Average glucose and HbA1c are closely linked measures that together provide a comprehensive picture of blood sugar control over time. HbA1c — or glycated haemoglobin — reflects your average glucose levels across the preceding two to three months, making it an essential tool for diagnosing type 2 diabetes and monitoring long-term glycaemic management. Used routinely across NHS services and recommended by NICE, understanding what your HbA1c result means, how it relates to your average glucose, and what factors can influence it empowers you to take informed steps towards better diabetes care.

What Is HbA1c and How Does It Reflect Average Glucose?

HbA1c measures the proportion of haemoglobin with glucose attached, reflecting average glucose levels over two to three months; it is the primary NHS tool for diagnosing and monitoring type 2 diabetes, reported in mmol/mol in the UK.

HbA1c — formally known as glycated haemoglobin — is a blood test that provides a reliable estimate of your average glucose levels over the preceding two to three months. It works by measuring the proportion of haemoglobin (the oxygen-carrying protein in red blood cells) that has glucose attached to it. Because red blood cells have a lifespan of approximately 90 to 120 days, the HbA1c result reflects a sustained picture of blood glucose control rather than a single snapshot in time.

When blood glucose levels are consistently elevated, more glucose molecules bind irreversibly to haemoglobin, producing a higher HbA1c reading. This relationship between average glucose and HbA1c is well established and forms a cornerstone of diabetes diagnosis and long-term management in the UK. The NHS and NICE recommend HbA1c testing as a primary tool for diagnosing type 2 diabetes and for monitoring glycaemic control in people already living with diabetes.

Important exceptions: when HbA1c should not be used for diagnosis

HbA1c is not appropriate for diagnosing diabetes in all circumstances. NICE, WHO, and Public Health England guidance identify the following situations where alternative tests (such as fasting plasma glucose, oral glucose tolerance test, or specialist assessment) should be used instead:

• Children and young people

• Suspected type 1 diabetes or any presentation with rapid onset of symptoms

• Pregnancy or possible gestational diabetes

• Haemoglobinopathies (e.g., sickle cell disease, thalassaemia) or haemolytic anaemia

• Recent blood transfusion

• Chronic kidney disease with use of erythropoiesis-stimulating agents (ESAs)

• Acute illness or use of high-dose corticosteroids

If any of these apply to you, your GP or specialist will arrange the most appropriate alternative test.

HbA1c is expressed in two ways in clinical practice:

• mmol/mol — the IFCC (International Federation of Clinical Chemistry) unit, now standard in the UK

• % — the older NGSP unit, still referenced in some international guidelines

For example, an HbA1c of 48 mmol/mol is equivalent to approximately 6.5%. Understanding which unit your result is reported in is important when interpreting your results or comparing them with published guidance. Your GP or diabetes care team can help translate these figures into a meaningful picture of your average glucose control.

It is also worth noting that if asymptomatic, a diagnosis of type 2 diabetes based on HbA1c ≥48 mmol/mol requires confirmation on a second sample before a diagnosis is made.

Understanding Your Results: HbA1c Ranges and What They Mean

An HbA1c below 42 mmol/mol is normal, 42–47 mmol/mol indicates prediabetes, and 48 mmol/mol or above is diagnostic of type 2 diabetes when confirmed on a repeat test.

NICE and NHS England provide clear reference ranges to help clinicians and patients interpret HbA1c results in the context of diabetes risk and management. These ranges are broadly categorised as follows:

• Below 42 mmol/mol (6.0%) — considered normal; low risk of type 2 diabetes

• 42–47 mmol/mol (6.0–6.4%) — non-diabetic hyperglycaemia (sometimes called prediabetes); indicates increased risk and warrants lifestyle intervention

• 48 mmol/mol (6.5%) or above — diagnostic of type 2 diabetes when confirmed on a repeat test (or alongside symptoms), in the absence of the exceptions listed above

For people already diagnosed with diabetes, NICE guidelines (NG28) recommend an individualised HbA1c target:

• 48 mmol/mol (6.5%) for those managed by lifestyle modification or metformin alone

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

• Less stringent targets may be appropriate for older adults, those with frailty, significant comorbidities, multimorbidity, or limited life expectancy — your care team will agree a personalised target with you

NICE recommends that HbA1c is checked every 3 to 6 months when treatment is being adjusted, and every 6 months once stable at target.

Some laboratory reports and diabetes management tools convert HbA1c into an estimated average glucose (eAG), expressed in mmol/L, to give a figure that aligns more intuitively with home blood glucose monitoring readings. This is an estimate based on population data (using the ADAG formula) and is not universally reported by UK laboratories. As an approximate guide, an HbA1c of 53 mmol/mol (7.0%) corresponds to an eAG of approximately 8.5 mmol/L. However, eAG values may be unreliable when HbA1c itself is affected by the confounders described in the next section, and should be interpreted with caution.

If your HbA1c result falls in the prediabetes range, this is an important opportunity — not a cause for alarm. Evidence consistently shows that structured lifestyle changes at this stage can delay or prevent progression to type 2 diabetes. Your GP may refer you to the NHS Diabetes Prevention Programme (NDPP), which is available to adults in England with an HbA1c of 42–47 mmol/mol. Referral can be made by your GP or, in many areas, via self-referral.

Factors That Can Affect HbA1c and Average Glucose Readings

Conditions altering red blood cell lifespan — such as iron deficiency anaemia, haemolytic anaemia, haemoglobinopathies, and recent blood transfusion — can falsely raise or lower HbA1c independently of actual average glucose.

