Anaemia and HbA1c levels are closely linked in ways that can significantly affect the accuracy of diabetes diagnosis and monitoring. HbA1c — the standard blood test used to assess average blood glucose over two to three months — relies on the normal lifespan and structure of red blood cells. When anaemia is present, red cell turnover and haemoglobin availability are altered, causing HbA1c results to read either falsely high or falsely low depending on the type of anaemia. Understanding this interference is essential for clinicians and patients alike, helping to ensure that diabetes is neither missed nor incorrectly diagnosed in those with underlying blood disorders.

How Anaemia Affects HbA1c Test Results

Anaemia distorts HbA1c readings by altering red blood cell turnover — haemolytic anaemia and blood loss cause falsely low results, while iron deficiency is associated with falsely elevated HbA1c.

HbA1c (glycated haemoglobin) is a widely used blood test that reflects average blood 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 become chemically bound to glucose. Because this process occurs continuously over the lifespan of a red blood cell (approximately 120 days), the test provides a reliable long-term picture of glycaemic control in most people.

However, anaemia can significantly distort HbA1c readings, either falsely raising or lowering them depending on the underlying mechanism. The key issue lies in how anaemia alters red blood cell turnover and haemoglobin availability. When red blood cells are destroyed or lost more rapidly than usual — for example in haemolytic anaemia, following acute blood loss, after a recent blood transfusion, or during erythropoietin (EPO) therapy — they spend less time in circulation, meaning there is less opportunity for glucose to bind to haemoglobin. This can produce a falsely low HbA1c, potentially masking poor glycaemic control.

Conversely, certain conditions associated with anaemia — such as iron deficiency — are associated with a falsely elevated HbA1c. Studies consistently show that HbA1c often falls measurably after iron repletion, even without changes in blood glucose. The precise mechanism is not fully established; proposed explanations include a shift towards an older red blood cell population and altered oxidative stress affecting glycation, but this remains an area of ongoing research. Clinicians should therefore treat the mechanism as not yet definitively confirmed.

This bidirectional interference makes it essential for clinicians to consider a patient's haematological status before interpreting HbA1c results in isolation. Misinterpretation of HbA1c in the context of anaemia can lead to:

• Underdiagnosis of diabetes or poor glycaemic control

• Overdiagnosis or unnecessary intensification of diabetes treatment

• Delayed identification of the underlying anaemia itself

Types of Anaemia Most Likely to Alter HbA1c Readings

Iron deficiency anaemia falsely raises HbA1c, haemolytic anaemias falsely lower it, and haemoglobin variants such as sickle cell disease and thalassaemia can produce unreliable or undetectable results depending on the assay used.

Not all forms of anaemia affect HbA1c to the same degree. The type and mechanism of anaemia are critical in determining the direction and magnitude of any distortion.

Iron deficiency anaemia is the most common form globally and is associated with falsely elevated HbA1c levels. Although the exact mechanism is not fully established, HbA1c frequently falls after iron repletion even without changes in blood glucose, confirming a clinically meaningful interference. Clinicians should be alert to this effect when interpreting HbA1c in patients with known or suspected iron deficiency.

Haemolytic anaemias — including those caused by autoimmune conditions, hereditary spherocytosis, or glucose-6-phosphate dehydrogenase (G6PD) deficiency — typically cause falsely low HbA1c readings. Because red blood cells are destroyed prematurely, there is insufficient time for glycation to accumulate, leading to an underestimate of true glycaemic exposure.

Haemoglobin variants, such as those seen in sickle cell disease (HbS) and thalassaemia, present a more complex challenge. The degree of interference depends on the specific variant and the HbA1c assay method used. The IFCC-standardised assays used in the UK are more robust than older methods, but interference can still occur with certain variants and is assay-dependent. Clinicians should consult their local laboratory for method-specific interference information when a haemoglobin variant is known or suspected, and laboratory reports will sometimes flag results as potentially unreliable.

Vitamin B12 and folate deficiency anaemias may also influence HbA1c, though the evidence is less consistent. These deficiencies affect red cell production and maturation, potentially altering cell lifespan and glycation dynamics.

Chronic kidney disease (CKD)-related anaemia has variable effects on HbA1c. Reduced red blood cell survival in CKD tends to lower HbA1c, and EPO therapy or increased reticulocytosis further increases red cell turnover, compounding this effect. Older assay methods were also susceptible to interference from carbamylated haemoglobin, though modern IFCC-standardised assays substantially reduce this problem. Clinicians should nonetheless interpret HbA1c with caution in patients with significant CKD.

