The accuracy of the HbA1c test is a key consideration for anyone being investigated for, or living with, diabetes. This widely used blood test measures average blood glucose levels over the preceding two to three months by assessing the proportion of glycated haemoglobin in red blood cells. Whilst HbA1c is generally reliable and convenient — requiring no fasting and reflecting longer-term glycaemic control — its accuracy can be affected by a range of physiological conditions, medications, and laboratory factors. Understanding when the test is appropriate, and when alternative tests may be needed, is essential for accurate diagnosis and effective diabetes management.

How the HbA1c Test Works and What It Measures

HbA1c measures the proportion of haemoglobin with glucose attached, providing an average of blood glucose control over 2–3 months; a result of 48 mmol/mol or above is diagnostic of type 2 diabetes in adults per NICE NG28.

The HbA1c test — formally known as the glycated haemoglobin test — is a blood test used to assess 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 glucose attached to it. Because red blood cells have a lifespan of approximately 120 days, the test provides a reliable window into longer-term blood sugar control, rather than a single-point snapshot. It is worth noting that the result is weighted towards the preceding four to eight weeks, as more recently formed red cells contribute proportionally more to the measurement.

When glucose circulates in the bloodstream, it binds irreversibly to haemoglobin in a process called glycation. The higher the blood glucose levels over time, the greater the percentage of glycated haemoglobin. Results are expressed as a percentage or in millimoles per mole (mmol/mol), with the latter being the standard unit used across NHS laboratories in the UK.

According to NICE guidance (NG28), an HbA1c of 48 mmol/mol (6.5%) or above is diagnostic of type 2 diabetes in adults. In asymptomatic individuals, a second confirmatory HbA1c of 48 mmol/mol or above on a separate sample is required before a diagnosis is made. In individuals with typical symptoms of diabetes (such as polyuria, polydipsia, or unexplained weight loss), a single abnormal result may be sufficient to confirm the diagnosis. A result between 42–47 mmol/mol (6.0–6.4%) indicates non-diabetic hyperglycaemia (sometimes called prediabetes), which carries an elevated risk of progression to type 2 diabetes.

For people already diagnosed with diabetes, HbA1c is used routinely to monitor blood glucose control and guide treatment decisions. NICE recommends measuring HbA1c every three to six months until levels are stable, then every six months thereafter.

HbA1c should not be used for diagnosis in the following situations:

• Children and young people

• Pregnancy (including suspected gestational diabetes)

• Suspected type 1 diabetes or rapid-onset hyperglycaemia

• Acute illness or following acute pancreatic damage

• Conditions affecting red blood cell survival (e.g., haemolytic anaemia, haemoglobinopathies, recent blood transfusion)

In these circumstances, fasting plasma glucose (FPG) or an oral glucose tolerance test (OGTT) should be used instead, in line with NICE and WHO guidance.

Factors That Can Affect HbA1c Accuracy

Red blood cell turnover is the most significant factor affecting HbA1c accuracy; shortened lifespan causes falsely low results, whilst lengthened lifespan — as in untreated iron deficiency — can cause falsely elevated readings.

Whilst the HbA1c test is widely regarded as a robust and reproducible measure, its accuracy can be influenced by several physiological and technical factors. Understanding these variables is important for both clinicians and patients when interpreting results.

One of the most significant factors is red blood cell turnover. Because HbA1c reflects glucose attachment over the lifespan of red blood cells, anything that shortens or lengthens that lifespan will alter the result:

• Shortened red cell lifespan (e.g., haemolytic anaemia or iron deficiency anaemia being actively treated) can lead to a falsely low HbA1c, as fewer older cells are present to accumulate glycation.

• Lengthened red cell lifespan (e.g., iron deficiency anaemia before treatment, vitamin B12 or folate deficiency) may produce a falsely elevated HbA1c.

