HbA1c 42 mmol/mol in percentage terms is approximately 6.0%, placing it at the lower boundary of the non-diabetic hyperglycaemia (NDH) range as defined by NICE and NHS England. This result is not a diagnosis of diabetes, but it does signal that blood glucose is running above the ideal level and that the risk of developing type 2 diabetes is elevated. Understanding what this figure means, how the two measurement units relate to each other, and what practical steps you can take is essential for protecting your long-term health. This article explains the conversion, the NHS reference ranges, and when to seek further advice from your GP.

What Does an HbA1c of 42 mmol/mol Mean in Percentage Terms?

An HbA1c of 42 mmol/mol equals approximately 6.0% and sits within the NHS non-diabetic hyperglycaemia (pre-diabetes) range of 42–47 mmol/mol, indicating elevated blood glucose risk without a diabetes diagnosis.

An HbA1c of 42 mmol/mol is equivalent to approximately 6.0% in the older percentage-based measurement system. In the UK, this result falls within the non-diabetic hyperglycaemia (NDH) range, which NICE and NHS England define as 42–47 mmol/mol (6.0–6.4%). This is sometimes referred to as the 'at risk' or 'pre-diabetes' range. It is not a diagnosis of diabetes, but it does indicate that blood glucose is higher than ideal and that the risk of developing type 2 diabetes is elevated.

HbA1c — or glycated haemoglobin — is a blood test that reflects your average blood glucose levels over the preceding two to three months. When glucose circulates in the bloodstream, it binds to haemoglobin (the protein in red blood cells that carries oxygen). The higher your blood glucose over time, the greater the proportion of haemoglobin that becomes glycated. A result of 42 mmol/mol (6.0%) therefore suggests that blood glucose has been running at the lower end of the elevated range, and that proactive steps to protect long-term health are worthwhile.

A single HbA1c result should always be interpreted alongside your overall clinical picture — including symptoms, weight, family history, and other metabolic markers. If your result is 42 mmol/mol, your GP or practice nurse will likely discuss lifestyle factors with you and may recommend a repeat test, typically after 12 months, to monitor any trend. This is not a cause for alarm, but it is a useful prompt to review diet, physical activity, and other modifiable risk factors. You may also be eligible for referral to the NHS Diabetes Prevention Programme (NHS DPP).

How HbA1c Units Are Converted: mmol/mol and Percentage Explained

The UK adopted mmol/mol reporting in 2011; 42 mmol/mol converts to approximately 6.0% using the formula % = (mmol/mol ÷ 10.929) + 2.15, with 48 mmol/mol (6.5%) marking the diagnostic threshold for type 2 diabetes.

The two units used to express HbA1c results — mmol/mol and percentage (%) — can cause confusion, particularly for patients who have received results in both formats over the years. The percentage system (NGSP/DCCT) was the original standard used in diabetes research and clinical practice for decades. In 2009, the International Federation of Clinical Chemistry (IFCC) introduced the mmol/mol system, which the UK formally adopted in 2011. Since then, NHS laboratories report HbA1c exclusively in mmol/mol, though many patients and some older resources still reference the percentage equivalent.

The conversion between the two units follows a standardised mathematical formula:

• % = (mmol/mol ÷ 10.929) + 2.15

• mmol/mol = (% − 2.15) × 10.929

Using this formula, an HbA1c of 42 mmol/mol converts to approximately 6.0%. Other common reference points include:

• 39 mmol/mol ≈ 5.7%

• 48 mmol/mol ≈ 6.5% (diagnostic threshold for type 2 diabetes)

• 53 mmol/mol ≈ 7.0%

Small rounding differences between laboratories are normal and do not affect clinical interpretation. For reliable conversions, use a validated tool such as the Diabetes UK HbA1c converter or the IFCC/NGSP conversion table, rather than performing manual calculations. When reviewing historical results or comparing with international guidelines (such as those from the American Diabetes Association, which still uses percentages), always ensure you are comparing like with like to avoid confusion about your diabetes risk status or treatment targets.

Understanding Your HbA1c Result Against NHS Reference Ranges

NICE defines normal as below 42 mmol/mol, NDH as 42–47 mmol/mol, and diabetes as 48 mmol/mol or above on a confirmatory second test; HbA1c is unreliable in pregnancy, haemoglobinopathies, and severe anaemia.

