Normal HbA1c levels for diabetes are a key measure of long-term blood glucose control, helping clinicians diagnose diabetes, identify prediabetes, and monitor treatment effectiveness. HbA1c — or glycated haemoglobin — reflects average blood glucose over the preceding two to three months. In the UK, results are expressed in mmol/mol, with a reading below 42 mmol/mol considered normal, 42–47 mmol/mol indicating prediabetes, and 48 mmol/mol or above meeting the NICE diagnostic threshold for type 2 diabetes. Understanding your HbA1c result, what influences it, and how to act on it is central to effective diabetes management.

What Is HbA1c and Why It Matters in Diabetes Care

HbA1c measures average blood glucose over two to three months; in the UK, 48 mmol/mol or above is the NICE diagnostic threshold for type 2 diabetes, while 42–47 mmol/mol indicates prediabetes.

HbA1c — formally known as glycated haemoglobin — is a blood test that reflects your average blood glucose levels over the preceding two to three months. When glucose circulates in the bloodstream, it binds to haemoglobin (the protein inside red blood cells that carries oxygen). The higher your blood glucose over time, the more glucose attaches to haemoglobin, producing a higher HbA1c reading. Because red blood cells live for approximately 120 days, the test provides a reliable window into longer-term glucose control rather than a single-moment snapshot.

In the UK, HbA1c is measured in millimoles per mole (mmol/mol), following international standardisation. Understanding the key thresholds is essential:

• Below 42 mmol/mol — considered normal (non-diabetic range)

• 42–47 mmol/mol — indicates prediabetes or non-diabetic hyperglycaemia (NDH), signalling increased risk

• 48 mmol/mol or above — used by NICE and NHS England as the diagnostic threshold for type 2 diabetes

Important diagnostic caveats: HbA1c should not be used to diagnose diabetes in children or young people, during pregnancy, in people with suspected type 1 diabetes, or where symptoms have developed rapidly or within the preceding two months. In these situations, plasma glucose testing is required. In asymptomatic adults, a diagnosis of type 2 diabetes based on HbA1c requires a confirmatory second HbA1c ≥48 mmol/mol (or a confirmatory plasma glucose test) on a separate occasion, unless clinical symptoms are present.

For people already diagnosed with diabetes, target HbA1c levels are individualised. NICE guidelines (NG28) generally recommend a target of 48 mmol/mol (6.5%) for most people with type 2 diabetes managed by lifestyle or metformin alone, rising to 53 mmol/mol (7.0%) for those on medications that carry a hypoglycaemia risk, such as sulphonylureas or insulin. For type 1 diabetes, NICE (NG17) recommends aiming for 48 mmol/mol where safely achievable. Targets should always be agreed individually with your diabetes team, taking into account your circumstances, preferences, and any comorbidities.

Regular HbA1c monitoring is a cornerstone of diabetes management in UK primary care. For people with diabetes, testing is typically performed every three to six months until levels are stable, then every six months thereafter. For those with non-diabetic hyperglycaemia, monitoring is usually recommended every six to twelve months. This helps clinicians and patients assess whether current treatment plans are effective and guides decisions about medication adjustments, lifestyle interventions, and referral to specialist services.

Factors That Can Affect Your HbA1c Result

Conditions altering red blood cell lifespan — such as haemolytic anaemia, iron-deficiency anaemia, haemoglobin variants, and advanced chronic kidney disease — can falsely raise or lower HbA1c, making alternative glucose tests necessary.

While HbA1c is a robust and widely used measure, several physiological and clinical factors can influence the result, sometimes leading to readings that do not accurately reflect true average glucose levels. Being aware of these variables is important for both patients and clinicians when interpreting results.

Conditions affecting red blood cell turnover are among the most significant confounders. Because HbA1c reflects glucose binding over the lifespan of red blood cells, anything that shortens or lengthens that lifespan will alter the result:

• Haemolytic anaemia, acute blood loss, or recent blood transfusion can falsely lower HbA1c by reducing the time red blood cells are exposed to glucose

• Iron-deficiency anaemia, vitamin B12 deficiency, or folate deficiency can falsely raise HbA1c; levels typically fall towards the true value once the deficiency is treated

• Erythropoietin therapy (used in chronic kidney disease) increases red cell production and may lower measured HbA1c independently of glucose control

• Haemoglobin variants such as sickle cell trait or haemoglobin C can interfere with certain laboratory assays, producing unreliable readings; the effect is method-specific, and UK laboratories will indicate when a variant may have affected the result

• Advanced chronic kidney disease (particularly stage 4–5), splenectomy, and the postpartum period can also affect red cell turnover and HbA1c reliability

Pregnancy is an important consideration. HbA1c is not recommended as the primary diagnostic tool for gestational diabetes in the UK, as physiological changes in red cell turnover during pregnancy can render results misleading. Oral glucose tolerance testing (OGTT) remains the standard approach (NICE NG3).

Certain medications may also influence HbA1c indirectly. Long-term corticosteroid use raises blood glucose and will therefore elevate HbA1c over time.

Finally, ethnicity can play a role. Some studies suggest that people of African, Caribbean, or South Asian heritage may have slightly higher HbA1c values at equivalent glucose levels, though this remains an area of ongoing research and does not currently alter UK diagnostic thresholds.

When HbA1c is considered unreliable due to any of the above factors, your GP or diabetes team may request alternative tests. These include fasting plasma glucose, an OGTT, or — in some specialist settings — alternative markers such as fructosamine or glycated albumin, which reflect shorter-term glucose control and are not affected by red cell lifespan. Structured glucose profiles or continuous glucose monitoring (CGM) may also be considered. It is important to note that the diagnostic caveats described in the previous section — particularly regarding children, pregnancy, and suspected type 1 diabetes — apply equally here: in these groups, plasma glucose testing rather than HbA1c should be used to diagnose diabetes.

