Can an HbA1c test be wrong? While the HbA1c test is one of the most reliable tools for diagnosing type 2 diabetes and monitoring long-term blood glucose control, it is not infallible. Certain medical conditions, physiological factors, and technical variables can cause results to appear falsely high or falsely low — meaning the reading may not accurately reflect your true average blood sugar levels. Understanding when and why HbA1c results can be misleading is essential for safe diagnosis and appropriate management, and helps both patients and clinicians make informed decisions about further testing.

How Accurate Is the HbA1c Test?

HbA1c is highly reliable when standardised correctly, but biological and technical factors can cause false results; it should always be interpreted alongside the full clinical picture, not in isolation.

The HbA1c test — formally known as glycated haemoglobin — is widely regarded as one of the most reliable tools for diagnosing type 2 diabetes and monitoring long-term blood glucose control. It works by measuring the proportion of haemoglobin molecules in the blood that have glucose attached to them, reflecting average blood sugar levels over the preceding two to three months. Because it captures a longer-term picture of glycaemia rather than a single moment in time, it complements rather than replaces acute glucose measurements such as fasting or random plasma glucose tests, each of which answers a different clinical question.

In the UK, HbA1c results are reported in IFCC units (mmol/mol), sometimes shown alongside the older percentage format. The key diagnostic thresholds are:

• ≥48 mmol/mol (6.5%) — diagnostic of diabetes (when confirmed appropriately; see below)

• 42–47 mmol/mol (6.0–6.4%) — indicates high risk of diabetes (sometimes called non-diabetic hyperglycaemia or prediabetes)

Modern HbA1c assays used in accredited UK laboratories are standardised against the International Federation of Clinical Chemistry (IFCC) reference system, with national guidance from the Association for Clinical Biochemistry and Laboratory Medicine (ACB) and the Royal College of Pathologists (RCPath). External quality assurance is provided through UK NEQAS, which helps ensure consistency across different testing sites.

That said, like all laboratory investigations, the HbA1c test is not infallible. Certain biological, physiological, and technical factors can cause the result to appear falsely elevated or falsely lowered — meaning the reading does not accurately reflect true average blood glucose levels. Understanding these limitations is important for both safe diagnosis and appropriate ongoing management. A single HbA1c result should always be interpreted in the context of the individual's full clinical picture, including symptoms, other test results, and relevant medical history.

Common Factors That Can Affect HbA1c Results

Recent blood loss, haemolytic anaemia, pregnancy, untreated iron deficiency, and certain medicines can all falsely lower or raise HbA1c by altering red blood cell lifespan or interfering with the assay.

Several physiological and clinical factors can influence HbA1c readings by altering red blood cell lifespan or interfering with the assay itself. Being aware of these helps both patients and clinicians interpret results more accurately.

Factors that may falsely lower HbA1c:

• Recent blood loss or blood donation: Losing red blood cells and replacing them with newer cells (which have had less time to accumulate glucose) can reduce the measured HbA1c.

• Haemolytic anaemia: Conditions that cause red blood cells to break down more rapidly than normal shorten their lifespan, meaning less time for glucose to attach — producing an artificially low reading.

• Pregnancy: Particularly in the second and third trimesters, increased red blood cell turnover can lower HbA1c. This is one reason HbA1c is not used to screen for or diagnose gestational diabetes in the UK; a 75 g oral glucose tolerance test (OGTT) is the standard approach (NICE NG3).

• Iron deficiency anaemia (after starting iron treatment): Commencing iron supplementation stimulates production of new red blood cells, which can lower a previously elevated HbA1c as the red cell population changes.

• Certain medicines: Drugs that cause haemolysis or stimulate red blood cell production can lower HbA1c. Examples with reasonable evidence include dapsone, ribavirin, interferon-alfa, and erythropoietin (epoetin). High-dose vitamin C or vitamin E may interfere with some assay methods.

Factors that may falsely raise HbA1c:

• Iron deficiency anaemia (untreated): When red blood cells survive longer than usual due to reduced production, they accumulate more glucose over time, potentially inflating the result. Treating the iron deficiency often brings the HbA1c down.

• Vitamin B12 or folate deficiency: These can similarly affect red blood cell turnover and lifespan, potentially raising HbA1c.

• Chronic kidney disease (CKD): Advanced CKD is associated with shortened red blood cell survival and the accumulation of carbamylated haemoglobin, which can interfere with some assay methods and produce misleading results in either direction. The effect of simple haemoconcentration (e.g., dehydration) on HbA1c is not well established, as HbA1c is a proportion of haemoglobin rather than an absolute concentration.

• Hypothyroidism: Reduced red blood cell turnover prolongs red cell lifespan, which may raise HbA1c independently of blood glucose levels.

