diabetes in adults where it is clinically appropriate to do so

• Identifying non-diabetic hyperglycaemia (prediabetes) in people at elevated risk

• Monitoring long-term glycaemic control in people already diagnosed with diabetes

• Assessing risk of microvascular and macrovascular complications in people with known diabetes, as HbA1c correlates with complication risk over time

The test is requested when a patient presents with symptoms suggestive of diabetes (such as increased thirst, frequent urination, or unexplained fatigue), as part of risk-based screening in people with relevant risk factors, or for ongoing monitoring in those with a confirmed diagnosis. It is not a standard component of a generic routine blood panel for all patients. The test requires only a small blood sample and does not require fasting in most circumstances.

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It is important to note that HbA1c is not suitable for diagnosis in all situations. It should not be used to diagnose diabetes in pregnancy or within two to three months postpartum, in children and young people, in people with suspected type 1 diabetes or rapid-onset symptoms, during acute illness, or where conditions affecting red blood cell turnover or assay interference are suspected. In these circumstances, alternative tests such as fasting plasma glucose or an oral glucose tolerance test (OGTT) are preferred.

HbA1c Normal Range: What NHS and NICE Guidelines Say

A normal HbA1c is below 42 mmol/mol (6.0%); 42–47 mmol/mol indicates prediabetes, and 48 mmol/mol or above is diagnostic of type 2 diabetes in adults per NICE NG28 and WHO guidance.

According to NHS and NICE guidance, the normal HbA1c range for a person without diabetes is generally considered to be below 42 mmol/mol (6.0%). Within this range, blood glucose levels are well regulated and the risk of developing type 2 diabetes or associated complications is considered low.

NICE NG28 (Type 2 Diabetes in Adults: Management) and WHO guidance define the following reference thresholds for adults:

• Normal (no diabetes risk): Below 42 mmol/mol (6.0%)

• Non-diabetic hyperglycaemia (prediabetes): 42–47 mmol/mol (6.0–6.4%)

• Diagnostic of type 2 diabetes: 48 mmol/mol (6.5%) or above

In asymptomatic adults, a result of 48 mmol/mol or above should be confirmed with a repeat HbA1c or a fasting plasma glucose before a diagnosis of type 2 diabetes is made, in line with NICE and WHO recommendations. If symptoms of diabetes are present, a single raised result may be sufficient.

These thresholds apply specifically to the diagnosis of type 2 diabetes in adults. They are not used to diagnose type 1 diabetes, gestational diabetes, or diabetes in children and young people, where different diagnostic criteria apply. HbA1c should also not be used as a diagnostic tool in the following situations, as results may be unreliable:

• Pregnancy and two to three months postpartum

• Children and young people

• Suspected type 1 diabetes or rapid-onset symptoms

• Acute illness

• Steroid-induced hyperglycaemia

• People receiving HIV treatment with certain antiretrovirals

• Advanced chronic kidney disease (CKD stage 4–5) or erythropoietin therapy

• Haemoglobinopathies, haemolytic anaemia, or recent blood transfusion

For people already living with type 2 diabetes, NICE NG28 recommends individualised HbA1c targets. For most adults managed with lifestyle modification or metformin alone, a target of 48 mmol/mol (6.5%) is recommended. For those taking medicines that carry a risk of hypoglycaemia — such as sulphonylureas or insulin — a slightly higher target of 53 mmol/mol (7.0%) is generally advised to reduce the risk of low blood sugar episodes. These targets should always be agreed with a GP or diabetes specialist and reviewed regularly.

Understanding Your HbA1c Results and What They Mean

A result below 42 mmol/mol is reassuring; 42–47 mmol/mol indicates prediabetes; and 48 mmol/mol or above on two occasions confirms type 2 diabetes in asymptomatic adults. Always discuss results with your GP for personalised interpretation.

Receiving an HbA1c result can feel confusing, particularly if you are unfamiliar with the units used. In the UK, results are now routinely reported in mmol/mol rather than as a percentage, though some laboratories and patient letters may include both. Understanding what your result means in context is essential before drawing any conclusions.

A result below 42 mmol/mol is generally reassuring and suggests that blood glucose levels have been well controlled over the preceding months. However, a 'normal' result does not entirely eliminate the possibility of other metabolic concerns, and it should always be interpreted alongside your overall clinical picture, including weight, blood pressure, cholesterol, and family history.