• Recent blood transfusion renders HbA1c unreliable for approximately three months, as the direction and magnitude of any bias is unpredictable. If HbA1c is required in this period, laboratory advice should be sought and an alternative test (such as fasting plasma glucose) considered.

Pregnancy is another important consideration. Physiological changes in red cell turnover — particularly in the second and third trimesters — can reduce HbA1c values, making the test unreliable for diagnosing gestational diabetes. In line with NICE NG3, a 75 g oral glucose tolerance test (OGTT) should be used to diagnose gestational diabetes mellitus (GDM); neither HbA1c nor fasting plasma glucose alone is recommended for this purpose in pregnancy.

Laboratory methodology also plays a role. Different assay methods used across NHS laboratories can produce slightly varying results, though standardisation programmes overseen by the UK National External Quality Assessment Service (UK NEQAS) help to minimise this variability. Pre-analytical factors — such as inappropriate storage or delayed transport of samples — can affect result reliability; EDTA whole blood samples are generally stable for a defined period when stored and transported correctly, in line with local laboratory and RCPath/ACB guidance. Clinicians should follow their laboratory's specimen handling instructions to ensure accurate results.

Conditions and Medications That May Skew HbA1c Results

Haemoglobin variants, chronic kidney disease, and medications such as hydroxyurea and erythropoiesis-stimulating agents can all skew HbA1c results, sometimes requiring alternative glycaemic markers such as fructosamine.

Several medical conditions and medications are known to interfere with the accuracy of HbA1c testing, and clinicians should be aware of these when interpreting results in affected patients.

Haemoglobin variants are among the most clinically significant sources of interference. Conditions such as sickle cell disease, sickle cell trait, and other haemoglobinopathies (e.g., HbC, HbE, HbD) can produce falsely low or falsely high HbA1c values depending on the assay method used. Many modern UK assays are designed to be variant-tolerant, but interference remains method-dependent. Clinicians should check whether their laboratory's HbA1c method has been validated for the relevant variant, and request a laboratory comment or an alternative marker (such as fructosamine or glycated albumin) where appropriate. In the UK, where haemoglobinopathies are more prevalent in certain ethnic communities, this is a particularly relevant consideration. UK NEQAS provides guidance on variant interference for HbA1c methods used in UK laboratories.

Chronic kidney disease (CKD) can also affect results. Uraemia associated with CKD may cause carbamylation of haemoglobin, which can interfere with some assay methods. Additionally, the anaemia commonly seen in CKD alters red cell turnover, potentially skewing HbA1c values.

With regard to medications:

• Erythropoiesis-stimulating agents (used in CKD and anaemia) increase red cell production, shortening average red cell age and potentially lowering HbA1c.

• Hydroxyurea, used in sickle cell disease, increases foetal haemoglobin (HbF), which can interfere with certain assay methods.

• Drugs that cause haemolysis — including dapsone, ribavirin, and some antiretrovirals — can lower HbA1c by shortening red cell lifespan. In patients taking these medicines, HbA1c should be interpreted alongside plasma glucose measurements.

• High-dose aspirin and vitamin C supplementation have been reported in some studies to affect glycation, though the clinical significance of this is uncertain and is not supported by current UK guidance; this effect is likely to be clinically insignificant in most patients.

It is also worth noting that liver disease and thyroid disorders may influence glucose metabolism and red cell dynamics, indirectly affecting HbA1c reliability. In any of these situations, clinicians should consider requesting laboratory advice and, where appropriate, using an alternative measure of glycaemic control.

How Reliable Is HbA1c Compared to Other Diabetes Tests

HbA1c is generally the most convenient and reproducible diagnostic test for diabetes in adults, but fasting plasma glucose or OGTT should be used when HbA1c is unreliable or clinically inappropriate.

The HbA1c test is generally considered one of the most convenient and reliable tools for diagnosing and monitoring diabetes in appropriate adult populations. Unlike fasting plasma glucose (FPG) or the oral glucose tolerance test (OGTT), it does not require fasting, can be taken at any time of day, and reflects longer-term glycaemic control rather than a single measurement that may be affected by recent food intake, stress, or illness.

