Causes of false high HbA1c are an important consideration for anyone who receives an unexpectedly elevated result, as several medical, pharmacological, and technical factors can artificially raise this widely used diabetes test. HbA1c measures average blood glucose over two to three months by assessing glycated haemoglobin in red blood cells — but conditions such as iron deficiency anaemia, haemoglobin variants, and certain laboratory interferences can distort the reading. Understanding why a falsely elevated result occurs helps clinicians and patients avoid misdiagnosis, unnecessary treatment escalation, and undue anxiety. This article explains the key causes, when to request a repeat or alternative test, and how to discuss an unexpected result with your GP.

What Is HbA1c and How Is It Measured in the NHS?

HbA1c measures average blood glucose over 90–120 days by quantifying glycated haemoglobin; in the UK, 48 mmol/mol or above on two occasions is diagnostic of type 2 diabetes under NICE guidance (NG28).

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HbA1c, or glycated haemoglobin, is a blood test used widely across the NHS to diagnose type 2 diabetes and monitor long-term blood glucose control in people already living with the condition. When glucose circulates in the bloodstream, it binds irreversibly to haemoglobin — the protein inside red blood cells that carries oxygen. The resulting compound is haemoglobin A1c. Because red blood cells have a lifespan of approximately 90 to 120 days, the HbA1c level reflects average blood glucose concentrations over the preceding two to three months.

In the UK, HbA1c is reported in millimoles per mole (mmol/mol), following standardisation aligned with the International Federation of Clinical Chemistry (IFCC). According to NICE guidance (NG28), a result of 48 mmol/mol (6.5%) or above on two separate occasions is diagnostic of type 2 diabetes in asymptomatic individuals, while a result of 42–47 mmol/mol (6.0–6.4%) indicates non-diabetic hyperglycaemia (sometimes called prediabetes).

For diagnosis, HbA1c must be measured on a venous blood sample analysed in an accredited laboratory. Point-of-care HbA1c devices should not be used for diagnostic purposes, as their wider analytical margins increase the risk of misclassification; NICE recommends laboratory-based testing for all diagnostic decisions.

Crucially, HbA1c is not appropriate for diagnosis in all circumstances. NICE and WHO guidance identifies several groups and situations where HbA1c should not be used diagnostically and where fasting plasma glucose or an oral glucose tolerance test (OGTT) should be used instead. These include:

• Children and young people

• Pregnant women or those who have recently been pregnant

• People with suspected type 1 diabetes

• People presenting with acute illness or rapid-onset hyperglycaemia

• People with haemolytic anaemia, haemoglobinopathies, or other conditions affecting red blood cell lifespan

• People with chronic kidney disease (CKD) stage 4 or 5

• People who have received a blood transfusion within the preceding two to three months

Understanding the causes of false high HbA1c is therefore essential for both clinicians and patients to avoid misdiagnosis or inappropriate treatment escalation.

Medical Conditions That Can Cause a Falsely Elevated HbA1c

Iron deficiency anaemia is the most clinically significant cause of falsely elevated HbA1c, as prolonged red blood cell survival increases haemoglobin's exposure to glucose without any true rise in blood glucose.

Several medical conditions can interfere with HbA1c measurements by altering red blood cell turnover, haemoglobin structure, or the glycation process itself. Recognising these conditions is critical to interpreting results correctly.

Iron deficiency anaemia is one of the most clinically significant causes of falsely elevated HbA1c. When iron stores are depleted, red blood cell production slows and existing cells survive longer in the circulation. This extended lifespan means haemoglobin is exposed to glucose for a longer period, artificially increasing the measured HbA1c — even if actual blood glucose levels are normal. Studies have consistently demonstrated that correcting iron deficiency leads to a meaningful reduction in HbA1c without any change in glycaemic status.

Other conditions associated with falsely high readings include:

• Vitamin B12 or folate deficiency, which similarly prolongs red blood cell lifespan by impairing normal cell turnover

• Splenectomy (surgical removal of the spleen), which reduces red blood cell clearance and extends their circulation time

• Haemoglobin variants such as haemoglobin D or E, which may interfere with certain laboratory assay methods and produce spuriously elevated readings depending on the analytical technique used. In the UK, haemoglobin S (sickle cell trait) and haemoglobin C are among the more prevalent variants; their effect on HbA1c — whether falsely high or falsely low — depends on the specific assay method used. Laboratories should be informed if a variant is known or suspected, so that an appropriate alternative method can be selected

Chronic kidney disease (CKD) has a complex and bidirectional effect on HbA1c. In advanced CKD (stages 4–5), reduced red blood cell lifespan and treatments such as erythropoiesis-stimulating agents (ESAs) more commonly cause falsely low HbA1c. However, carbamylated haemoglobin — formed from urea in uraemia — can interfere with some immunoassay platforms and produce spuriously elevated readings; the direction of interference is therefore method-dependent. HbA1c should not be used for diagnosis in CKD stage 4–5, and its reliability for monitoring should be discussed with the treating team.

