HbA1c is used as a test for diabetes because it provides a reliable measure of average blood glucose levels over the preceding two to three months, making it far more informative than a single glucose reading. By measuring the proportion of haemoglobin with glucose attached, it captures sustained hyperglycaemia — the defining feature of diabetes. Endorsed by NICE, the WHO, and Diabetes UK, HbA1c is the cornerstone of both diagnosis and long-term monitoring in most adults. This article explains how the test works, why it is preferred, and when alternative tests may be needed.

What Is HbA1c and What Does It Measure?

HbA1c measures the proportion of haemoglobin with glucose attached, reflecting average blood glucose over the preceding 90–120 days. A result of 48 mmol/mol or above on two occasions is diagnostic of type 2 diabetes in asymptomatic adults.

HbA1c, or glycated haemoglobin, is a 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 found in red blood cells — that has glucose attached to it. Because red blood cells have a lifespan of approximately 90 to 120 days, the HbA1c result provides a reliable window into longer-term blood glucose control, rather than a single snapshot in time.

When blood glucose levels are consistently elevated, more glucose binds irreversibly to haemoglobin through a process called glycation. The higher the average blood glucose over the preceding months, the higher the HbA1c reading. This makes it a particularly useful marker for identifying sustained hyperglycaemia, which is the hallmark of diabetes mellitus.

In the UK, HbA1c results are reported in millimoles per mole (mmol/mol) in line with the International Federation of Clinical Chemistry (IFCC) standardisation. According to NICE (NG28) and WHO guidance:

• An HbA1c of 48 mmol/mol (6.5%) or above on two separate occasions is diagnostic of type 2 diabetes in asymptomatic individuals.

• A level of 42–47 mmol/mol (6.0–6.4%) indicates non-diabetic hyperglycaemia (sometimes referred to as prediabetes), signalling an elevated risk of developing type 2 diabetes.

• Levels below 42 mmol/mol are generally considered within the normal range in most adults.

Importantly, HbA1c is not appropriate for diagnosing diabetes in the following groups, where plasma glucose criteria (fasting plasma glucose, oral glucose tolerance test, or random glucose with symptoms) should be used instead:

• Children and young people (NICE NG18)

• Pregnant women — physiological changes in pregnancy alter red blood cell dynamics, making HbA1c unreliable (NICE NG3)

• People with suspected type 1 diabetes or acute-onset hyperglycaemia

• People with conditions affecting red blood cell survival (see below)

Understanding what HbA1c measures — and recognising both its strengths and its limitations — is fundamental to appreciating why it has become the preferred diagnostic and monitoring tool for diabetes in most adults in clinical practice.

Why HbA1c Is Used to Diagnose and Monitor Diabetes

HbA1c is preferred because it captures sustained hyperglycaemia, requires no fasting, and is endorsed by NICE and the WHO for both diagnosis and ongoing monitoring. Monitoring frequency is every 3–6 months when treatment is being adjusted, and every 6 months once stable.

HbA1c has become the cornerstone test for both diagnosing and monitoring diabetes for several important clinical and practical reasons. Unlike a fasting plasma glucose test, HbA1c does not require the patient to fast beforehand, making it more convenient and reducing the risk of inaccurate results caused by recent food intake. A single blood draw is sufficient, and the sample is stable at room temperature, simplifying laboratory processing.

From a diagnostic standpoint, HbA1c is endorsed by NICE, the World Health Organisation (WHO), and Diabetes UK as a reliable marker because it captures sustained hyperglycaemia rather than transient fluctuations. A person under stress or with an acute illness may have a temporarily elevated fasting glucose, but their HbA1c will only be raised if blood glucose has been consistently high over weeks to months — reducing the risk of misdiagnosis.

For ongoing monitoring of established diabetes, HbA1c is equally valuable. NICE recommends the following monitoring frequencies:

Type 2 diabetes (NICE NG28):

• Every 3 to 6 months when treatment is being adjusted or targets are not being met.

• Every 6 months once stable and on a consistent treatment regimen.

Type 1 diabetes (NICE NG17):

• Every 3 to 6 months for all adults; at least every 6 months once stable.

Regarding HbA1c targets, these should always be individualised based on age, comorbidities, frailty, and risk of hypoglycaemia. As a general guide (NICE NG28):

• 48 mmol/mol (6.5%) is the recommended target for adults with type 2 diabetes managed by lifestyle measures and/or medications not associated with hypoglycaemia (e.g., metformin).

• 53 mmol/mol (7.0%) is recommended for adults with type 2 diabetes taking medications associated with hypoglycaemia (e.g., sulfonylureas or insulin).

• For adults with type 1 diabetes, NICE NG17 recommends aiming for 48 mmol/mol (6.5%) or below where this is achievable without problematic hypoglycaemia, though targets are always agreed individually. Continuous glucose monitoring (CGM) time-in-range metrics are increasingly used alongside HbA1c to inform management in type 1 diabetes.

By tracking HbA1c over time, clinicians can assess whether lifestyle changes, oral medications, or insulin therapy are achieving adequate glucose control, and adjust treatment plans accordingly — making it an indispensable tool in long-term diabetes management.

Factors That Can Affect HbA1c Accuracy

Several conditions can cause falsely low or high HbA1c results, including haemolytic anaemia, iron deficiency, haemoglobin variants, and advanced chronic kidney disease. When results are inconsistent with the clinical picture, fasting plasma glucose or an OGTT should be considered.