In summary, the anaemia types most likely to cause clinically significant HbA1c distortion include:

• Iron deficiency anaemia (falsely elevated)

• Haemolytic anaemias (falsely low)

• Sickle cell disease and thalassaemia (unreliable or undetectable results; assay-dependent)

• CKD-related anaemia and EPO therapy (generally falsely low)

Clinical Implications for Diabetes Diagnosis and Monitoring

Anaemia-related HbA1c distortion can lead to misdiagnosis of diabetes, inappropriate treatment intensification, or missed poor glycaemic control, making it essential to interpret HbA1c alongside the full clinical picture.

The interference of anaemia with HbA1c has meaningful consequences for both the diagnosis of diabetes and the ongoing monitoring of patients already living with the condition.

In the UK, NICE guidelines and the Public Health England/Diabetes UK consensus (implementing the WHO 2011 recommendations) recommend an HbA1c threshold of 48 mmol/mol (6.5%) or above for the diagnosis of type 2 diabetes in asymptomatic adults. Importantly, in the absence of symptoms, a single raised HbA1c result is not sufficient — a second confirmatory test (either a repeat HbA1c or a plasma glucose measurement) is required before a diagnosis of diabetes is made. Where classic symptoms of hyperglycaemia are present, a random plasma glucose of 11.1 mmol/L or above is itself diagnostic and does not require confirmation.

If anaemia is present and causing a falsely elevated HbA1c, a patient without diabetes could potentially be misdiagnosed — leading to unnecessary treatment, patient anxiety, and inappropriate clinical pathways. Equally concerning is the scenario where haemolytic anaemia produces a falsely low HbA1c in a person with poorly controlled diabetes, causing a clinician to incorrectly conclude that glycaemic management is satisfactory when blood glucose levels may in fact be significantly elevated.

For patients already diagnosed with diabetes, HbA1c is used to guide treatment decisions, including the initiation or intensification of glucose-lowering therapies. Distorted results in the context of anaemia can therefore lead to:

• Under-treatment of hyperglycaemia, increasing the risk of long-term complications such as nephropathy, retinopathy, and neuropathy

• Over-treatment, raising the risk of hypoglycaemia, particularly in older or frail patients

• Inaccurate assessment of treatment efficacy, complicating shared decision-making

Clinicians should always consider the patient's full clinical picture — including recent full blood count, haematinics, and any known haemoglobinopathy — before acting on an HbA1c result that appears inconsistent with other clinical findings or self-monitored blood glucose readings.

NICE and UK Clinical Guidance on HbA1c Interpretation in Anaemia

NICE explicitly states HbA1c should not be used for diabetes diagnosis in patients with haemoglobinopathies, active haemolysis, recent transfusion, or untreated iron, B12, or folate deficiency.

NICE and authoritative UK clinical bodies acknowledge that HbA1c may not be a reliable diagnostic or monitoring tool in certain clinical circumstances, including the presence of anaemia and haemoglobin disorders. NICE guideline NG28 (Type 2 diabetes in adults) and the Public Health England/Diabetes UK consensus on the use of HbA1c for diagnosis explicitly state that HbA1c should not be used for diagnosis in conditions that affect red cell turnover or haemoglobin structure. The Royal College of Pathologists (RCPath) and the Association for Clinical Biochemistry and Laboratory Medicine (ACB) provide additional laboratory-level guidance on assay-specific interferences.

Key situations in which HbA1c is not appropriate for diagnosing diabetes include:

• Known haemoglobinopathies (e.g., sickle cell trait, HbC, thalassaemia)

• Active haemolysis of any cause

• Recent significant blood loss or transfusion (within the preceding three months)

• Iron, B12, or folate deficiency that has not yet been treated

• Pregnancy — HbA1c must not be used to diagnose gestational diabetes; NICE guideline NG3 (Diabetes in pregnancy) recommends an oral glucose tolerance test (OGTT) at 24–28 weeks of gestation (or earlier if risk factors are identified)

• Children and young people

• Suspected type 1 diabetes

• Acutely unwell patients

In these situations, NICE recommends using alternative glycaemic markers (discussed in the following section) and ensuring that the underlying haematological condition is investigated and managed concurrently. Clinical judgement remains essential — an HbA1c result should always be interpreted alongside the patient's symptoms, plasma glucose measurements, and other relevant investigations.

The UK uses the IFCC-standardised method for HbA1c measurement, which is more specific than older DCCT-aligned methods and less susceptible to interference from some haemoglobin variants. Nevertheless, no assay is entirely immune to the effects of significant anaemia. Clinicians should consult their local laboratory regarding method-specific interference flags, particularly when a haemoglobin variant is known or suspected.