Whilst HbA1c is a robust and widely validated marker of average glucose, several physiological and clinical factors can influence its accuracy. Being aware of these 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 attachment over the lifespan of red blood cells, anything that shortens or lengthens that lifespan will alter the result:

• Iron deficiency anaemia tends to increase HbA1c independently of blood glucose levels

• Haemolytic anaemia tends to decrease HbA1c, as red blood cells are destroyed more rapidly

• Haemoglobinopathies (such as sickle cell disease or thalassaemia) can cause assay-specific interference, with the direction of effect depending on the laboratory method used

• Recent blood transfusion renders HbA1c unreliable for approximately three months, as donor red blood cells alter the measured result in an unpredictable direction

• Pregnancy — particularly in the second and third trimesters — increases red blood cell turnover, which may lower HbA1c independently of glucose levels; HbA1c is not used to diagnose gestational diabetes (see NICE NG3)

In these circumstances, alternative measures of average glucose — such as fructosamine or continuous glucose monitoring (CGM) — may be more appropriate, and your GP or specialist will advise accordingly.

Other conditions that can affect HbA1c reliability include advanced chronic kidney disease, significant liver disease, and asplenia or splenectomy.

Medications can also influence HbA1c. Hydroxycarbamide (used in sickle cell disease and certain other conditions) is a well-recognised cause of falsely low HbA1c. Erythropoiesis-stimulating agents used in chronic kidney disease may lower HbA1c by increasing red blood cell production. Dapsone has also been associated with haemolysis and altered readings. Claims that high-dose vitamin C or E supplements affect HbA1c are based on limited evidence and may be assay-dependent; if you take high-dose supplements, mention this to your GP or diabetes care team.

It is also worth noting that ethnicity can influence HbA1c levels independently of average glucose. Some studies suggest that people of South Asian, African, or Afro-Caribbean heritage may have slightly higher HbA1c values at equivalent glucose levels. However, current UK guidance does not recommend adjusting diagnostic thresholds for ethnicity; clinicians should interpret results in the full clinical context. If you have concerns about the accuracy of your HbA1c, discuss them with your GP or diabetes care team, who can arrange additional testing if clinically indicated.

How to Lower Your Average Glucose and Improve Your HbA1c

Reducing refined carbohydrates, increasing physical activity, managing weight, and using evidence-based medications such as metformin or SGLT-2 inhibitors are the most effective strategies for lowering average glucose and HbA1c.

Improving your average glucose and HbA1c involves a combination of lifestyle modification, appropriate medication management, and regular monitoring. Even modest reductions in HbA1c are associated with meaningful reductions in the risk of diabetes-related complications, including cardiovascular disease, kidney disease, and retinopathy.

Dietary changes are one of the most effective tools available. Evidence supports the following approaches:

• Reducing intake of refined carbohydrates and added sugars, which cause rapid rises in blood glucose

• Increasing dietary fibre through vegetables, pulses, and wholegrains to slow glucose absorption

• Following a Mediterranean-style diet or a low-carbohydrate diet, both of which have demonstrated HbA1c-lowering effects in clinical trials

• Maintaining regular meal timings to avoid large fluctuations in blood glucose throughout the day

Physical activity plays an equally important role. Both aerobic exercise (such as brisk walking, cycling, or swimming) and resistance training improve insulin sensitivity and help muscles utilise glucose more efficiently. In line with UK Chief Medical Officers' guidelines and NICE recommendations for adults with type 2 diabetes:

• Aim for at least 150 minutes of moderate-intensity aerobic activity per week (or 75 minutes of vigorous-intensity activity), spread across the week

• Include muscle-strengthening activities on at least 2 days per week (such as resistance exercises, yoga, or heavy gardening)

• Reduce prolonged periods of sitting or sedentary behaviour where possible

Weight management is particularly relevant, as excess adiposity — especially visceral fat — is strongly associated with insulin resistance and elevated average glucose. Even a 5–10% reduction in body weight has been shown to produce clinically significant improvements in HbA1c.

Medication plays an important role for many people. Metformin remains the first-line pharmacological agent for type 2 diabetes in the UK. In line with NICE NG28 and relevant technology appraisals, additional agents are selected based on individual clinical circumstances:

• SGLT-2 inhibitors are recommended in preference for people with established cardiovascular disease, heart failure, or chronic kidney disease, given their additional protective effects beyond glucose lowering

• GLP-1 receptor agonists are considered in specific circumstances, including when further weight reduction is needed, when HbA1c remains above target on other agents, or as part of dual or triple therapy — eligibility criteria are defined in NICE guidance

• All treatment choices should be agreed with your GP or diabetes care team, taking into account your individual circumstances, preferences, and any other health conditions

If you experience any suspected side effects from your diabetes medicines — including hypoglycaemia — you can report these to the MHRA Yellow Card scheme (available at yellowcard.mhra.gov.uk or via the Yellow Card app), in addition to informing your GP or pharmacist.

Experiencing these side effects? Our pharmacists can help you navigate them →

When to seek urgent help

Contact your GP promptly if your HbA1c remains above your agreed target despite lifestyle efforts, or if you experience unexplained weight loss or recurrent hypoglycaemia.

Seek same-day urgent assessment if you or someone you care for develops symptoms suggesting type 1 diabetes or rapid-onset hyperglycaemia (such as sudden excessive thirst, frequent urination, unexplained weight loss, or fatigue), as this requires specialist evaluation without delay.

Call 999 or go to A&E immediately if you experience symptoms that may indicate diabetic ketoacidosis (DKA), including abdominal pain, vomiting, rapid or deep breathing, confusion, or a fruity smell on the breath.

Not sure if this is normal? Chat with one of our pharmacists →

If you are pregnant and have concerns about raised blood glucose, contact your midwife, GP, or obstetric diabetes team urgently, as gestational hyperglycaemia requires prompt specialist management (NICE NG3).

Frequently Asked Questions