NICE (NG28) and NHS England define clear reference ranges for HbA1c to help clinicians and patients interpret results meaningfully. These ranges are:

• Below 42 mmol/mol (below 6.0%): Normal — not indicative of diabetes or non-diabetic hyperglycaemia

• 42–47 mmol/mol (6.0–6.4%): Non-diabetic hyperglycaemia (NDH) — sometimes called 'pre-diabetes' or 'at risk'

• 48 mmol/mol (6.5%) or above: Indicative of type 2 diabetes (when confirmed on a second test in the absence of symptoms)

An HbA1c of 42 mmol/mol falls within the NDH range. Individuals in this band are at significantly elevated risk of progressing to type 2 diabetes and are eligible for referral to the NHS Diabetes Prevention Programme (NHS DPP) in England. Similar structured prevention programmes are available across the devolved nations.

It is important to recognise that HbA1c has certain limitations as a diagnostic tool and should not be used in the following situations, where alternative tests such as fasting plasma glucose or an oral glucose tolerance test (OGTT) are more appropriate:

• Pregnancy (including postpartum) and suspected gestational diabetes

• Children and young people

• Suspected rapid-onset type 1 diabetes

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

• Significant anaemia (haemolytic or iron deficiency anaemia)

• Chronic kidney disease (CKD) stage 4–5

• Recent blood transfusion

In these circumstances, HbA1c may produce falsely low or falsely high results. Your GP will take these factors into account when interpreting your result and selecting the most appropriate test.

Steps to Maintain or Improve Your HbA1c Level

Reducing refined carbohydrates, achieving at least 150 minutes of moderate aerobic activity per week, and attending the NHS Diabetes Prevention Programme are evidence-based interventions that can lower HbA1c from the NDH range.

If your HbA1c is 42 mmol/mol, lifestyle modifications can be highly effective in keeping it within the normal range or reducing it further. Evidence consistently shows that even modest improvements in diet and physical activity can have a meaningful impact on blood glucose regulation and long-term metabolic health.

Dietary changes are among the most impactful interventions:

• Reduce intake of refined carbohydrates and added sugars (white bread, sugary drinks, pastries)

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

• Follow a balanced eating pattern — the NHS Eatwell Guide provides a practical framework

• Consider reducing portion sizes if you are overweight, as even a 5–10% reduction in body weight can meaningfully lower HbA1c

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), alongside muscle-strengthening activities on two or more days per week. Regular movement helps muscles use glucose more efficiently, reducing the burden on insulin-producing cells.

Other modifiable factors include:

• Stopping smoking, which is associated with insulin resistance

• Limiting alcohol intake in line with NHS guidelines (no more than 14 units per week, spread across the week)

• Managing stress, as chronic elevation of stress hormones such as cortisol can raise blood glucose

• Ensuring adequate sleep, as poor sleep quality is linked to impaired glucose metabolism

If you are referred to the NHS Diabetes Prevention Programme, attending this structured lifestyle education course has been shown in UK evaluations to reduce the risk of progressing to type 2 diabetes. Structured lifestyle programmes of this kind are recommended by NICE as an effective intervention for people with NDH.

When to Speak to Your GP About Your HbA1c Result

Speak to your GP if your HbA1c is rising towards 48 mmol/mol, you develop symptoms such as increased thirst or frequent urination, or you have additional risk factors such as a family history of type 2 diabetes or a BMI above 25 kg/m².

An HbA1c of 42 mmol/mol does not require urgent medical attention, but it merits a conversation with your GP or practice nurse — particularly if it represents a rise from a previous lower reading. Monitoring trends over time is often more informative than a single result. For most people with NDH (42–47 mmol/mol), UK practice is to repeat HbA1c annually. Earlier retesting — for example, after three to six months — may be appropriate if your result is close to 48 mmol/mol, if your risk factors change, if you start a new medicine that can affect blood glucose, or if you develop symptoms suggestive of diabetes.

Contact your GP promptly if you experience any of the following symptoms, which may suggest blood glucose levels are rising:

• Increased thirst or a persistently dry mouth

• Frequent urination, particularly at night

• Unexplained fatigue or lack of energy

• Blurred vision

• Slow-healing wounds or recurrent infections

• Unintentional weight loss

These symptoms can sometimes precede a formal diagnosis of diabetes and should not be ignored, even if a recent HbA1c appeared reassuring. It is also worth discussing your result with your GP if you have additional risk factors such as a family history of type 2 diabetes, a BMI above 25 kg/m², a history of gestational diabetes, or polycystic ovary syndrome (PCOS), as these may lower the threshold for closer monitoring or earlier intervention.

If you are taking medicines that can raise blood glucose — such as systemic corticosteroids, thiazide diuretics, or atypical antipsychotics — inform your GP, as these may influence your HbA1c trajectory and require additional monitoring. If you are pregnant, planning a pregnancy, or have recently given birth, speak to your GP or midwife, as HbA1c is not used to diagnose gestational diabetes and different assessment pathways apply.

Open communication with your healthcare team remains the most effective safeguard for your long-term metabolic health.

Frequently Asked Questions