HbA1c (haemoglobin A1c) blood results that fall outside the normal range are an important clinical signal that should never be overlooked. This glycated haemoglobin test reflects your average blood glucose levels over the preceding two to three months, offering a far broader picture than a single finger-prick reading. In the UK, results are reported in mmol/mol and interpreted against NHS and NICE-defined thresholds. Whether your result is unexpectedly high — suggesting prediabetes or poorly controlled diabetes — or unusually low, understanding what it means and what to do next is essential for protecting your long-term health.

What Does an Out-of-Range HbA1c Result Mean?

An out-of-range HbA1c means average blood glucose over two to three months has fallen outside the normal reference interval, potentially indicating prediabetes, diabetes, or inadequate glycaemic control — but it is a clinical indicator requiring further assessment, not a diagnosis in itself.

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HbA1c — formally known as glycated haemoglobin or haemoglobin A1c — is a blood test that reflects your average blood glucose (sugar) levels over the preceding two to three months. Unlike a standard finger-prick glucose reading, which captures a single moment in time, HbA1c provides a broader picture of how well blood sugar has been controlled over a sustained period. It is measured in millimoles per mole (mmol/mol) in the UK, following standardisation to the IFCC (International Federation of Clinical Chemistry) reporting format.

When a result is described as 'out of range', it means the value falls either below or above the reference interval considered normal or target for a given individual. A raised HbA1c suggests that blood glucose has been consistently elevated, which may indicate prediabetes or type 2 diabetes, or that existing diabetes is not being adequately managed. Conversely, a very low HbA1c in someone treated for diabetes may signal episodes of hypoglycaemia (low blood sugar), which carry their own clinical risks.

It is important to understand that an out-of-range result is not a diagnosis in itself — it is a clinical indicator that warrants further assessment. Results must always be interpreted in the context of an individual's medical history, symptoms, medications, and other laboratory findings.

HbA1c is not appropriate for diagnosing diabetes in all situations. It should not be used as a diagnostic test in children and young people, during pregnancy, where type 1 diabetes is suspected, during acute severe illness, or where red blood cell turnover is abnormal (for example, following a recent blood transfusion, in haemolytic anaemia, or in people with haemoglobin variants such as sickle cell trait or thalassaemia). In these circumstances, fasting plasma glucose or an oral glucose tolerance test (OGTT) should be used instead, in line with NHS and WHO guidance adopted in the UK. If you have received an unexpected HbA1c result, your GP or diabetes care team is best placed to explain what it means for you personally.

Understanding HbA1c Reference Ranges Used in the UK

UK NHS and NICE guidelines define normal HbA1c as below 42 mmol/mol, prediabetes as 42–47 mmol/mol, and type 2 diabetes as 48 mmol/mol or above, with individualised treatment targets for those already diagnosed.

In the UK, HbA1c thresholds are defined by NHS and NICE (National Institute for Health and Care Excellence) guidelines (NICE NG28), and are expressed in mmol/mol. Understanding these thresholds helps contextualise whether a result is within a healthy range, indicative of prediabetes, or consistent with a diagnosis of diabetes:

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

• 42–47 mmol/mol — classified as prediabetes or 'non-diabetic hyperglycaemia' (NDH); indicates increased risk of developing type 2 diabetes

• 48 mmol/mol or above — diagnostic of type 2 diabetes (when confirmed on a second test, unless symptoms are present)

These diagnostic thresholds apply only where HbA1c is a reliable measure (see the section above for situations where it should not be used for diagnosis).

For people already diagnosed with diabetes, NICE recommends individualised HbA1c targets. For most adults with type 2 diabetes managed with lifestyle measures or a single non-hypoglycaemic drug, a target of 48 mmol/mol is advised. For those on insulin or drugs that carry a hypoglycaemia risk, a target of 53 mmol/mol may be more appropriate to reduce the risk of dangerous low blood sugar episodes. Targets should always be personalised, taking into account hypoglycaemia risk, comorbidities, frailty, limited life expectancy, and individual preferences — a decision made jointly between the patient and their care team.

It is worth noting that HbA1c reference ranges can vary slightly between laboratories, and some clinical systems may still display older percentage-based values (e.g., 6.5% equates to approximately 48 mmol/mol). If you are unsure which unit your result is reported in, ask your healthcare provider for clarification. Certain conditions — discussed in the next section — can also affect the reliability of HbA1c as a measure, meaning the result must sometimes be interpreted with caution.

Common Causes of Abnormal HbA1c Levels

The most common cause of a raised HbA1c is persistently elevated blood glucose from prediabetes or diabetes, but conditions such as iron deficiency anaemia, haemolytic anaemia, haemoglobin variants, and chronic kidney disease can produce falsely high or low results.

