A low HbA1c level can be just as clinically significant as a high one, yet it often receives far less attention. HbA1c — glycated haemoglobin — is a blood test used across the NHS to reflect average blood glucose levels over the preceding two to three months. While most people are familiar with elevated readings as a marker of diabetes risk, a result that falls below the expected range may indicate over-treatment, an underlying medical condition, or a laboratory artefact affecting red blood cell biology. This article explains what a low HbA1c means in the UK context, its common causes, associated symptoms, and when to seek advice from your GP or diabetes care team.

Understanding HbA1c and What Your Result Means

HbA1c measures the proportion of glycated haemoglobin in red blood cells, reflecting average blood glucose over the preceding two to three months. In the UK, results are reported in mmol/mol following NHS IFCC standardisation.

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HbA1c — formally known as glycated haemoglobin — is a blood test that reflects your average blood glucose (sugar) levels over the preceding two to three months. When glucose circulates in the bloodstream, it binds to haemoglobin, the protein found inside red blood cells. Because red blood cells have a lifespan of roughly 120 days, the proportion of haemoglobin that has become glycated provides a reliable snapshot of longer-term glucose control, weighted most strongly towards the preceding eight to twelve weeks, rather than reflecting a single moment in time.

In the UK, HbA1c results are reported in millimoles per mole (mmol/mol), following the IFCC (International Federation of Clinical Chemistry) standardisation adopted by the NHS. You may occasionally see older percentage figures referenced in literature, but NHS laboratories now use mmol/mol as standard. A result is typically requested as part of a routine health check, diabetes screening, or ongoing diabetes management.

It is important to be aware that HbA1c is not appropriate for diagnosing diabetes in all situations. It should not be used to diagnose diabetes in children, during pregnancy, or where type 1 diabetes is suspected. It is also unreliable in people with haemoglobin variants (such as sickle cell trait or thalassaemia), haemolytic or iron deficiency anaemia, chronic kidney disease (CKD), or following a recent blood transfusion. In these circumstances, alternative tests — such as an oral glucose tolerance test (OGTT), fasting plasma glucose, capillary glucose profiles, or fructosamine measurement — are more appropriate.

Understanding your HbA1c result is important because both high and low values carry clinical significance. Most public awareness focuses on elevated HbA1c — a marker of poorly controlled diabetes — but a result that falls below the expected range also warrants attention. Whether you have diabetes or not, a low HbA1c reading can sometimes indicate an underlying health issue that requires further investigation. Your GP or diabetes care team is best placed to interpret your result in the context of your full medical history, current medications, and symptoms.

What Is Considered a Low HbA1c Level in the UK?

In the UK, an HbA1c below 42 mmol/mol is considered normal for adults without diabetes. For people with diabetes, a result significantly below the agreed target — especially below 48 mmol/mol on insulin or sulphonylureas — may indicate hypoglycaemia or over-treatment.

In the UK, the NHS and NICE define the following HbA1c diagnostic thresholds for adults:

• Below 42 mmol/mol (6.0%): Normal — not indicative of diabetes

• 42–47 mmol/mol (6.0–6.4%): Non-diabetic hyperglycaemia (NDH), sometimes referred to as 'prediabetes' or the 'at risk' range

• 48 mmol/mol (6.5%) or above: Diagnostic threshold for type 2 diabetes (when confirmed on a second test in the absence of symptoms, or on a single test when symptoms are present)

For people without diabetes, an HbA1c below 42 mmol/mol is generally within the normal range and is not automatically cause for concern. If a result appears unexpectedly low compared to previous readings, your GP will interpret it in the context of your clinical history and your local laboratory's reference intervals, rather than against a single fixed cut-off.

For people with diabetes, NICE guidelines recommend an individualised HbA1c target, typically between 48–58 mmol/mol (6.5–7.5%), depending on treatment regimen and individual risk factors. An HbA1c that falls significantly below the agreed target — particularly below 48 mmol/mol in someone taking insulin or sulphonylureas — may suggest episodes of hypoglycaemia (low blood sugar), over-treatment, or other physiological changes affecting red blood cell turnover.

It is important to note that HbA1c is not always a reliable measure. Haemolytic anaemia shortens red blood cell survival and typically lowers HbA1c, producing a falsely reassuring result. Iron deficiency anaemia, by contrast, tends to raise HbA1c, with levels often falling after iron replacement therapy — though the precise effect can vary by assay method. Haemoglobin variants, recent blood transfusions, CKD, and erythropoietin therapy can all affect HbA1c accuracy. In such cases, alternative monitoring methods — such as fructosamine testing, OGTT, or continuous glucose monitoring (CGM) — may be more appropriate. These limitations should also be considered when HbA1c is used for diagnosis, not only for monitoring.

