How HbA1c is formed is central to understanding one of the most important tests in diabetes care. HbA1c — glycated haemoglobin — forms when glucose in the bloodstream binds non-enzymatically to haemoglobin inside red blood cells, a continuous process directly proportional to blood glucose concentration. Because red blood cells survive for roughly 90 to 120 days, the resulting measurement reflects average blood glucose over the preceding two to three months. In the UK, HbA1c is expressed in mmol/mol per IFCC standardisation and is used by the NHS both to diagnose type 2 diabetes and to monitor long-term glycaemic control in line with NICE guidance.

What Is HbA1c and Why Does It Matter?

HbA1c forms when glucose binds non-enzymatically to haemoglobin in red blood cells, reflecting average blood glucose over two to three months. Chronically elevated HbA1c is strongly associated with microvascular and macrovascular complications of diabetes.

HbA1c — formally known as glycated haemoglobin — is a form of haemoglobin that forms when glucose in the bloodstream binds to haemoglobin molecules inside red blood cells. Understanding how HbA1c is formed begins with a straightforward biochemical process: glucose attaches to the amino acid valine at the N-terminal end of the beta chain of haemoglobin A through a non-enzymatic reaction called glycation. This reaction is not controlled by enzymes and occurs continuously and proportionally to the concentration of glucose in the blood. The higher the blood glucose concentration over time, the greater the proportion of haemoglobin that becomes glycated.

Because red blood cells have a lifespan of approximately 90 to 120 days, the HbA1c measurement reflects average blood glucose levels over roughly the preceding two to three months. Importantly, the result is weighted towards the most recent four to eight weeks of glycaemia, meaning that recent improvements or deteriorations in blood glucose control will have a proportionally greater influence on the reading than changes earlier in the period. This makes HbA1c a far more informative marker than a single fasting glucose reading, which only captures a snapshot in time.

In the UK, results are expressed in millimoles per mole (mmol/mol) in line with the International Federation of Clinical Chemistry (IFCC) standardisation, as recommended by NHS and WHO guidance. The percentage (%) figure may also be shown for context.

HbA1c matters clinically because chronically elevated blood glucose — and therefore elevated HbA1c — is strongly associated with the development of long-term complications of diabetes, including:

• Microvascular damage — affecting the kidneys (nephropathy), eyes (retinopathy), and nerves (neuropathy)

• Macrovascular disease — increasing the risk of heart attack and stroke

• Poor wound healing and increased susceptibility to infection

For people living with diabetes, regular HbA1c monitoring is a cornerstone of ongoing management, helping clinicians and patients assess whether blood glucose control is within a safe and therapeutic range, in line with NICE guidance (NG28 for type 2 diabetes; NG17 for type 1 diabetes).

What Affects HbA1c Levels Over Time

Any condition altering blood glucose concentration or red blood cell lifespan can affect HbA1c — including anaemia, chronic kidney disease, haemoglobin variants, and pregnancy. When HbA1c is unreliable, plasma glucose, OGTT, or fructosamine should be used instead.

Because HbA1c is formed continuously as long as red blood cells are circulating and glucose is present in the blood, any factor that influences either blood glucose concentration or red blood cell lifespan can affect the result. This is an important consideration when interpreting test findings, as certain medical conditions and lifestyle factors can cause HbA1c readings to be misleadingly high or low.

Factors that may raise HbA1c include:

• Consistently elevated blood glucose due to poorly controlled type 1 or type 2 diabetes

• Iron deficiency anaemia, which can prolong red blood cell survival and allow more glycation to occur

• Vitamin B12 or folate deficiency, which may similarly affect red blood cell turnover

• Certain medicines, including corticosteroids, which raise blood glucose levels

Factors that may lower HbA1c include:

• Haemolytic anaemia or other conditions that shorten red blood cell lifespan, reducing the time available for glycation

• Chronic kidney disease (CKD): CKD commonly lowers HbA1c due to shortened red blood cell survival, though the effect can vary depending on the degree of anaemia, use of erythropoietin (EPO) therapy, and the laboratory assay used — results should therefore be interpreted with caution and corroborated with plasma glucose or continuous glucose monitoring (CGM) where possible

• Recent blood transfusions or significant blood loss, which introduce new, unglycated red blood cells

• EPO therapy, which increases red blood cell turnover

• Splenectomy, which prolongs red blood cell lifespan and may raise HbA1c

• Haemoglobin variants (such as sickle cell trait or haemoglobin C), which can interfere with some laboratory assay methods — the degree of interference is assay-dependent, and modern IFCC-aligned methods may mitigate this; it is advisable to check with your local laboratory if a variant is known or suspected

• Pregnancy, particularly in the second and third trimesters, due to increased red blood cell turnover

When HbA1c is known to be unreliable due to any of the above conditions, plasma glucose measurements, the oral glucose tolerance test (OGTT), or alternative markers such as fructosamine may be used instead, in line with local laboratory and clinical guidance.

Lifestyle factors also play a meaningful role. Regular physical activity, a balanced diet low in refined carbohydrates, adequate sleep, and effective stress management all contribute to better glycaemic control and, consequently, lower HbA1c values over time. Conversely, a sedentary lifestyle, high-sugar dietary patterns, and psychological stress can contribute to rising levels.