Patients with any of the above conditions who receive an unexpectedly high HbA1c should have the result interpreted cautiously. Clinicians should consider whether the underlying condition may be contributing to the reading before initiating or intensifying diabetes treatment.

Medications and Supplements That May Affect HbA1c Results

Medicines causing haemolytic anaemia, such as dapsone and ribavirin, typically lower HbA1c, while iron, B12, and folate supplementation can normalise a previously falsely elevated result by restoring red blood cell turnover.

Certain medicines and supplements can influence HbA1c values through their effects on red blood cell lifespan or haemoglobin glycation rates. It is important to understand both the direction and the mechanism of any interference.

Iron, vitamin B12, and folate supplements, when taken to treat deficiency, typically lower a previously falsely elevated HbA1c by restoring normal red blood cell turnover. Conversely, during the transitional period after starting supplementation, results may be inconsistent. HbA1c should therefore be interpreted in the context of any recent changes to supplementation.

Medicines that cause haemolytic anaemia — by accelerating red blood cell destruction — generally produce falsely low HbA1c, because cells are cleared before they accumulate significant glycation. Clinically relevant examples include:

• Dapsone, used in dermatological and infectious conditions, which can cause haemolytic anaemia and is therefore associated with spuriously low rather than high HbA1c

• Ribavirin, used in hepatitis C treatment, which similarly causes haemolytic anaemia and tends to lower measured HbA1c

Conversely, medicines that prolong red blood cell survival may contribute to falsely elevated HbA1c by extending the period during which haemoglobin is exposed to glucose. This is the mechanism underlying the effect of iron, B12, and folate repletion on previously low-turnover cells.

Some older literature has suggested that high-dose aspirin or vitamin E supplementation may influence glycation, but the evidence is weak, the clinical significance at standard therapeutic doses is not established, and neither is reflected in current UK guidance. These associations should not be used to explain an unexpected HbA1c result without further investigation.

Patients and clinicians should always disclose all current medications and supplements — including over-the-counter products and herbal remedies — when interpreting HbA1c results. If a medicine is suspected of causing an adverse effect on a blood test result, this can be reported to the MHRA via the Yellow Card Scheme (yellowcard.mhra.gov.uk).

Laboratory and Technical Factors Behind Inaccurate HbA1c Readings

Haemoglobin variants and carbamylated haemoglobin in uraemia are the most important technical causes of inaccurate HbA1c, with the direction of interference depending on the specific assay method used.

Beyond patient-related factors, the accuracy of HbA1c measurement can be affected by the analytical methods used in the laboratory and the conditions under which the sample is processed. The NHS uses a range of accredited assay platforms, and while most are highly reliable, no method is entirely immune to interference.

Haemoglobin variants represent one of the most important technical challenges. Conditions such as sickle cell trait, haemoglobin C trait, and other haemoglobinopathies can interfere with specific assay methods — either falsely elevating or falsely lowering the result depending on the technique employed. High-performance liquid chromatography (HPLC), immunoassay, and capillary electrophoresis each have different susceptibility profiles to haemoglobin variants. Laboratories accredited under the UK National External Quality Assessment Service (UK NEQAS) are expected to flag potential interference and use appropriate alternative methods.

Carbamylated haemoglobin, formed in uraemia when urea reacts with haemoglobin, can interfere with some immunoassay platforms and produce spuriously elevated HbA1c readings. This is particularly relevant in patients with advanced CKD and is another reason why HbA1c is unreliable for diagnosis or monitoring in this group.

Other technical factors that may contribute to inaccurate readings include:

• Sample storage and transport delays: HbA1c is generally stable, but prolonged storage at incorrect temperatures can affect results

• Sample haemolysis: Damaged red blood cells in the sample tube can interfere with some assay platforms

• Lipaemia or hyperbilirubinaemia: Elevated lipids or bilirubin in the blood sample may interfere with certain colorimetric assay methods

• Calibration errors or equipment malfunction: Though rare in accredited laboratories, these remain a theoretical source of error

For diagnostic purposes, point-of-care HbA1c devices should not be used. NICE guidance (NG28) is explicit that HbA1c used for diagnosis must be measured on a venous blood sample in an accredited laboratory, to minimise the risk of misclassification. Point-of-care devices may be used for monitoring in some clinical settings, but their wider analytical margins make them unsuitable for establishing a new diagnosis.

