The glycated haemoglobin HbA1c test is one of the most important blood tests used in UK healthcare, providing a reliable measure of average blood glucose levels over the preceding two to three months. Unlike a fasting glucose test, it requires no preparation, making it straightforward to use in NHS primary care. The HbA1c test is central to diagnosing type 2 diabetes and prediabetes, monitoring established diabetes, and guiding treatment decisions. This article explains how the test works, what your results mean, NICE-recommended target ranges, factors that can affect accuracy, and what to do after receiving your result.

What Is the Glycated Haemoglobin HbA1c Test?

The HbA1c test measures the percentage of haemoglobin glycated by glucose over 90–120 days, providing a reliable indicator of longer-term blood glucose control without requiring the patient to fast.

The glycated haemoglobin HbA1c test is a blood test that measures the average level of blood glucose (sugar) over the preceding two to three months. It works by detecting the proportion of haemoglobin — the oxygen-carrying protein found inside red blood cells — that has become chemically bonded to glucose, a process known as glycation. Because red blood cells have a lifespan of approximately 90 to 120 days, the HbA1c result reflects sustained glucose exposure rather than a single point-in-time measurement, making it a particularly reliable indicator of longer-term glycaemic control.

Unlike a fasting plasma glucose test, the HbA1c test does not require the patient to fast beforehand, which makes it more convenient and easier to standardise across clinical settings. The test is performed using a venous blood sample, typically taken at a GP surgery or NHS laboratory, and results are reported in millimoles per mole (mmol/mol) in the UK, in line with the International Federation of Clinical Chemistry (IFCC) standardisation adopted by the NHS. Some UK laboratory reports may also display a percentage figure alongside the mmol/mol value; this percentage uses the older DCCT (Diabetes Control and Complications Trial) scale. The diagnostic thresholds and treatment targets described in this article refer to the IFCC mmol/mol values used in current UK practice.

The HbA1c test plays a central role in both the diagnosis and ongoing monitoring of type 2 diabetes, and is also used in the assessment of type 1 diabetes management. It is one of the most frequently requested blood tests in NHS primary care in England, reflecting its clinical importance in identifying and managing one of the country's most prevalent long-term conditions (NICE NG28). Understanding what the test measures — and what it does not — is essential for interpreting results accurately and making informed decisions about care.

When and Why the HbA1c Test Is Requested on the NHS

The NHS requests HbA1c primarily to diagnose type 2 diabetes, identify prediabetes, and monitor glycaemic control in people with established diabetes, though it is not appropriate in pregnancy, children, or acutely unwell patients.

The HbA1c test is requested in a variety of clinical contexts across NHS primary and secondary care. The most common reasons include:

• Diagnosing type 2 diabetes: NICE guideline NG28 recommends HbA1c as the preferred diagnostic test for type 2 diabetes in adults who are not acutely unwell and do not have symptoms suggestive of type 1 diabetes.

• Identifying prediabetes (non-diabetic hyperglycaemia): An HbA1c in the range of 42–47 mmol/mol indicates an elevated risk of developing type 2 diabetes and warrants lifestyle intervention and regular monitoring.

• Monitoring established diabetes: For people already diagnosed with diabetes, the HbA1c test is used routinely — typically every three to six months until glycaemic control is stable, and then every six months thereafter, in line with NICE NG28.

• Cardiovascular risk assessment: HbA1c may be included as part of a broader cardiovascular risk review, particularly in patients with hypertension, obesity, or a family history of diabetes.

GPs may also request the test opportunistically when a patient presents with symptoms that could suggest undiagnosed diabetes, such as increased thirst, frequent urination, unexplained weight loss, or persistent fatigue. Within the NHS Health Check programme for adults aged 40 to 74, HbA1c or fasting plasma glucose may be offered to those identified as being at increased risk of diabetes through a validated risk filter.

It is important to note that HbA1c is not appropriate for diagnosing diabetes in certain groups, including:

• Pregnant women (use fasting plasma glucose or oral glucose tolerance test instead)

• Children and young people

• People with suspected type 1 diabetes

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

• Individuals who are acutely unwell

• People whose glycaemia may be changing rapidly, for example those taking high-dose corticosteroids

In these circumstances, alternative tests such as fasting plasma glucose or an oral glucose tolerance test (OGTT) are preferred (NICE NG28; WHO 2011 HbA1c diagnostic criteria).

Understanding Your HbA1c Results and NICE Target Ranges

A result below 42 mmol/mol is normal, 42–47 mmol/mol indicates prediabetes, and 48 mmol/mol or above confirms type 2 diabetes; NICE recommends individualised targets of 48–53 mmol/mol for people with established diabetes.

HbA1c results in the UK are expressed in millimoles per mole (mmol/mol). Understanding where your result falls within established reference ranges is key to interpreting what it means for your health:

• Below 42 mmol/mol: Considered normal; does not indicate diabetes or prediabetes in most adults.

• 42–47 mmol/mol: Indicates non-diabetic hyperglycaemia (sometimes called prediabetes). This range signals an increased risk of developing type 2 diabetes and warrants lifestyle modification and annual monitoring.

• 48 mmol/mol or above: Diagnostic of type 2 diabetes when confirmed on a second test in an asymptomatic individual, or on a single test in someone with classic symptoms (NICE NG28; WHO 2011).

For people already living with diabetes, NICE sets individualised target ranges rather than a single universal figure:

• A target of 48 mmol/mol is recommended for most adults with type 2 diabetes managed by lifestyle or a single non-hypoglycaemic drug (NICE NG28).

• A target of 53 mmol/mol may be appropriate for those on medications that carry a risk of hypoglycaemia, such as sulphonylureas or insulin (NICE NG28).

