HbA1c less than 5% (below 31 mmol/mol) sits at the lower end of the non-diabetic reference range and, for most healthy adults, reflects efficient glucose regulation rather than a medical problem. However, a notably low result is not always straightforward to interpret — certain conditions affecting red blood cell lifespan, haemoglobin variants, and glucose-lowering medications can all influence the reading. This article explains what an HbA1c below 5% means, when it may warrant further investigation, and how UK clinicians interpret it alongside other blood tests, in line with NHS and NICE guidance.

What an HbA1c Below 5% (31 mmol/mol) Means

An HbA1c below 5% (31 mmol/mol) falls within the lower end of the non-diabetic reference range and usually reflects efficient glucose regulation, though laboratory reference intervals vary slightly and the broader clinical context must always be considered.

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HbA1c, or glycated haemoglobin, is a blood test that reflects your average blood glucose levels over the preceding two to three months. It measures the proportion of haemoglobin — the protein in red blood cells that carries oxygen — that has become bound to glucose. The result is expressed either as a percentage or in millimoles per mole (mmol/mol), following the IFCC standardisation adopted across the UK.

An HbA1c result below 5% (less than 31 mmol/mol) sits at the lower end of the non-diabetic reference interval. Most UK laboratories define the non-diabetic reference range as approximately 20–42 mmol/mol (roughly 4–6%), so a result of 31 mmol/mol is generally within — rather than below — this range, though it is towards the lower end. Reference intervals can vary slightly between laboratories, so it is always worth checking the range provided on your own results report.

For most healthy adults without diabetes, an HbA1c in the lower part of the normal range simply reflects efficient glucose regulation. However, a notably low result may prompt clinicians to consider whether underlying factors — such as conditions affecting red blood cell turnover or certain assay interferences — are influencing the measurement. Understanding what this result means requires looking beyond the number itself and considering the broader clinical picture.

Is a Very Low HbA1c a Cause for Concern?

A very low HbA1c is not inherently dangerous for most people, but in those treated for diabetes it may indicate over-treatment and hypoglycaemia risk, particularly with insulin or sulphonylureas; conditions reducing red blood cell lifespan can also produce a falsely low result.

For the majority of people, a low HbA1c result is not inherently dangerous and may simply reflect a healthy metabolic profile, a plant-rich or low-carbohydrate diet, regular physical activity, or a naturally efficient glucose metabolism. However, in certain contexts, a persistently low HbA1c can signal an underlying issue that merits further investigation.

One important consideration is that a very low HbA1c may not always accurately reflect true blood glucose control. Conditions that shorten red blood cell lifespan — such as haemolytic anaemia or recent blood transfusion — can artificially lower the HbA1c reading. In these situations, the result may be misleadingly low, not because blood glucose is genuinely reduced, but because glucose has had less time to bind to haemoglobin.

In people being treated for type 1 or type 2 diabetes, an HbA1c below 5% (31 mmol/mol) may indicate over-treatment, raising the risk of hypoglycaemia (abnormally low blood sugar). The risk of hypoglycaemia is greatest with insulin and sulphonylureas (such as gliclazide). SGLT-2 inhibitors (such as dapagliflozin) used alone rarely cause hypoglycaemia, though the risk increases when they are combined with insulin or a sulphonylurea.

Symptoms of hypoglycaemia include:

• Shakiness or trembling

• Sweating and pallor

• Confusion or difficulty concentrating

• Palpitations

• In severe cases, seizure or loss of consciousness

If someone experiences severe hypoglycaemia — including confusion, seizure, or loss of consciousness — call 999 immediately. Do not leave them alone and do not give anything by mouth if they are unconscious.

NICE guidelines for diabetes management (NG28 and NG17) emphasise that treatment targets should be individualised, and that excessively tight glucose control can carry its own risks, particularly in older adults or those with cardiovascular disease. A very low HbA1c in a person on glucose-lowering medication should therefore be reviewed promptly by a clinician. If you believe a medication may have caused or contributed to hypoglycaemia, this can be reported via the MHRA Yellow Card scheme (yellowcard.mhra.gov.uk).

Conditions and Factors That Can Lower HbA1c

Haemolytic anaemia, recent blood transfusion, haemoglobin variants, pregnancy, and erythropoietin therapy are the most common causes of a falsely low HbA1c; glucose-lowering medications such as insulin and sulphonylureas can also produce a genuinely low result.