A result between 42 and 47 mmol/mol places an individual in the non-diabetic hyperglycaemia (prediabetes) range. This does not mean diabetes is inevitable — with appropriate lifestyle changes, many people in this range can return to normal glucose levels. NHS England's Diabetes Prevention Programme (NDPP) is specifically designed to support people in this category.

A result of 48 mmol/mol or above on two separate occasions is diagnostic of type 2 diabetes in an asymptomatic adult. If symptoms of diabetes are present, a single raised result may be sufficient for diagnosis.

Key points to remember when reviewing your result:

• Context matters — one result alone rarely tells the full story

• Trends over time are often more informative than a single reading

• Always discuss your result with your GP or practice nurse, who can explain what it means for your individual health

• Do not self-diagnose based on an online result alone

Seek same-day urgent medical advice if you or someone you know experiences rapid-onset symptoms such as extreme thirst, frequent urination, unexplained weight loss, abdominal pain, vomiting, drowsiness, deep or rapid breathing, or a fruity smell on the breath. These may indicate diabetic ketoacidosis (DKA) or another hyperglycaemic emergency — particularly in younger people or those with possible type 1 diabetes — and require prompt assessment. In this situation, do not wait for an HbA1c result; blood glucose and ketone testing are more appropriate.

Raised HbA1c Levels: Prediabetes and Diabetes Thresholds

HbA1c of 42–47 mmol/mol indicates prediabetes with insulin resistance, while 48 mmol/mol or above confirms type 2 diabetes requiring formal management. NICE recommends NDPP referral for prediabetes and a stepwise pharmacological approach for diabetes.

When HbA1c levels rise above the normal range, it indicates that blood glucose has been consistently elevated over recent months. This can occur gradually and without obvious symptoms, which is why risk-based screening is so important — particularly for individuals with known risk factors such as obesity, a sedentary lifestyle, a family history of type 2 diabetes, or a history of gestational diabetes.

Non-diabetic hyperglycaemia / prediabetes (42–47 mmol/mol) is a clinically significant finding. At this stage, the pancreas is still producing insulin, but the body's cells are becoming less responsive to it — a process known as insulin resistance. Without intervention, this can progress to type 2 diabetes, though this is not inevitable. In line with NICE PH38 (Type 2 Diabetes: Prevention in People at High Risk), people in this range should be offered:

• Referral to the NHS Diabetes Prevention Programme (NDPP), a free, evidence-based structured lifestyle intervention for eligible adults

• Dietary advice focused on reducing refined carbohydrates and increasing fibre intake

• Physical activity guidance — at least 150 minutes of moderate-intensity exercise per week

• Annual HbA1c monitoring to track progression or improvement

Type 2 diabetes (48 mmol/mol or above) requires formal clinical management. Initial treatment typically involves lifestyle modification alongside metformin, which works by reducing hepatic glucose production and improving insulin sensitivity. NICE NG28 outlines a stepwise approach to pharmacological management, escalating through additional agents if glycaemic targets are not met. Notably, NICE recommends considering SGLT-2 inhibitors at an earlier stage in people with established atherosclerotic cardiovascular disease, heart failure, or chronic kidney disease, given their cardio-renal protective benefits. Other agents — including GLP-1 receptor agonists and DPP-4 inhibitors — may also be considered depending on individual clinical circumstances.

It is important to emphasise that a raised HbA1c is not a life sentence. With appropriate support, many people achieve significant improvements in their glucose levels and overall metabolic health.

Factors That Can Affect Your HbA1c Reading

Haemolytic anaemia and recent blood transfusion can falsely lower HbA1c, while iron deficiency anaemia and advanced CKD can falsely raise it. Clinicians may request alternative tests such as fasting plasma glucose or OGTT when these factors are present.

While the HbA1c test is widely regarded as a reliable marker of long-term glycaemic control, several physiological and clinical factors can influence the result — either falsely raising or lowering it. Clinicians are trained to account for these variables, but it is useful for patients to be aware of them.

