Pre-diabetic range HbA1c in mmol/mol is a key marker used across the NHS to identify people at risk of developing type 2 diabetes before it fully develops. In the UK, an HbA1c of 42–47 mmol/mol indicates prediabetes — also called non-diabetic hyperglycaemia — meaning blood glucose is elevated but has not yet reached the diagnostic threshold for diabetes. Understanding what this result means, how it is measured, and what steps to take next can make a significant difference to long-term health outcomes. This article explains the UK thresholds, how the NHS diagnoses and monitors prediabetes, and the lifestyle changes and support available.

What HbA1c Levels Indicate Prediabetes in the UK?

In the UK, the prediabetic HbA1c range is 42–47 mmol/mol; below 42 mmol/mol is normal, and 48 mmol/mol or above indicates type 2 diabetes requiring confirmatory testing.

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In the UK, prediabetes — also referred to as non-diabetic hyperglycaemia (NDH) — is identified through a blood test measuring glycated haemoglobin, known as HbA1c. This test reflects average blood glucose levels over the preceding two to three months, making it a reliable indicator of longer-term glucose control rather than a single-point measurement.

According to NICE and NHS England, the prediabetic range for HbA1c is 42–47 mmol/mol. A result within this range suggests that blood glucose is higher than normal but has not yet reached the threshold for a type 2 diabetes diagnosis. It is an important warning sign that warrants clinical attention and lifestyle intervention.

For context, the diagnostic thresholds are as follows:

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

• 42–47 mmol/mol — prediabetes (non-diabetic hyperglycaemia)

• 48 mmol/mol or above — indicative of type 2 diabetes (requires a second confirmatory laboratory test in asymptomatic individuals)

In addition to HbA1c, fasting plasma glucose (FPG) is also used in UK referral pathways. An FPG of 5.5–6.9 mmol/L is commonly used alongside HbA1c to identify people eligible for referral to the NHS Diabetes Prevention Programme (NHS DPP).

It is important to note that HbA1c is not suitable for diagnosing prediabetes or diabetes in all situations. It should not be used in pregnancy, in children and young people, where type 1 diabetes is suspected, or in conditions that affect red blood cell turnover or haemoglobin structure. In these circumstances, alternative tests such as fasting plasma glucose or an oral glucose tolerance test (OGTT) are used instead. Diagnostic HbA1c must always be measured from a venous blood sample analysed in an accredited laboratory — point-of-care testing (POCT) devices should not be used for diagnosis.

Prediabetes is not inevitable — with appropriate intervention, many people successfully return their HbA1c to the normal range.

Understanding HbA1c Measurements in mmol/mol

UK HbA1c results are reported in mmol/mol (IFCC units); 42–47 mmol/mol is the prediabetic range, replacing the older percentage-based system still used in some international sources.

HbA1c is measured in millimoles per mole (mmol/mol) in the UK, following the adoption of the International Federation of Clinical Chemistry (IFCC) standardised reporting units. This replaced the older percentage-based system (NGSP/DCCT units) previously used, which is still referenced in older literature and some international sources. If you have seen HbA1c expressed as a percentage — for example, 6.0% — this broadly corresponds to approximately 42 mmol/mol.

The test works by measuring the proportion of haemoglobin in red blood cells that has glucose attached to it. Because red blood cells have a lifespan of roughly 90–120 days, the HbA1c result provides a meaningful window into average blood glucose control over that period.

Understanding your result in mmol/mol is straightforward once you know the reference ranges:

• Below 42 mmol/mol — normal glucose regulation

• 42–47 mmol/mol — prediabetic range (non-diabetic hyperglycaemia)

• 48 mmol/mol and above — diabetes range

For most adults without conditions affecting red blood cell turnover, HbA1c is a well-validated test for identifying prediabetes and type 2 diabetes. However, per NICE and WHO guidance, fasting plasma glucose or an OGTT should be used when HbA1c is unreliable or not recommended — for example, in pregnancy, in children and young people, where type 1 diabetes is suspected, or when haemoglobin variants or anaemia may interfere with the result. Diagnostic HbA1c should always be performed using an accredited laboratory method, not a point-of-care device.