Several medical conditions and physiological factors can result in a genuinely low or artificially reduced HbA1c. Understanding these is essential for accurate clinical interpretation.

Conditions affecting red blood cell lifespan are among the most common causes of a falsely low HbA1c:

• Haemolytic anaemia — where red blood cells are destroyed prematurely, reducing the time available for glycation and producing a spuriously low result

• Recent blood transfusion — introduces donor red blood cells that have not been exposed to the patient's glucose levels, lowering the measured HbA1c

• Acute blood loss followed by recovery — rapid production of new red blood cells can similarly reduce the proportion of glycated haemoglobin

• Haemoglobin variants (e.g., sickle cell trait, thalassaemia) — these can interfere with certain HbA1c assay methods, producing unreliable results that may be falsely low or falsely high depending on the platform used

• Pregnancy — increased red blood cell turnover during pregnancy can lower HbA1c, making it a less reliable marker in this group

• Chronic kidney disease (CKD) treated with erythropoietin — erythropoietin therapy stimulates red blood cell production, increasing turnover and potentially lowering HbA1c

• Hypersplenism — accelerated red blood cell destruction by an enlarged spleen can also reduce HbA1c

A note on iron deficiency anaemia (IDA): Unlike haemolytic conditions, iron deficiency anaemia typically causes a spuriously raised HbA1c, because iron-deficient red blood cells have a longer lifespan, allowing more time for glycation. HbA1c often falls after iron repletion. IDA should therefore not be considered a cause of a low HbA1c.

Lifestyle and dietary factors may also contribute to a genuinely low HbA1c in otherwise healthy individuals:

• A low-carbohydrate or calorie-restricted diet

• High levels of physical activity, which improves insulin sensitivity

• Naturally low fasting glucose levels

Medications are another important consideration. Glucose-lowering drugs used in diabetes management — particularly insulin and sulphonylureas (such as gliclazide) — can reduce HbA1c significantly and carry a meaningful risk of hypoglycaemia if the dose is too high or dietary intake is insufficient. SGLT-2 inhibitors alone are less likely to cause hypoglycaemia but may do so in combination with insulin or sulphonylureas. Clinicians should review the medication regimen if a very low result is recorded in a patient receiving these treatments.

When to Speak to Your GP About Your HbA1c Result

Speak to your GP if a low HbA1c is accompanied by recurrent dizziness, shakiness, pallor, or unexplained fatigue; those on diabetes medication should not adjust their dose without first consulting their diabetes care team.

If you have received an HbA1c result below 5% (31 mmol/mol) and are not currently being treated for diabetes, it is reasonable to discuss the result with your GP, particularly if you have been experiencing symptoms that might suggest low blood sugar or an underlying health condition. In many cases, a low result in an otherwise well individual will not require further action, but your GP can help contextualise the finding.

You should contact your GP promptly if you are experiencing any of the following alongside a low HbA1c:

• Recurrent episodes of dizziness, shakiness, or sweating, especially before meals

• Unexplained fatigue or weakness

• Pallor or breathlessness that may suggest anaemia

• Unintentional weight loss

• A known haemoglobin disorder or family history of one

If you or someone with you experiences severe hypoglycaemia — including seizure, loss of consciousness, or inability to swallow safely — call 999 immediately. Do not attempt to give food or drink to someone who is unconscious.

If you are already being treated for type 1 or type 2 diabetes and your HbA1c has fallen below your agreed target range, do not adjust your medication without speaking to your diabetes care team first. Your GP or diabetes nurse can review your treatment plan and make any necessary adjustments safely.

For people who are pregnant, HbA1c is not the primary monitoring tool recommended by NICE (NG3), as physiological changes — including increased red blood cell turnover — make it less reliable, particularly in the second and third trimesters. HbA1c is also not used to diagnose gestational diabetes; an oral glucose tolerance test (OGTT) is the recommended approach. If you are pregnant and concerned about your blood glucose levels, speak to your midwife or obstetric team.

HbA1c should also not be used as the sole diagnostic test in children and young people, those with suspected type 1 diabetes or rapid-onset hyperglycaemia, or those with haemoglobinopathies or significant anaemia. In these situations, fasting plasma glucose or an OGTT is more appropriate. Always discuss your results with your healthcare team before drawing conclusions or making changes to your lifestyle or medication.