Alternative Tests for Monitoring Blood Sugar in Anaemic Patients

Fasting plasma glucose and OGTT are the recommended diagnostic alternatives when HbA1c is unreliable; fructosamine and glycated albumin offer haemoglobin-independent monitoring options, though neither is standardised for routine NHS use.

When HbA1c is deemed unreliable due to anaemia or a haemoglobin disorder, clinicians have several alternative tools available to assess glycaemic status. The choice of alternative depends on the clinical context, the type of anaemia, and the purpose of testing (diagnosis versus ongoing monitoring).

Fasting plasma glucose (FPG) and oral glucose tolerance testing (OGTT) are the recommended alternatives for diagnosing diabetes when HbA1c is unreliable, in line with WHO 2011 criteria adopted in the UK. These tests measure actual blood glucose concentrations and are not affected by haemoglobin structure or red cell lifespan. A fasting glucose of 7.0 mmol/L or above, or a two-hour post-load glucose of 11.1 mmol/L or above on OGTT, is diagnostic of diabetes. As noted above, in asymptomatic individuals a confirmatory second test is required.

Fructosamine reflects average blood glucose over the preceding two to three weeks by measuring glycated serum proteins (primarily albumin). Because it is independent of haemoglobin, it is unaffected by anaemia or haemoglobin variants. However, fructosamine is influenced by conditions that alter serum albumin levels (such as liver disease or nephrotic syndrome), and it is not currently standardised or routinely commissioned across the NHS. Its use should be guided by specialist advice.

Glycated albumin offers a similar shorter-term glycaemic window (approximately two to four weeks) and is likewise unaffected by red cell disorders. It is used more routinely in some countries but is not yet part of standard NHS practice and is not currently standardised for routine UK use.

Continuous glucose monitoring (CGM) and self-monitored blood glucose (SMBG) provide real-time glucose data and can be invaluable in patients where HbA1c is unreliable, offering detailed insight into glucose variability, time in range, and hypoglycaemic episodes. However, it is important to note that CGM and SMBG are not accepted as diagnostic tests for diabetes; diagnosis must be based on plasma glucose measurements (FPG or OGTT) or, where appropriate, HbA1c.

If you are taking glucose-lowering medicines and experience symptoms that may represent a side effect — including hypoglycaemia — you or your healthcare professional can report this via the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk.

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When to Seek Further Assessment From Your GP or Specialist

Patients with anaemia and diabetes should seek GP or specialist review if HbA1c results are inconsistent with home glucose readings, a new haemoglobin disorder is identified, or a recent transfusion or blood loss may have invalidated results.

If you have been diagnosed with anaemia — or if you are living with diabetes and have recently been told your blood count is abnormal — it is important to discuss with your GP or diabetes care team how this may affect the interpretation of your HbA1c results. Do not assume that a 'normal' HbA1c result means your blood sugar is well controlled if you also have a known blood disorder.

You should seek prompt assessment from your GP or specialist if you experience any of the following:

• Symptoms of anaemia alongside poorly explained changes in your HbA1c, such as unexplained fatigue, pallor, breathlessness, or dizziness

• HbA1c results that seem inconsistent with your home blood glucose readings or how you feel day to day

• A new diagnosis of a haemoglobin disorder (e.g., sickle cell trait, thalassaemia minor) that has not previously been factored into your diabetes monitoring plan

• Recent blood transfusion or significant blood loss, which can temporarily invalidate HbA1c results for up to three months

• Pregnancy — if you are pregnant or planning a pregnancy, HbA1c should not be used to diagnose gestational diabetes. In line with NICE guideline NG3 (Diabetes in pregnancy), testing for gestational diabetes is carried out using an OGTT, typically at 24–28 weeks of gestation or earlier if you have identified risk factors. Speak to your midwife or obstetric team about the appropriate screening pathway.

Your GP may refer you to a haematologist if a complex haemoglobin variant is suspected, or to a diabetes specialist (diabetologist or endocrinologist) if alternative glycaemic monitoring strategies are needed. In some cases, a joint clinic approach between haematology and diabetes services may be most appropriate. Your local laboratory can also advise on whether your specific HbA1c assay is likely to be affected by a known haemoglobin variant.

Ultimately, the relationship between anaemia and HbA1c levels underscores the importance of holistic, individualised care. No single test should be interpreted in isolation, and open communication between patients and their clinical teams is essential to ensure accurate diagnosis and safe, effective management of both conditions.

Frequently Asked Questions