It is important not to compare UK mmol/mol results directly with percentage figures without using a validated conversion tool, as this can cause confusion. The NHS consistently uses mmol/mol in clinical communications, and patients are encouraged to ask their healthcare provider to explain their result clearly in these units.

How the NHS Diagnoses and Monitors Prediabetes

The NHS uses HbA1c as the primary diagnostic tool for prediabetes, with results of 42–47 mmol/mol triggering referral to the NHS Diabetes Prevention Programme and annual repeat testing.

The NHS identifies individuals at risk of prediabetes through several routes, including the NHS Health Check programme (offered to adults aged 40–74 in England), GP opportunistic testing, and referrals prompted by risk factors such as obesity, a family history of type 2 diabetes, or a previous diagnosis of gestational diabetes. The HbA1c blood test is the primary diagnostic tool used in these settings, provided it is appropriate for the individual.

When a result falls in the prediabetic range of 42–47 mmol/mol — or when fasting plasma glucose is 5.5–6.9 mmol/L — the NHS recommends referral to the NHS Diabetes Prevention Programme (NHS DPP), also known as Healthier You. This evidence-based behavioural change programme provides personalised support around diet, physical activity, and weight management, and has been shown to reduce the risk of progression to type 2 diabetes significantly. To be eligible, the result should generally have been recorded within the preceding 12 months. The programme is available in group or digital formats, and self-referral is possible in some areas.

The framework for identifying and managing non-diabetic hyperglycaemia is set out in NICE guideline PH38 (Type 2 diabetes: prevention in people at high risk), alongside NHS England's NHS DPP service specification. These emphasise a preventative approach, with GPs and practice nurses playing a central role in communicating results clearly and ensuring patients understand the significance of their HbA1c level.

Monitoring frequency for people with prediabetes typically involves a repeat HbA1c test every 12 months, though this interval may be shorter — for example, every 3–6 months — if values are rising towards the diabetes threshold or individual risk is increasing. The aims of monitoring are to:

• Track whether HbA1c is improving, stable, or worsening

• Identify progression to type 2 diabetes at the earliest opportunity

• Motivate and support ongoing lifestyle changes

All diagnostic and monitoring HbA1c tests should be performed using venous laboratory samples; point-of-care devices are not appropriate for these purposes.

Factors That Can Affect Your HbA1c Result

Iron deficiency anaemia, haemolytic anaemia, haemoglobin variants, and pregnancy can falsely raise or lower HbA1c, making alternative tests such as fasting plasma glucose more appropriate in these situations.

While HbA1c is a robust and widely used test, several physiological and clinical factors can influence the result, potentially leading to falsely elevated or falsely lowered readings. It is important for both patients and clinicians to be aware of these variables when interpreting results.

Factors that may falsely lower HbA1c:

• Haemolytic anaemia (increased red blood cell turnover reduces the time available for glucose to attach to haemoglobin)

• Recent blood transfusion

• Some haemoglobin variants, depending on the assay used

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

Factors that may falsely raise HbA1c:

• Iron deficiency anaemia (the predominant effect is to raise HbA1c, as reduced red cell turnover prolongs haemoglobin exposure to glucose; values typically fall after iron replacement)

• Vitamin B12 or folate deficiency

• Some haemoglobin variants, depending on the assay used

Situations where HbA1c is unreliable:

• Advanced chronic kidney disease (CKD) and dialysis: anaemia associated with CKD often reduces red cell survival, making HbA1c unreliable and frequently lower than expected; interpretation requires specialist or laboratory input

• Haemoglobin variants (e.g., sickle cell trait, haemoglobin C): the direction and magnitude of interference depends on the specific assay; the laboratory should be informed if a variant is suspected

In these situations, alternative tests such as fasting plasma glucose or an oral glucose tolerance test (OGTT) may be more appropriate. Note that OGTT is generally reserved for specific indications — such as pregnancy or where other tests are inconclusive — rather than routine use. Your GP or practice nurse will take these factors into account when reviewing your results.