The most common reason for a raised HbA1c is persistently elevated blood glucose, typically associated with type 2 diabetes, prediabetes, or poorly controlled type 1 diabetes. Lifestyle factors such as a diet high in refined carbohydrates and sugars, physical inactivity, obesity, and chronic stress can all contribute to elevated glucose levels over time. Certain medications — including corticosteroids, antipsychotics, and some immunosuppressants — are also known to raise blood glucose and may therefore increase HbA1c.

However, HbA1c is not always a straightforward measure. Several medical conditions can cause falsely elevated or falsely low results, making interpretation more complex. The direction of the interference often depends on the specific laboratory assay method used, so it is important to check local laboratory notes when results appear inconsistent:

• Falsely high HbA1c may occur in iron deficiency anaemia or vitamin B12 deficiency

• Falsely low HbA1c can result from haemolytic anaemia, recent blood transfusion, acute blood loss, erythropoietin (EPO) therapy, or splenectomy

• Haemoglobin variants (such as sickle cell trait or thalassaemia) and chronic kidney disease (CKD) can produce either falsely high or falsely low results depending on the assay method and the degree of associated anaemia or altered red cell turnover; your laboratory or GP can advise on the reliability of your specific result in these circumstances

Where HbA1c may be unreliable, alternative approaches to assessing glycaemic control include fasting plasma glucose, an oral glucose tolerance test (OGTT), fructosamine measurement, or continuous glucose monitoring (CGM). It is also worth noting that ethnicity can influence the relationship between HbA1c and average glucose levels; some studies suggest that HbA1c may be slightly higher in people of South Asian or African-Caribbean heritage at equivalent glucose concentrations, though there is no current NICE guidance recommending different diagnostic thresholds by ethnicity.

If your HbA1c result appears inconsistent with your symptoms or other test results, your GP may investigate further to rule out these confounding factors before drawing clinical conclusions.

When to Seek Medical Advice About Your HbA1c

Contact your GP promptly if your HbA1c is 42 mmol/mol or above, rising despite treatment, or unexpectedly low with hypoglycaemia symptoms; call 999 or go to A&E for suspected diabetic ketoacidosis or severe hypoglycaemia.

If you have received an HbA1c result that falls outside the normal range, it is important not to ignore it — even if you feel well. Many people with prediabetes or early type 2 diabetes experience no obvious symptoms, which is why routine blood testing is so valuable. Prompt engagement with your GP or practice nurse allows for timely intervention, which can significantly reduce the risk of long-term complications.

Contact your GP promptly if your HbA1c result is:

• 42 mmol/mol or above (particularly if you have not previously been told you are at risk)

• Rising despite existing diabetes treatment

• Unexpectedly low and you are experiencing symptoms of hypoglycaemia, such as shakiness, sweating, confusion, or palpitations

Seek emergency medical attention (call 999 or go to A&E) if you experience:

• Symptoms of diabetic ketoacidosis (DKA) — including excessive thirst, frequent urination, nausea, vomiting, abdominal pain, or fruity-smelling breath

• Severe hypoglycaemia — loss of consciousness, seizures, or inability to swallow

If you are unsure whether your symptoms require urgent attention, call NHS 111, which can provide immediate advice and direct you to the most appropriate care.

For those already under diabetes care, a significantly out-of-range HbA1c at a routine review should prompt a structured conversation with your diabetes team about medication adjustment, lifestyle factors, or referral to specialist services. The NHS Diabetes Prevention Programme (NHS DPP) is available to people identified with non-diabetic hyperglycaemia (typically HbA1c 42–47 mmol/mol, or an equivalent fasting plasma glucose result) and offers evidence-based support to reduce the risk of progression to type 2 diabetes. Eligibility criteria may vary slightly by local area; your GP can advise and refer you directly to this programme.

How HbA1c Results Guide Diabetes Management on the NHS

NICE guidelines recommend HbA1c monitoring every three to six months when treatment is changing and every six months once stable, with results directly informing stepwise medication decisions including metformin, SGLT-2 inhibitors, GLP-1 agonists, and insulin.

HbA1c is central to diabetes care pathways across the NHS. NICE guidelines — NG17 for type 1 diabetes in adults and NG28 for type 2 diabetes in adults — recommend that HbA1c is measured every three to six months when treatment has recently changed or targets are not being met, and every six months once diabetes is stable and well controlled. These regular checks allow clinicians to assess whether current management is effective and to make timely adjustments.