How HbA1c Is Interpreted Alongside Other Blood Tests

A low HbA1c is routinely interpreted alongside a full blood count, fasting plasma glucose, reticulocyte count, and — where HbA1c is unreliable — fructosamine or an OGTT, to ensure accurate assessment of glycaemic control.

HbA1c is a valuable but not infallible marker of blood glucose control. Because it can be influenced by factors unrelated to glucose metabolism — such as red blood cell disorders, haemoglobin variants, and certain medications — clinicians routinely interpret it alongside other investigations to build a complete clinical picture.

When a low HbA1c is identified, the following tests may be considered:

• Full blood count (FBC) — to assess for anaemia, including haemolytic anaemia or iron deficiency, which can affect HbA1c accuracy

• Fasting plasma glucose — a direct measure of blood glucose at a single point in time, useful for cross-referencing the HbA1c result

• Oral glucose tolerance test (OGTT) — recommended when HbA1c may be unreliable due to haemoglobinopathy, anaemia, or pregnancy

• Haemoglobin electrophoresis — if a haemoglobin variant is suspected, this test can identify abnormal forms that may interfere with HbA1c assays

• Reticulocyte count — elevated levels suggest increased red blood cell turnover, which may explain a falsely low HbA1c

• Fructosamine or glycated albumin — alternative markers of medium-term glycaemic control that are not affected by red blood cell lifespan; useful when HbA1c is unreliable

• Continuous glucose monitoring (CGM) — in people with diabetes, CGM provides a more granular view of glucose fluctuations and can reveal hypoglycaemic episodes that an HbA1c alone might not capture

UKAS-accredited NHS laboratories use IFCC-aligned methods for HbA1c measurement, with external quality assurance provided through schemes such as UK NEQAS. However, not all assay platforms perform equally well in the presence of haemoglobin variants. If there is clinical suspicion of a variant, the laboratory should be informed so that an appropriate method can be selected. Interpreting HbA1c in isolation, without considering the patient's full clinical and haematological context, risks both under- and over-diagnosis of glycaemic abnormalities.

Understanding Your HbA1c Result: NHS Guidance

NHS and NICE guidance classifies an HbA1c below 42 mmol/mol as within the non-diabetic range; a result below 31 mmol/mol requires no treatment in most people, but HbA1c is not appropriate as a diagnostic test in pregnancy, children, or those with haemoglobinopathies.

The NHS uses HbA1c as a key diagnostic and monitoring tool for diabetes and non-diabetic hyperglycaemia (NDH). According to NHS and NICE guidance, the following thresholds are used in clinical practice:

• Below 42 mmol/mol (6%) — within the non-diabetic range; diabetes is unlikely

• 42–47 mmol/mol (6.0–6.4%) — non-diabetic hyperglycaemia (NDH), sometimes referred to as 'prediabetes'; indicates increased risk of developing type 2 diabetes

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

A result below 31 mmol/mol (5%) therefore sits at the lower end of the non-diabetic reference interval. For most people without diabetes, this is not a cause for concern and does not require treatment. However, as outlined throughout this article, context is everything.

It is important to note that HbA1c is not appropriate as a diagnostic test in the following situations:

• Children and young people

• Pregnancy (including for the diagnosis of gestational diabetes, where an OGTT is used instead)

• Suspected type 1 diabetes or rapid-onset hyperglycaemia

• People with haemoglobinopathies, haemolytic anaemia, or other conditions affecting red blood cell lifespan In these circumstances, fasting plasma glucose or an OGTT should be used, in line with NICE (NG28, NG3) and WHO 2011 guidance.

The NHS recommends that HbA1c testing is carried out in UKAS-accredited laboratories using standardised methods, and that results are always interpreted by a qualified clinician who can account for individual factors. Patients are encouraged to ask their GP or practice nurse to explain their results clearly, including what the number means for their specific situation.

If you are managing a long-term condition such as diabetes, your HbA1c will typically be checked every three to six months as part of your annual diabetes review. The goal is not simply to achieve the lowest possible number, but to maintain a target range that balances effective glucose control with quality of life and safety. NICE guidance (NG28, NG17) consistently emphasises individualised care — recognising that the right HbA1c target for one person may not be appropriate for another. Always discuss your results with your healthcare team before drawing conclusions or making changes to your lifestyle or medication.

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