It is also worth noting that people of South Asian, Black African, or Black Caribbean heritage are at higher risk of developing type 2 diabetes at a lower BMI and at a younger age than the general population. The diagnostic HbA1c thresholds themselves are standardised and do not differ by ethnicity, but awareness of this increased risk means that screening and preventative action may be particularly important for these groups. If you have concerns about how your background may affect your risk, discussing this with your GP is advisable.

Reducing Your HbA1c: Lifestyle Changes and NHS Support

Reducing refined carbohydrates, increasing physical activity to at least 150 minutes of moderate aerobic exercise per week, and managing weight can meaningfully lower HbA1c and reduce progression to type 2 diabetes.

The encouraging news for those with a prediabetic HbA1c result is that the condition is often reversible through sustained lifestyle modification. Evidence consistently shows that changes to diet, physical activity, and body weight can meaningfully reduce HbA1c and lower the risk of progression to type 2 diabetes.

Dietary changes associated with improved HbA1c include:

• Reducing intake of refined carbohydrates and added sugars

• Increasing dietary fibre through wholegrains, vegetables, and legumes

• Choosing lower glycaemic index (GI) foods

• Reducing portion sizes and overall caloric intake if overweight

Physical activity plays an equally important role. The UK Chief Medical Officers' physical activity guidelines recommend at least 150 minutes of moderate-intensity aerobic activity per week (such as brisk walking, cycling, or swimming), or 75 minutes of vigorous-intensity activity, alongside muscle-strengthening activities on two or more days per week. Reducing prolonged sedentary time is also beneficial. Even modest increases in daily movement can have a measurable impact on blood glucose regulation.

Weight management is particularly significant — losing 5–10% of body weight in those who are overweight has been shown to substantially reduce HbA1c and diabetes risk.

For people at high risk where lifestyle change alone is insufficient or not possible, NICE guideline PH38 indicates that a GP may consider metformin as an option. This is a clinical decision made on an individual basis and is not appropriate for everyone.

NHS support available includes:

• Healthier You NHS DPP — a free behavioural change programme to prevent progression to type 2 diabetes, available via GP referral or self-referral in some areas, in group or digital formats

• NHS Better Health online tools and apps

• Referral to a registered dietitian for personalised dietary advice

Small, consistent changes are more sustainable than drastic short-term measures, and healthcare professionals can help tailor a realistic plan to individual circumstances.

When to Seek Further Advice From Your GP

Contact your GP promptly if your HbA1c is 42 mmol/mol or above without follow-up, or if you develop symptoms such as increased thirst, frequent urination, or unexplained fatigue suggesting worsening blood glucose.

If you have received an HbA1c result in the prediabetic range, it is important not to ignore it, even if you feel well. Prediabetes is largely asymptomatic, which is precisely why routine testing and follow-up are so valuable. Knowing your result gives you the opportunity to act before type 2 diabetes develops.

You should contact your GP or practice nurse promptly if:

• Your HbA1c result is 42 mmol/mol or above and you have not yet received a follow-up appointment

• You experience symptoms that may suggest worsening blood glucose levels, such as increased thirst, frequent urination, unexplained fatigue, or blurred vision

• You are pregnant or planning a pregnancy, as glucose management is particularly important during this time

• You have other risk factors such as cardiovascular disease, polycystic ovary syndrome (PCOS), or a strong family history of type 2 diabetes

• You are struggling to make lifestyle changes and would benefit from structured support or referral

• You have not had a repeat HbA1c test within the past 12 months following a prediabetic result

Seek same-day medical help or contact NHS 111 urgently if you or someone else develops symptoms that may suggest type 1 diabetes or acute hyperglycaemia, including rapid or unexplained weight loss, vomiting, abdominal pain, deep or rapid breathing, extreme thirst, confusion, or drowsiness. These symptoms are particularly important to act on promptly in children and young people. In such cases, a single diagnostic test result may be sufficient to confirm diabetes without waiting for a repeat test, per NICE guidance.

Remember that a prediabetic HbA1c result is not a diagnosis of diabetes — it is a signal that warrants attention and action. With the right support from NHS services and a commitment to lifestyle change, many people successfully bring their HbA1c back into the normal range. Open, honest conversations with your GP are the best starting point for understanding your individual risk and the steps you can take to protect your long-term health.

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