For people with type 2 diabetes, HbA1c results directly inform treatment decisions. If lifestyle measures alone are insufficient to achieve target levels, NICE NG28 recommends initiating metformin as first-line pharmacological therapy, provided there are no contraindications. Metformin works by reducing hepatic glucose production and improving insulin sensitivity, and is generally well tolerated. If HbA1c remains above target despite metformin, additional agents may be added based on individual cardiovascular and renal risk profiles. These include:

• SGLT-2 inhibitors (e.g., empagliflozin, dapagliflozin) — particularly recommended where established cardiovascular disease or chronic kidney disease is present; note that these medicines carry a small risk of diabetic ketoacidosis (DKA), and patients should be counselled accordingly (see MHRA Drug Safety Update on SGLT-2 inhibitors and DKA risk)

• GLP-1 receptor agonists (e.g., semaglutide, liraglutide) — may be preferred where weight loss is a priority or cardiovascular risk is high

• DPP-4 inhibitors (e.g., sitagliptin) — a well-tolerated option with a low hypoglycaemia risk

• Sulfonylureas (e.g., gliclazide) — effective but carry a higher risk of hypoglycaemia and weight gain

• Insulin — initiated when other agents are insufficient or in specific clinical circumstances, with the type and regimen tailored to the individual

SGLT-2 inhibitors require adequate renal function (eGFR thresholds vary by agent; check current SmPC and NICE guidance) and are not appropriate for all patients. Your prescriber will consider your full clinical picture before recommending any medication.

For people with type 1 diabetes, HbA1c is used alongside continuous glucose monitoring (CGM) data to guide insulin dose adjustments and assess overall glycaemic variability. NICE NG17 recommends offering all adults with type 1 diabetes a choice of real-time CGM (RT-CGM) or intermittently scanned CGM (isCGM/flash), with the decision personalised to the individual's needs and preferences. HbA1c alone does not capture the full picture of glucose fluctuations, including time in range or hypoglycaemia frequency.

Across both diabetes types, HbA1c results are also used to stratify risk for complications such as retinopathy, nephropathy, and cardiovascular disease, informing referral decisions and screening intervals.

Steps to Help Bring Your HbA1c Back Into a Healthy Range

Evidence-based steps to lower HbA1c include reducing refined carbohydrates, achieving at least 150 minutes of moderate aerobic activity weekly, adhering to prescribed medications, and engaging with NHS structured education programmes such as DESMOND or DAFNE.

If your HbA1c is above the recommended range, there are several evidence-based steps you can take — in partnership with your healthcare team — to help bring it back towards a healthier level. Small, consistent changes can have a meaningful impact on blood glucose control over time.

Dietary adjustments:

• Reduce intake of refined carbohydrates, sugary drinks, and ultra-processed foods

• Increase fibre-rich foods such as vegetables, pulses, wholegrains, and fruit (in moderate portions)

• A range of dietary approaches can improve HbA1c, including Mediterranean-style eating, reduced-carbohydrate diets, lower glycaemic index (GI) diets, and calorie-deficit approaches. NICE NG28 supports individualised dietary advice rather than a single prescribed pattern; discuss the most suitable approach for you with your GP or a registered dietitian before making significant changes

• Important: If you are taking an SGLT-2 inhibitor (such as empagliflozin or dapagliflozin), you should avoid very-low-carbohydrate or ketogenic diets, as these can increase the risk of diabetic ketoacidosis (DKA). Always seek medical advice before making major dietary changes if you are on this class of medication

Physical activity:

• Aim for at least 150 minutes of moderate-intensity aerobic activity per week, plus strengthening activities on at least two days per week, in line with the UK Chief Medical Officers' Physical Activity Guidelines

• Resistance exercise (e.g., weight training) has also been shown to improve insulin sensitivity

• Even breaking up prolonged sitting with short walks can help reduce post-meal glucose spikes

Medication adherence:

• Take prescribed diabetes medications consistently and at the correct times

• Discuss any side effects or concerns with your GP rather than stopping medication without guidance

• If you think you are experiencing a side effect from any medicine, you can report it via the MHRA Yellow Card Scheme at yellowcard.mhra.gov.uk or through the Yellow Card app

Monitoring and support:

• Regular self-monitoring of blood glucose (where appropriate) can help identify patterns and guide behaviour

• Structured diabetes education programmes such as DESMOND (for type 2 diabetes) or DAFNE (for type 1 diabetes) are available on the NHS and provide practical skills for self-management; ask your GP or diabetes team for a referral

Remember, improving HbA1c is a gradual process. Even a modest reduction — for example, from 58 to 50 mmol/mol — can significantly lower the risk of diabetes-related complications. Work with your care team to set realistic, personalised goals.

Frequently Asked Questions