• Certain medicines: Hydroxyurea (hydroxycarbamide) can affect HbA1c through its effects on haemoglobin and red cell production.

It is worth noting that age and ethnicity can influence baseline HbA1c values to a modest degree; however, UK diagnostic thresholds (≥48 mmol/mol) are not adjusted by ethnicity, and the same cut-offs apply across all groups.

Medical Conditions That May Cause a Misleading HbA1c Reading

Haemoglobin variants, advanced chronic kidney disease, recent blood transfusion, liver disease, and thyroid disorders are well-documented causes of clinically significant HbA1c inaccuracy.

Beyond everyday physiological factors, a number of specific medical conditions are well-documented to interfere with HbA1c measurement in clinically significant ways. The most important relate to abnormalities in haemoglobin structure or red blood cell lifespan.

Haemoglobin variants are among the most significant causes of inaccurate HbA1c results. Conditions such as sickle cell trait, sickle cell disease, haemoglobin C disease, and thalassaemia involve structural changes to haemoglobin molecules. Depending on the specific variant and the laboratory method used, these can cause HbA1c to be falsely elevated, falsely lowered, or even undetectable. This is particularly relevant in the UK, where haemoglobin variants are more prevalent in individuals of African, Caribbean, South Asian, and Mediterranean heritage. NICE guidance explicitly acknowledges that HbA1c may be unreliable in people with haemoglobin variants, and laboratories should flag potential interference when a variant is known or suspected.

Chronic kidney disease (CKD): Advanced CKD is associated with shortened red blood cell survival and the presence of carbamylated haemoglobin, which can interfere with some assay methods, potentially producing misleading results in either direction. Patients receiving haemodialysis or erythropoietin (epoetin) therapy are particularly affected, and alternative measures of glycaemic control are often more appropriate in this group.

Recent blood transfusion: Transfused red blood cells dilute the patient's own cells and can substantially alter HbA1c for up to two to three months, making the result unreliable for both diagnosis and monitoring.

Liver disease may affect the test through altered red blood cell metabolism and turnover, though the degree of effect varies with the severity of liver impairment.

Thyroid disorders have bidirectional effects: hyperthyroidism is associated with increased red blood cell turnover, which may lower HbA1c independently of blood glucose levels, whilst hypothyroidism may raise it through the opposite mechanism.

Splenomegaly (an enlarged spleen) can accelerate red blood cell destruction, shortening their lifespan and potentially reducing HbA1c readings. Conversely, asplenia or splenectomy may prolong red cell survival and raise HbA1c.

In all these situations, clinicians should consider whether the HbA1c result is a true reflection of glycaemic control, and alternative testing strategies may be warranted.

What the NHS and NICE Say About HbA1c Limitations

NICE and WHO advise that HbA1c should not be used to diagnose diabetes in pregnancy, suspected type 1 diabetes, children, or those with haemoglobin variants; alternative tests such as OGTT or fasting plasma glucose are recommended.

Both NHS guidance and NICE clinical guidelines acknowledge that HbA1c is not universally applicable and should not be used in isolation when certain conditions are present. NICE guideline NG28 (Type 2 diabetes in adults), NICE guideline NG17 (Type 1 diabetes in adults), and NICE guideline NG3 (Diabetes in pregnancy) all note that HbA1c may be unreliable or inappropriate in specific clinical circumstances, and that alternative diagnostic approaches should be considered. The WHO 2011 guidance on the use of HbA1c for diagnosis similarly sets out these caveats.

NICE and WHO advise that HbA1c should not be used for diagnosing diabetes in the following situations:

• People with symptoms of diabetes lasting fewer than two months, or with rapid-onset hyperglycaemia (where a plasma glucose test is more appropriate)

• Suspected type 1 diabetes at any age (urgent clinical assessment and plasma glucose testing are required)

• Children and young people

• Pregnant women (75 g OGTT is the standard approach for gestational diabetes in the UK)

• People with haemoglobin variants or conditions affecting red blood cell turnover

• People who have recently received a blood transfusion

• People with acute illness

• People with acute pancreatic damage or following pancreatectomy

• People who have recently started medicines known to cause rapid rises in blood glucose, such as corticosteroids or antipsychotics

Where there is clinical uncertainty, NICE recommends repeating the test or using an alternative measure such as fasting plasma glucose or a two-hour OGTT. Importantly, in asymptomatic individuals, a diagnosis of diabetes based on HbA1c should be confirmed with a repeat test — ideally using the same method and laboratory — before any diagnosis is made. A single elevated result in the absence of symptoms is not sufficient on its own.