Common Causes of a Low HbA1c Reading

Low HbA1c results arise either from genuinely reduced blood glucose — due to over-treatment, dietary changes, or rare conditions such as insulinoma — or from falsely low readings caused by haemolytic anaemia, CKD, haemoglobin variants, or recent blood transfusion.

A low HbA1c can arise from several different causes, which broadly fall into two categories: those related to genuinely low blood glucose levels, and those that produce a falsely low result due to factors affecting red blood cell biology.

Causes of genuinely low blood glucose (leading to low HbA1c):

• Diabetes over-treatment: Excessive doses of insulin, sulphonylureas (such as gliclazide or glibenclamide), or other glucose-lowering medications can drive blood glucose — and consequently HbA1c — below target

• Dietary changes: Significant caloric restriction, prolonged fasting, or a very low-carbohydrate diet can reduce average glucose levels

• Increased physical activity: Regular, intensive exercise improves insulin sensitivity and can lower average blood glucose

• Alcohol excess: Alcohol can suppress hepatic glucose production and contribute to hypoglycaemia, particularly in people taking insulin or sulphonylureas

• Severe hepatic disease: The liver plays a central role in glucose regulation; significant liver impairment can cause persistent low blood glucose

• Insulinoma: A rare insulin-secreting tumour of the pancreas that causes persistent hypoglycaemia

• Adrenal insufficiency or other hormonal disorders: Conditions such as Addison's disease or hypopituitarism can impair glucose regulation

• Sepsis or critical illness: Severe illness can disrupt glucose homeostasis and lead to hypoglycaemia

Causes of falsely low HbA1c (not reflecting true glucose levels):

• Haemolytic anaemia: Accelerated destruction of red blood cells shortens their lifespan, reducing the time available for glycation and producing a falsely low result

• Iron deficiency anaemia: Typically raises HbA1c rather than lowering it; levels may fall after iron replacement — this effect is assay-dependent and should be interpreted with caution

• Haemoglobin variants: Conditions such as sickle cell trait or thalassaemia may interfere with certain HbA1c assay methods

• Recent blood transfusion or acute blood loss: Introduces donor red blood cells that have not been exposed to the patient's glucose levels, or dilutes glycated cells

• Chronic kidney disease (CKD): Associated with shortened red blood cell survival and erythropoietin therapy, both of which can lower HbA1c independently of glucose control

• Erythropoietin therapy: Stimulates production of new red blood cells with less time for glycation, lowering HbA1c

• Pregnancy and the postpartum period: Physiological changes in red blood cell turnover affect HbA1c reliability

Identifying the underlying cause is essential, as management differs considerably depending on whether the low result reflects genuine hypoglycaemia or a laboratory artefact.

Symptoms and Health Risks Associated With Low HbA1c

A low HbA1c reflecting true hypoglycaemia can cause shakiness, sweating, confusion, and in severe cases seizures or loss of consciousness. Recurrent hypoglycaemia may lead to hypoglycaemia unawareness, a recognised patient safety concern.

When a low HbA1c reflects genuinely reduced average blood glucose levels, the most immediate concern is hypoglycaemia — episodes where blood glucose drops below 4 mmol/L. Hypoglycaemia can range from mild to life-threatening, and its symptoms are important to recognise:

Mild to moderate hypoglycaemia symptoms:

• Shakiness, trembling, or palpitations

• Sweating and pallor

• Hunger and nausea

• Dizziness, light-headedness, or difficulty concentrating

• Irritability or mood changes

Severe hypoglycaemia symptoms:

• Confusion or disorientation

• Seizures

• Loss of consciousness

For people with diabetes, recurrent hypoglycaemia carries significant risks, including hypoglycaemia unawareness — a condition where the body's warning signals become blunted over time, making dangerous episodes harder to detect. This is a recognised patient safety concern and should be discussed promptly with a diabetes care team.

NICE guidance (NG28 and NG17) emphasises the importance of individualising HbA1c targets, particularly in older adults, those with frailty, or those with multiple long-term conditions, where the priority is to avoid hypoglycaemia rather than to achieve the lowest possible HbA1c. Observational studies have suggested a U-shaped association between HbA1c and cardiovascular outcomes — meaning both very high and very low values may be associated with poorer outcomes — but this evidence is observational and does not establish causation. There is currently no definitive NICE or NHS guidance linking low HbA1c directly to specific long-term outcomes in people without diabetes.

In people without diabetes, a persistently low HbA1c — particularly when accompanied by symptoms of low blood sugar — warrants investigation to exclude rare but serious conditions such as insulinoma or adrenal insufficiency. Prompt assessment helps ensure that any underlying cause is identified and appropriately managed.