However, each test has its own strengths and limitations:

• Fasting plasma glucose (FPG): Straightforward and widely available, but requires an overnight fast and reflects only current glucose levels. A diagnostic threshold of ≥7.0 mmol/L on two separate occasions (or once with symptoms) confirms diabetes in adults. A single elevated result in an asymptomatic individual should be confirmed with a repeat test.

• Oral glucose tolerance test (OGTT): A two-hour plasma glucose of ≥11.1 mmol/L following a 75 g glucose load confirms diabetes. The OGTT is the recommended diagnostic test for gestational diabetes (per NICE NG3) and is also used when HbA1c is unreliable or inappropriate. It is more time-consuming and less practical for routine adult screening, but is particularly useful for detecting isolated post-prandial hyperglycaemia that HbA1c may miss.

• Continuous glucose monitoring (CGM): Provides real-time glucose data and is increasingly used in type 1 diabetes management, but is not a diagnostic tool and does not replace HbA1c in clinical practice.

NHS and NICE guidance acknowledge that HbA1c may miss some cases of diabetes — particularly in individuals with conditions that affect red cell lifespan, as outlined above. Where HbA1c is unreliable or gives a borderline result, NICE recommends confirming the diagnosis using a different method — typically FPG or OGTT. When HbA1c is unsuitable (for example, in pregnancy or in the presence of a haemoglobinopathy), FPG or OGTT should be used as the primary diagnostic test.

The overall consensus, supported by NICE NG28 and WHO guidance, is that for the majority of adults, HbA1c offers a practical, reproducible, and clinically meaningful measure of glycaemic status.

Talking to Your GP About Interpreting Your HbA1c Result

Patients should discuss any HbA1c result with their GP in the context of symptoms, medications, and conditions that may affect accuracy, as personalised targets and alternative tests may be required.

Receiving an HbA1c result — whether as part of a routine health check, a diabetes review, or an investigation for symptoms — can raise questions, and it is entirely appropriate to discuss your result in detail with your GP or diabetes care team. Understanding the context of your result is just as important as the number itself.

Seek urgent medical attention — do not wait for an HbA1c result — if you experience marked thirst, frequent urination, unexplained weight loss, extreme fatigue, abdominal pain, vomiting, or drowsiness. These may be signs of new-onset diabetes or diabetic ketoacidosis (DKA), which require same-day assessment.

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If your result falls in the non-diabetic hyperglycaemia (prediabetes) range (42–47 mmol/mol), your GP may recommend lifestyle modifications, including dietary changes, increased physical activity, and weight management, as these have been shown to significantly reduce the risk of progression to type 2 diabetes. The NHS Diabetes Prevention Programme (NHS DPP) offers structured support for people in this category. For some individuals at very high risk, NICE guidance also supports consideration of metformin alongside lifestyle measures, where appropriate.

If you have a condition or are taking a medication known to affect HbA1c accuracy — such as a haemoglobin variant, anaemia, or chronic kidney disease — it is important to raise this with your GP. They may recommend an alternative or supplementary test to ensure an accurate picture of your blood glucose control. You should also speak to your GP if:

• Your HbA1c result does not seem consistent with your symptoms or home glucose readings.

• You are pregnant or planning a pregnancy, as different diagnostic criteria and tests apply.

• You have recently had a blood transfusion or started treatment for anaemia.

For people already managing diabetes, HbA1c targets are individualised. Typical targets are around 48–53 mmol/mol, though your personal target may differ depending on your age, treatment regimen, and risk of hypoglycaemia, in line with NICE NG28. HbA1c should always be interpreted alongside other clinical information — including blood pressure, cholesterol, kidney function, and your overall wellbeing. Your GP or diabetes nurse can help you understand what your personal target should be and how best to achieve it.

Frequently Asked Questions