UK laboratories follow IFCC standardisation for HbA1c assays, with national guidance from the ACB and RCPath, and participate in external quality assurance through UK NEQAS. Results are reported in mmol/mol (with percentage shown in parallel at some sites). Patients can be reassured that NHS laboratories are subject to rigorous quality assurance programmes, but clinical context remains essential when interpreting any result.

When Your GP May Recommend Additional or Alternative Tests

GPs may use fasting plasma glucose, OGTT, fructosamine, or continuous glucose monitoring when HbA1c is unreliable; urgent same-day assessment is required if type 1 diabetes or DKA is suspected.

If your GP has reason to believe that your HbA1c result may not accurately reflect your blood glucose levels, they have several alternative or complementary investigations available. The decision to pursue further testing will depend on your individual clinical circumstances, symptoms, and medical history.

Alternative tests your GP may consider include:

• Fasting plasma glucose: A blood test taken after an overnight fast of at least eight hours. A result of 7.0 mmol/L or above on two separate occasions (or once if symptoms are present) is diagnostic of diabetes.

• Random plasma glucose: Useful when symptoms of diabetes are present; a result of 11.1 mmol/L or above alongside classic symptoms is diagnostic. In the absence of symptoms, a confirmatory test on a separate day is required before a diagnosis can be made.

• Oral glucose tolerance test (OGTT): Involves measuring blood glucose before and two hours after consuming a standard 75 g glucose drink. This is the preferred test for diagnosing gestational diabetes in the UK (NICE NG3) and is also used in other situations where HbA1c is unreliable.

• Fructosamine test: This measures glycated proteins over a shorter period (approximately two to three weeks) and can be useful when HbA1c is unreliable — for example, in haemolytic anaemia or with haemoglobin variants.

• Continuous glucose monitoring (CGM) or self-monitored blood glucose: In people already diagnosed with diabetes, these tools can provide a more detailed picture of day-to-day glucose patterns that HbA1c alone cannot capture.

If type 1 diabetes is suspected — particularly with significant symptoms, weight loss, or ketones present — your GP should arrange urgent same-day assessment, which may include blood or urine ketone testing and prompt specialist referral. This is because type 1 diabetes can deteriorate rapidly and HbA1c is not appropriate for its diagnosis.

Your GP may also refer you to a specialist — such as a diabetologist or haematologist — if there is a suspected haemoglobin variant or complex underlying condition affecting the interpretation of your results. Open communication with your healthcare team about any relevant medical history is key to ensuring the most appropriate test is chosen.

What to Do If You Think Your HbA1c Result Is Incorrect

Speak to your GP if you suspect an inaccurate HbA1c result; seek emergency care immediately if you have symptoms of DKA such as vomiting, confusion, rapid breathing, or a fruity breath odour.

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If you have concerns about the accuracy of your HbA1c result, the most important first step is to speak with your GP or diabetes care team. Do not attempt to self-diagnose or dismiss a result without professional guidance, as both falsely reassuring and falsely alarming results carry potential risks if acted upon inappropriately.

Seek urgent or same-day medical attention if you experience any of the following, as these may indicate a serious and rapidly developing condition such as diabetic ketoacidosis (DKA) or hyperosmolar hyperglycaemic state (HHS):

• Severe thirst and passing large amounts of urine, particularly with dehydration

• Abdominal pain, nausea, or vomiting

• Drowsiness, confusion, or difficulty staying awake

• Rapid or laboured breathing

• A fruity or acetone smell on the breath

• A very high reading on a home blood glucose or ketone meter

If you are concerned about any of these symptoms, call 999 or go to your nearest emergency department immediately. Do not wait for a routine GP appointment.

Contact your GP promptly (non-urgent) if:

• You have symptoms of diabetes (such as increased thirst, frequent urination, unexplained weight loss, or fatigue) but your HbA1c result is within the normal range

• You have a known haemoglobin variant (such as sickle cell trait) and have not been informed whether this was accounted for in your result

• You have recently had a blood transfusion, started iron supplementation, or been diagnosed with anaemia

• Your result has changed significantly from a previous reading without an obvious explanation

• You are pregnant or planning to become pregnant

When you attend your appointment, it can be helpful to bring a list of all current medicines, any recent illnesses or hospital admissions, and details of any relevant family history. This will assist your GP in deciding whether a repeat HbA1c or an alternative test is appropriate.

Remember that in asymptomatic individuals, a diagnosis of diabetes based on HbA1c should be confirmed with a repeat test — preferably using the same method and laboratory — before any diagnosis is formally made. If you remain concerned after speaking with your GP, you are entitled to ask for a second opinion or a referral to a specialist diabetes service. Staying informed and engaged with your care is always encouraged.

Frequently Asked Questions