If you are taking insulin or a sulphonylurea and experience symptoms of hypoglycaemia, you can report suspected side effects to the MHRA Yellow Card scheme (available at yellowcard.mhra.gov.uk). This helps the MHRA monitor the safety of medicines used in the UK.

When to Speak to Your GP About Your HbA1c Result

Contact your GP if your HbA1c is unexpectedly low, you experience recurrent hypoglycaemia symptoms, or your result has dropped below your agreed target without explanation. Call 999 or attend A&E for severe hypoglycaemia involving loss of consciousness or seizures.

Knowing when to seek medical advice about your HbA1c result is an important aspect of self-care. As a general guide, you should contact your GP or diabetes care team if:

• Your HbA1c result is unexpectedly low compared to previous readings, particularly if you have diabetes

• You are experiencing symptoms consistent with hypoglycaemia, such as shakiness, sweating, confusion, or palpitations — especially if these occur regularly or without obvious cause

• You have diabetes and your HbA1c has dropped below your agreed target range without a clear explanation (such as a planned dietary change or medication adjustment)

• You do not have diabetes but have received a low HbA1c result alongside symptoms such as persistent fatigue, dizziness, or unexplained weight loss

• You are taking medications known to lower blood glucose (insulin, sulphonylureas) and are concerned about the frequency or severity of hypoglycaemic episodes

If you are unsure whether your symptoms require urgent attention, you can call NHS 111 for advice at any time.

Seek urgent medical attention if you or someone you care for experiences severe hypoglycaemia — including loss of consciousness, seizures, or an inability to swallow safely. In such cases, call 999 or attend your nearest A&E department.

It is also worth raising any concerns about HbA1c accuracy if you have a known haematological condition, have recently had a blood transfusion, have CKD, or have been diagnosed with a haemoglobin variant. In these circumstances, your GP can arrange alternative tests — such as an OGTT, fasting plasma glucose, fructosamine measurement, or CGM — to assess glucose control more reliably. Open communication with your healthcare team ensures that your results are interpreted correctly and that any necessary adjustments to your care plan are made safely.

How HbA1c Levels Are Monitored and Managed on the NHS

NICE recommends HbA1c testing every three to six months for people with diabetes, with frequency adjusted to clinical need. Management of a low result may include medication dose reduction, dietitian referral, CGM, or specialist referral for suspected underlying conditions.

On the NHS, HbA1c monitoring frequency is guided by NICE recommendations and tailored to individual clinical need. For people with type 2 diabetes (NICE NG28), HbA1c testing is recommended every three to six months when treatment is being adjusted, and every six months once stable. For type 1 diabetes (NICE NG17), testing is recommended every three to six months, with more frequent testing if clinically indicated — for example, following a medication change or during periods of instability. People identified as having non-diabetic hyperglycaemia (NDH) are usually offered annual monitoring.

When a low HbA1c is identified in someone with diabetes, the clinical response will depend on the likely cause:

• Medication review: If over-treatment is suspected, the prescribing clinician may reduce the dose of insulin or glucose-lowering tablets, particularly sulphonylureas, which carry a recognised hypoglycaemia risk. Suspected adverse effects from diabetes medicines, including hypoglycaemia, can be reported to the MHRA Yellow Card scheme.

• Dietary and lifestyle assessment: A referral to a dietitian or structured education programme may be offered — the NHS Diabetes Prevention Programme (NDPP) for those with NDH, DESMOND for people with type 2 diabetes, or DAFNE for people with type 1 diabetes.

• Continuous glucose monitoring (CGM): NICE recommends that CGM should be offered to all adults with type 1 diabetes. For adults with type 2 diabetes on insulin, CGM or flash glucose monitoring should be considered in specified circumstances — for example, recurrent hypoglycaemia, hypoglycaemia unawareness, or an inability to self-monitor safely. CGM provides real-time glucose data that complements HbA1c results and is increasingly available on the NHS.

• Specialist referral: If an underlying condition such as insulinoma or adrenal insufficiency is suspected, referral to an endocrinologist will be arranged.

For individuals without diabetes who receive an unexpectedly low HbA1c, further blood tests — including a full blood count, iron studies, and fasting glucose — are typically arranged to investigate potential causes. Where HbA1c is considered unreliable (for example, in the presence of haemoglobinopathy, haemolysis, CKD, or recent transfusion), an OGTT, fructosamine, or capillary glucose profile may be used instead.

Key sources used in this article include NICE NG28 (Type 2 diabetes in adults: management), NICE NG17 (Type 1 diabetes in adults: diagnosis and management), the NHS HbA1c test information pages, and Diabetes UK patient resources. The NHS Long Term Plan continues to expand access to diabetes technology and structured support, ensuring that both high and low HbA1c results are managed within a comprehensive, patient-centred framework.

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