Whilst HbA1c is a robust and widely validated test, several physiological and medical factors can affect its accuracy, potentially leading to misleading results. Clinicians must be aware of these variables to interpret results correctly and avoid misdiagnosis or inappropriate treatment changes. The direction of interference can vary depending on the laboratory assay used and the individual's clinical circumstances.

Conditions that may cause a falsely low HbA1c include:

• Haemolytic anaemia — increased red blood cell turnover means cells are replaced more rapidly, reducing the time available for glycation.

• Recent blood transfusion — donor red blood cells dilute the patient's own glycated haemoglobin, artificially lowering the result.

• Haemoglobin variants (e.g., sickle cell trait, haemoglobin C or E) — these can interfere with certain laboratory assay methods used to measure HbA1c (RCPath guidance advises assay-specific interpretation).

• Pregnancy (particularly the second and third trimesters) — shortened red blood cell lifespan reduces HbA1c reliability; plasma glucose criteria should be used for diagnosis instead (NICE NG3).

Conditions that may cause a falsely high or otherwise unreliable HbA1c include:

• Iron deficiency anaemia (untreated) — reduced red blood cell turnover prolongs cell lifespan, allowing more time for glycation and potentially raising HbA1c independently of true glycaemia. Following iron supplementation, HbA1c may fall as new red blood cells are produced, without any genuine improvement in glucose control. HbA1c results should therefore be interpreted cautiously during transitions in iron status.

• Vitamin B12 or folate deficiency — similarly reduces red cell turnover and may artefactually elevate HbA1c.

• Splenectomy — removal of the spleen prolongs red blood cell survival, increasing glycation time.

• Advanced chronic kidney disease (CKD) — in advanced CKD, HbA1c may be unreliable in either direction. Uraemia can cause carbamylation of haemoglobin (interfering with some assays), whilst anaemia of CKD and erythropoietin therapy alter red cell turnover. Self-monitored blood glucose (SMBG) or CGM, and in selected cases fructosamine, may provide more reliable information in this group.

In clinical practice, if a patient's HbA1c result appears inconsistent with their symptoms, self-monitored blood glucose readings, or overall clinical picture, it is important to consider these confounding factors. In such cases, alternative diagnostic tests — such as fasting plasma glucose or an oral glucose tolerance test — may be more appropriate (NICE NG28; WHO 2011). Patients should always inform their GP or diabetes team of any relevant medical history that could influence test interpretation.

When Other Tests May Be Used Alongside HbA1c

HbA1c should not be used for diagnosis in children, pregnant women, or those with suspected type 1 diabetes; plasma glucose criteria or an OGTT should be used instead. CGM and C-peptide testing complement HbA1c in specific clinical circumstances.

Although HbA1c is the primary test for diagnosing and monitoring diabetes in most adults in the UK, it is not universally applicable, and there are specific clinical circumstances in which alternative or complementary tests are recommended by NICE and the NHS.

HbA1c should not be used for diagnosis in the following groups; plasma glucose criteria should be used instead:

• Children and young people — diagnosis is based on clinical features and plasma glucose (NICE NG18).

• Pregnant women — gestational diabetes is diagnosed using the oral glucose tolerance test (OGTT), as physiological changes in pregnancy make HbA1c unreliable (NICE NG3).

• People with suspected type 1 diabetes or acute-onset hyperglycaemia — a random plasma glucose of 11.1 mmol/L or above in the presence of classic symptoms (excessive thirst, polyuria, unexplained weight loss) is sufficient for diagnosis without awaiting HbA1c confirmation.

• People with conditions affecting red blood cell survival (as described above).

Urgent safety advice: If type 1 diabetes or diabetic ketoacidosis (DKA) is suspected — particularly in someone with rapid-onset symptoms, vomiting, abdominal pain, drowsiness, or rapid breathing — same-day urgent assessment is required. Do not delay clinical assessment, ketone testing, or treatment whilst awaiting an HbA1c result. Contact your GP urgently or call 999 if symptoms are severe.

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Fasting plasma glucose (FPG) and the oral glucose tolerance test (OGTT) remain important diagnostic tools when HbA1c results are unreliable due to haemoglobin variants or conditions affecting red blood cell turnover.

Continuous glucose monitoring (CGM) and self-monitoring of blood glucose (SMBG) are increasingly used alongside HbA1c, particularly in people with type 1 diabetes or those on insulin therapy. These tools provide real-time glucose data and can identify patterns such as nocturnal hypoglycaemia or post-meal glucose spikes that HbA1c alone cannot detect. NICE NG17 recommends that CGM (real-time or intermittently scanned) is offered to all adults with type 1 diabetes. Access is also expanding for some people with type 2 diabetes on insulin, in line with NICE NG28 and NHS England policy.

C-peptide testing may be used to help distinguish between type 1 and type 2 diabetes, particularly in younger patients or those with atypical presentations, as it reflects endogenous insulin production. Pancreatic autoantibody testing (e.g., GAD antibodies, IA-2 antibodies) is also used alongside C-peptide to support diabetes classification and guide decisions about insulin therapy.

In summary, whilst HbA1c remains the preferred test for most adults, a holistic approach to diabetes diagnosis and monitoring — incorporating clinical judgement, patient history, and complementary investigations where necessary — ensures the most accurate and safe management of this complex, long-term condition. If you have concerns about your blood glucose levels or diabetes risk, speak to your GP, who can advise on the most appropriate tests for your individual circumstances.

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