It is worth noting that HbA1c does not capture short-term glucose fluctuations or episodes of hypoglycaemia, which is why it is sometimes used alongside other monitoring tools such as CGM in clinical practice. Further information on interpreting HbA1c is available from the NHS HbA1c test page and Diabetes UK.

How the NHS Uses HbA1c to Diagnose Diabetes

The NHS uses HbA1c to diagnose type 2 diabetes in non-pregnant adults, with a threshold of 48 mmol/mol or above. A result of 42–47 mmol/mol indicates non-diabetic hyperglycaemia, qualifying for the NHS Diabetes Prevention Programme.

In the United Kingdom, HbA1c testing has been used as a diagnostic tool for type 2 diabetes since 2011, following the World Health Organization (WHO) report on the use of glycated haemoglobin in the diagnosis of diabetes mellitus, and subsequent adoption by NHS England and NICE. For routine diagnosis in non-pregnant adults, HbA1c has largely replaced the oral glucose tolerance test (OGTT) owing to its convenience — no fasting is required — and its reliability as a marker of sustained hyperglycaemia. However, the OGTT remains the standard method for diagnosing gestational diabetes and is also preferred in situations where HbA1c is known to be unreliable (see above).

According to NICE guidance (NG28 and related guidance), the diagnostic thresholds are as follows:

• HbA1c of 48 mmol/mol (6.5%) or above — diagnostic of type 2 diabetes (when confirmed on a second sample in an asymptomatic person; a single result is sufficient if typical symptoms are present)

• HbA1c of 42–47 mmol/mol (6.0–6.4%) — indicates non-diabetic hyperglycaemia (NDH), representing a significantly elevated risk of progressing to type 2 diabetes

• HbA1c below 42 mmol/mol (6.0%) — considered within the normal range

For individuals in the non-diabetic hyperglycaemia range, the NHS Diabetes Prevention Programme (NHS DPP) — also known as the Healthier You programme — offers structured lifestyle intervention to reduce the risk of progression. NICE recommends that these individuals are offered annual HbA1c monitoring to detect any deterioration early.

HbA1c is not appropriate for diagnosing diabetes in certain groups, including:

• Pregnant women (OGTT per NICE NG3 is the standard approach)

• People with suspected type 1 diabetes

• Children and young people

• Those with conditions affecting red blood cell lifespan or haemoglobin variants

• Those who are acutely unwell or have recently started corticosteroid therapy

In these situations, plasma glucose measurements remain the preferred diagnostic method. Clinicians should always interpret HbA1c results in the context of the individual's full clinical picture, in line with WHO 2011 and NICE guidance.

Understanding Your HbA1c Test Results

NICE recommends individualised HbA1c targets — typically 48 mmol/mol for those on lifestyle or non-hypoglycaemic therapy, and 53 mmol/mol for those at risk of hypoglycaemia. Results should always be interpreted alongside the full clinical picture by your diabetes care team.

Receiving an HbA1c result can feel confusing, particularly if you are unfamiliar with what the numbers mean in practice. Whether you are being monitored for existing diabetes or screened as part of a routine health check, understanding your result empowers you to take an active role in your health.

For people already diagnosed with type 2 diabetes, NICE (NG28) recommends an individualised HbA1c target, typically:

• 48 mmol/mol (6.5%) for those managed by lifestyle or a single non-hypoglycaemic drug

• 53 mmol/mol (7.0%) for those on medicines that carry a risk of hypoglycaemia, such as sulphonylureas or insulin

For people with type 1 diabetes, NICE guidance (NG17) suggests aiming for an HbA1c of 48 mmol/mol (6.5%) or below if this can be achieved safely without problematic hypoglycaemia.

Targets should always be individualised. Where the risks of tight glucose control outweigh the benefits — for example in older adults, those with frailty, significant comorbidities, or a limited life expectancy — less stringent targets may be more appropriate. Your diabetes care team will discuss what is right for you.

It is equally important not to over-interpret a single result. HbA1c should be viewed as one piece of a broader clinical picture. Your GP or diabetes care team will consider your symptoms, weight, blood pressure, kidney function, and other factors alongside your HbA1c when making treatment decisions.

When to contact your GP or diabetes team:

• Your HbA1c has risen significantly since your last test

• You are experiencing symptoms of high blood glucose — such as increased thirst, frequent urination, or unexplained fatigue

• You are concerned about hypoglycaemia or your current medication regimen

• You are pregnant or planning a pregnancy, as tighter glucose control is essential

Seek urgent same-day medical attention if you or someone you know experiences symptoms that may suggest diabetic ketoacidosis (DKA) or new-onset type 1 diabetes, including abdominal pain, vomiting, drowsiness, rapid or deep breathing, or the smell of ketones on the breath. Call 999 or go to your nearest emergency department if symptoms are severe.

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If you think you have experienced a side effect from a diabetes medicine, you can report it to the Medicines and Healthcare products Regulatory Agency (MHRA) via the Yellow Card scheme at yellowcard.mhra.gov.uk or through the Yellow Card app.

Regular testing — typically every three to six months for those with diabetes, and annually for those at risk — allows timely adjustments to treatment and lifestyle, helping to reduce the long-term risk of complications. If you are unsure what your result means, always ask your healthcare provider for a clear explanation tailored to your individual circumstances. Further information is available from the NHS HbA1c test page, NICE, and Diabetes UK.

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