Conditions and circumstances that may falsely lower HbA1c:

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

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

• Acute blood loss

• Haemoglobin variants (e.g., HbS, HbC, HbE) — more common in people of African, Caribbean, or South Asian heritage; these can interfere with certain HbA1c assay methods. Where haemoglobin variants are suspected, a laboratory method validated for that variant should be used, or an alternative test considered

• Erythropoietin (EPO) therapy in advanced CKD — stimulates new red blood cell production, shortening average cell age

Conditions and circumstances that may falsely raise HbA1c:

• Iron deficiency anaemia (untreated) — reduced red blood cell turnover prolongs glycation exposure, potentially elevating HbA1c. Importantly, HbA1c may fall after iron replacement therapy as red cell turnover normalises; results during or shortly after iron treatment should therefore be interpreted with caution

• Vitamin B12 or folate deficiency

• Advanced chronic kidney disease (CKD stage 4–5) — affects red blood cell lifespan and assay accuracy

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

• Severe liver disease

Other important circumstances affecting HbA1c reliability:

• Pregnancy and two to three months postpartum — physiological changes in red cell turnover make HbA1c unreliable for diagnosis

• Steroid therapy — can cause acute hyperglycaemia not reflected proportionately in HbA1c

• HIV treatment with certain antiretrovirals — some agents affect red blood cell parameters

• Acute illness — may not be reflected accurately in HbA1c

Certain medicines, including hydroxyurea and dapsone, may also interfere with results.

If your clinician suspects that any of these factors may be affecting your HbA1c, they may request alternative tests — such as a fasting plasma glucose, a fructosamine measurement, or an oral glucose tolerance test (OGTT) — to obtain a more accurate assessment of your glucose metabolism.

What to Do If Your HbA1c Result Is Outside the Normal Range

If your HbA1c is raised, speak with your GP promptly; prediabetes warrants NDPP referral and lifestyle changes, while a diabetic-range result requires confirmatory testing and a personalised management plan per NICE NG28.

If your HbA1c result falls outside the normal range, the most important first step is to speak with your GP or practice nurse as soon as possible. Avoid drawing firm conclusions from an online search alone, as results must always be interpreted in the context of your full medical history, current medications, and any relevant symptoms.

If your result is in the non-diabetic hyperglycaemia range (42–47 mmol/mol):

• Your GP may refer you to the NHS Diabetes Prevention Programme (NDPP), a free, evidence-based lifestyle intervention for eligible adults

• Focus on dietary changes — reducing sugar, refined carbohydrates, and ultra-processed foods while increasing vegetables, wholegrains, and lean protein

• Aim for regular physical activity — even modest increases in movement can meaningfully improve insulin sensitivity

• Your HbA1c should be rechecked annually to monitor for progression or improvement, in line with NICE PH38

If your result is in the diabetic range (48 mmol/mol or above):

• A repeat HbA1c or fasting plasma glucose will usually be arranged to confirm the diagnosis in asymptomatic adults, in line with NICE NG28 and WHO guidance

• Your GP will discuss a personalised management plan, which may include lifestyle changes, medication, and referral to a diabetes specialist nurse or dietitian

• You should be offered structured diabetes education — such as the DESMOND or X-PERT programme for type 2 diabetes — ideally within six months of diagnosis, as recommended by NICE NG28

Seek same-day urgent medical advice if you experience rapid-onset symptoms such as extreme thirst, frequent urination, unexplained weight loss, abdominal pain, vomiting, drowsiness, deep or rapid breathing, fruity-smelling breath, or confusion. These may indicate diabetic ketoacidosis (DKA) or another hyperglycaemic emergency — particularly in younger people or those with possible type 1 diabetes — and require immediate assessment. Do not wait for an HbA1c result in this situation.

If you are prescribed medicines for diabetes that carry a risk of hypoglycaemia — such as insulin or a sulphonylurea — and you experience suspected side effects, these can be reported to the MHRA via the Yellow Card scheme (yellowcard.mhra.gov.uk). Your GP or pharmacist can advise you further.

Finally, remember that an abnormal HbA1c result is a prompt for action, not a cause for alarm. Early identification and intervention significantly reduce the risk of long-term complications, including cardiovascular disease, kidney disease, and neuropathy. Working collaboratively with your healthcare team gives you the best chance of achieving and maintaining good metabolic health.

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