What Happens If Your HbA1c Is Too High or Too Low

A persistently high HbA1c increases the risk of microvascular and macrovascular complications, while an excessively low HbA1c in those on insulin or sulphonylureas raises the risk of hypoglycaemia; targets should be individualised.

A persistently elevated HbA1c indicates that blood glucose levels have been running higher than the recommended target over recent months. This matters because sustained hyperglycaemia is directly linked to the development and progression of diabetes-related complications. These include:

• Microvascular complications — diabetic retinopathy (eye disease), nephropathy (kidney disease), and peripheral neuropathy (nerve damage)

• Macrovascular complications — increased risk of cardiovascular disease, stroke, and peripheral arterial disease

Large-scale trials, including the UK Prospective Diabetes Study (UKPDS 35), demonstrated that each 1% (approximately 11 mmol/mol) reduction in HbA1c is associated with meaningful reductions in diabetes-related complications. This underscores why achieving and maintaining a personalised HbA1c target is a clinical priority, not merely a numerical goal.

However, it is equally important to recognise that an HbA1c that is too low can also pose risks, particularly in people treated with insulin or sulphonylureas. Overly tight glucose control in these individuals increases the risk of hypoglycaemia (low blood sugar), which can cause symptoms ranging from shakiness, sweating, and confusion to, in severe cases, loss of consciousness. For older adults, those with frailty, cardiovascular disease, or hypoglycaemia unawareness, NICE guidance (NG28) supports individualising and relaxing HbA1c targets rather than pursuing the lowest achievable value. The appropriate target for you should be agreed with your diabetes team based on your overall health, comorbidities, and personal circumstances.

When to seek urgent help — red flags:

• Severe hypoglycaemia (confusion, seizures, loss of consciousness, or inability to treat yourself): call 999 immediately

• Symptoms of diabetic ketoacidosis (DKA) — including abdominal pain, vomiting, deep or rapid breathing, drowsiness, or confusion, particularly if blood glucose is markedly elevated or ketones are detected: seek urgent same-day medical care or call 999

• Symptoms of hyperosmolar hyperglycaemic state (HHS) — extreme thirst, very high blood glucose, progressive drowsiness or confusion, particularly in older people with type 2 diabetes: seek urgent same-day medical care or call 999

• Suspected type 1 diabetes in an adult (new-onset classic symptoms of thirst, polyuria, unexplained weight loss, or ketonuria): contact your GP for same-day specialist assessment; in children or young people, immediate referral to a paediatric diabetes team is required

If your HbA1c result has risen significantly since your last test, or if you are experiencing symptoms such as increased thirst, frequent urination, unexplained fatigue, or recurrent infections, contact your GP or diabetes nurse promptly. If you are experiencing frequent hypoglycaemic episodes, this warrants an urgent review of your medication regimen. Do not adjust insulin or other diabetes medications without professional guidance.

How to Work With Your GP or Diabetes Team to Manage HbA1c

Effective HbA1c management combines lifestyle changes — diet, physical activity, and weight management — with pharmacological treatment guided by NICE (NG28), including metformin first-line and SGLT-2 inhibitors for those with cardiovascular or renal comorbidities.

Managing HbA1c effectively is rarely about a single intervention — it requires a collaborative, ongoing relationship between you and your healthcare team. In UK primary care, people with type 2 diabetes are typically reviewed at least annually through the NHS Diabetes Annual Review, which encompasses nine care processes including HbA1c testing alongside checks for blood pressure, cholesterol, kidney function, foot health, and body mass index. Retinal screening is delivered separately through the NHS Diabetic Eye Screening Programme. Those with less well-controlled diabetes or those on complex medication regimens may be reviewed more frequently.

Lifestyle modifications remain the foundation of HbA1c management and should not be underestimated:

• Dietary changes — reducing refined carbohydrates and sugary foods, increasing fibre, and following a balanced diet can meaningfully lower HbA1c. Referral to a dietitian or a structured education programme such as the NHS Diabetes Prevention Programme or DESMOND (for type 2 diabetes) may be offered

• Physical activity — regular aerobic exercise improves insulin sensitivity; even 150 minutes of moderate activity per week, as recommended by the UK Chief Medical Officers, can contribute to HbA1c reduction

• Weight management — for people with type 2 diabetes who are overweight, even modest weight loss of 5–10% of body weight can produce clinically significant improvements in HbA1c

When lifestyle measures alone are insufficient, your GP will discuss pharmacological options. Metformin remains the first-line medication for most people with type 2 diabetes in the UK, per NICE guidance (NG28). If HbA1c remains above target, additional agents may be added. Notably, updated NICE guidance recommends SGLT-2 inhibitors (such as empagliflozin or dapagliflozin) as a priority treatment option — and in some cases alongside or instead of metformin — for people with type 2 diabetes who also have established cardiovascular disease, heart failure, or chronic kidney disease, where they offer benefits beyond glucose lowering. Other options include GLP-1 receptor agonists, DPP-4 inhibitors, or insulin, depending on individual clinical circumstances and patient preference.

If you experience side effects from any diabetes medication, report these to your GP or diabetes nurse. You can also report suspected adverse drug reactions directly to the MHRA via the Yellow Card scheme (available at yellowcard.mhra.gov.uk).

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Open communication with your diabetes team is essential. If you are struggling to understand your results, finding it difficult to make lifestyle changes, or experiencing side effects from medication, raise these concerns at your next appointment — or sooner if needed. Shared decision-making, where your values and preferences are central to the treatment plan, is a core principle of NICE-aligned diabetes care and leads to better long-term outcomes.

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