When to Request a Repeat or Alternative Test for Diabetes Diagnosis

A repeat or alternative test is indicated when HbA1c is unexpectedly high, a known interfering condition is present, or there is a discrepancy between the result and clinical presentation; fasting plasma glucose or OGTT should be used when HbA1c is unreliable.

NICE guidance (NG28) is clear that in asymptomatic individuals, a diagnosis of type 2 diabetes should not be made on the basis of a single HbA1c result alone. A repeat test is required to confirm the diagnosis, unless the individual is symptomatic of hyperglycaemia (such as polyuria, polydipsia, or unexplained weight loss), in which case a single elevated result may be sufficient alongside clinical assessment.

A repeat or alternative test should be strongly considered in the following circumstances:

• The HbA1c result is unexpectedly high in someone with no risk factors for diabetes and no symptoms

• The patient has a known condition that may cause false elevation, such as iron deficiency anaemia, a haemoglobin variant, or has recently undergone splenectomy

• The patient is taking medicines known to interfere with HbA1c assays

• There is a discrepancy between the HbA1c result and self-monitored blood glucose readings or clinical presentation

• The laboratory has flagged a potential haemoglobin variant or assay interference

• The patient falls into a group where HbA1c is not appropriate for diagnosis (see the first section of this article)

In situations where HbA1c is considered unreliable, alternative tests for diabetes diagnosis include:

• Fasting plasma glucose (≥7.0 mmol/L is diagnostic)

• Random plasma glucose (≥11.1 mmol/L in the presence of symptoms is diagnostic)

• 75 g oral glucose tolerance test (OGTT), with a two-hour plasma glucose of ≥11.1 mmol/L being diagnostic

These tests measure glucose directly and are not subject to the same haemoglobin-related interferences.

Important safety note: If someone presents with symptoms suggestive of type 1 diabetes — including rapid-onset thirst, polyuria, unexplained weight loss, or fatigue — or with features of a hyperglycaemic emergency such as diabetic ketoacidosis (DKA) or hyperosmolar hyperglycaemic state (HHS), including vomiting, drowsiness, confusion, or severe dehydration, same-day urgent assessment is required. These presentations should not be managed with routine repeat testing. Clinicians should follow local urgent referral pathways and NICE guidance (NG17 for type 1 diabetes in adults).

Clinicians should document the rationale for choosing an alternative test clearly in the patient's records, and patients should be reassured that requesting further testing reflects thorough, patient-centred care.

Talking to Your GP About an Unexpected HbA1c Result

Patients with an unexpectedly high HbA1c should inform their GP of all medications, supplements, and relevant medical history, as additional tests such as iron studies or haemoglobin electrophoresis may be needed to contextualise the result.

Receiving an unexpectedly high HbA1c result can be understandably worrying, particularly if you have no symptoms and have not previously been told you are at risk of diabetes. It is important to remember that a single result does not automatically confirm a diagnosis, and there are several legitimate medical and technical reasons why a reading may be higher than expected. Raising your concerns with your GP is always the right course of action.

Seek urgent same-day medical attention — by calling 999 or going to your nearest A&E — if you or someone you know has symptoms that may suggest a serious hyperglycaemic emergency, including vomiting, drowsiness or confusion, rapid breathing, severe thirst, or signs of dehydration. Do not wait for a routine appointment in these circumstances.

For a routine appointment about an unexpected result, it may be helpful to:

• Mention any recent illnesses, particularly anaemia, infections, or conditions affecting your blood

• Bring a list of all current medications and supplements, including over-the-counter products and herbal remedies

• Ask whether a repeat test or alternative glucose test would be appropriate given your individual circumstances

• Enquire whether your ethnic background or family history might suggest the possibility of a haemoglobin variant that could affect the result

Your GP may arrange additional blood tests, such as a full blood count, iron studies, or haemoglobin electrophoresis, to help contextualise the HbA1c result. If a haemoglobin variant is identified, your GP can request that the laboratory use an alternative assay method less susceptible to interference. In some situations — for example, if you have advanced kidney disease or a condition that makes HbA1c unreliable — your clinician may use an alternative monitoring marker such as fructosamine or glycated albumin to assess glucose control.

It is equally important not to dismiss a raised HbA1c without proper investigation. Even if a false elevation is suspected, your GP will want to rule out genuine hyperglycaemia before concluding that the result is inaccurate. Open, informed dialogue between patient and clinician — supported by an understanding of the causes of false high HbA1c — leads to the most accurate diagnosis and the safest outcomes. If you are ever uncertain about your results or feel your concerns have not been addressed, you are entitled to ask for a second opinion or a referral to a diabetes specialist.

Frequently Asked Questions