• For people with type 1 diabetes, NICE guideline NG17 recommends a target of 48 mmol/mol, acknowledging that individual circumstances, hypoglycaemia risk, and patient preference should inform the agreed target.

It is worth emphasising that HbA1c targets should always be agreed collaboratively between the patient and their healthcare team. A lower HbA1c is not always better — overly tight control can increase the risk of hypoglycaemia, particularly in older adults, those with frailty, or those with complex health needs or limited life expectancy, where less intensive targets may be more appropriate. In pregnancy, glycaemic targets and monitoring approaches differ significantly and should be guided by a specialist team. The goal is to achieve the best possible balance between glycaemic control and quality of life.

Factors That Can Affect HbA1c Accuracy

Haemolytic anaemia, iron deficiency, haemoglobin variants, recent blood transfusion, and chronic kidney disease can all falsely raise or lower HbA1c results, requiring alternative tests when accuracy is in doubt.

Although the HbA1c test is highly reliable in most clinical situations, a number of physiological and pathological factors can affect its accuracy, potentially leading to falsely elevated or falsely low results. Clinicians and patients should be aware of these limitations when interpreting findings.

Conditions that may cause falsely low HbA1c results include:

• Haemolytic anaemia and other conditions that shorten red blood cell lifespan, as younger red blood cells have had less time to accumulate glycated haemoglobin.

• Recent blood transfusion, which introduces donor red blood cells and dilutes the patient's own glycated haemoglobin.

• Haemoglobin variants such as sickle cell trait or haemoglobin C, which can interfere with certain laboratory assay methods.

• Iron supplementation: after treatment for iron deficiency anaemia, HbA1c values may fall as new red blood cells are produced.

Conditions that may cause falsely elevated HbA1c results include:

• Iron deficiency anaemia (before treatment), as reduced red blood cell turnover can lead to older cells accumulating more glycation, raising the measured HbA1c.

• Vitamin B12 or folate deficiency, which similarly affects red blood cell lifespan.

Chronic kidney disease (CKD) can bias HbA1c results in either direction — upwards or downwards — depending on the degree of anaemia, whether erythropoietin (EPO) therapy is being used, and the specific laboratory assay method employed. HbA1c should therefore be interpreted with particular caution in people with CKD, and alternative measures considered if there is doubt about its reliability (NICE NG28).

Additionally, ethnicity may influence baseline HbA1c levels independently of blood glucose, with some research suggesting slightly higher HbA1c values in people of South Asian or African-Caribbean descent compared with white European populations at equivalent glucose levels. UK diagnostic thresholds do not currently vary by ethnicity, and clinicians apply the same cut-offs across all groups, though this context may be taken into account when interpreting borderline results.

When HbA1c accuracy is in doubt, alternative measures such as fasting plasma glucose, an oral glucose tolerance test (OGTT), continuous glucose monitoring (CGM), or fructosamine testing may be considered. Always discuss any concerns about your result with your GP or diabetes care team.

Next Steps After Your HbA1c Test Result

Next steps depend on your result: normal results may require periodic monitoring if risk factors are present, prediabetes warrants referral to the NHS Diabetes Prevention Programme, and a diabetic result prompts a management plan including lifestyle changes and likely metformin.

Receiving your HbA1c result is the beginning of a clinical conversation, not the end of one. The appropriate next steps depend on where your result falls and your individual health context.

If your result is in the normal range (below 42 mmol/mol): No immediate action is usually required. However, if you have risk factors for type 2 diabetes — such as obesity, a family history of diabetes, or a history of gestational diabetes — your GP may recommend periodic retesting, typically every one to three years.

If your result indicates non-diabetic hyperglycaemia (42–47 mmol/mol): You should be referred to or offered a structured lifestyle programme, such as the NHS Diabetes Prevention Programme (NHS DPP), which provides evidence-based support around diet, physical activity, and weight management. Your HbA1c should be rechecked annually. Modest reductions in body weight and increases in physical activity have been shown to significantly reduce the risk of progression to type 2 diabetes.

If your result is diagnostic of type 2 diabetes (48 mmol/mol or above): Your GP will discuss a management plan that may include:

• Lifestyle modification as a first-line intervention

• Metformin, the most commonly prescribed first-line medication for type 2 diabetes in the UK, which works by reducing hepatic glucose production and improving insulin sensitivity

• Referral to a diabetes structured education programme such as DESMOND

• Regular monitoring of HbA1c (every three to six months until stable, then every six months), blood pressure, kidney function, and cholesterol (NICE NG28)

Important: diagnosis and monitoring in pregnancy and in children follow different pathways and should be guided by the relevant specialist team. HbA1c is not used to diagnose or monitor diabetes in these groups in the same way as in non-pregnant adults.

Urgent red-flag advice: If you or someone you are with develops symptoms that may suggest type 1 diabetes or diabetic ketoacidosis (DKA) — including severe thirst, very frequent urination, unexplained weight loss, abdominal pain, vomiting, drowsiness, or rapid or laboured breathing — seek same-day urgent medical assessment. Do not wait for an HbA1c result, as this test is not appropriate for diagnosing or managing acute presentations. Call 999 or go to your nearest emergency department if symptoms are severe (NICE NG17).

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When to contact your GP promptly: You should seek advice if you experience symptoms of hypoglycaemia (shakiness, sweating, confusion), symptoms of hyperglycaemia (excessive thirst, frequent urination, blurred vision), or if you are unsure how to interpret your result. Never adjust diabetes medication without first consulting your healthcare team. Regular engagement with your GP or diabetes nurse is the most effective way to maintain good long-term glycaemic control and reduce the risk of complications.

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