The cut-off value of HbA1c is a cornerstone of diabetes diagnosis and management in UK clinical practice. HbA1c, or glycated haemoglobin, reflects average blood glucose levels over the preceding 8–12 weeks, making it a reliable marker for both diagnosing type 2 diabetes and prediabetes and monitoring long-term glycaemic control. Understanding the specific diagnostic thresholds — expressed in mmol/mol under NHS IFCC standardisation — is essential for patients and clinicians alike. This article explains the key cut-off values, when HbA1c may be unreliable, what an abnormal result means in practice, and how treatment targets are applied across different patient groups.

What Is HbA1c and Why Is It Measured?

HbA1c measures the proportion of haemoglobin that has become glycated due to circulating blood glucose, providing a reliable indicator of glycaemic control over approximately 8–12 weeks. It is used to both diagnose diabetes and prediabetes and to monitor ongoing glycaemic management.

HbA1c, or glycated haemoglobin, is a blood marker that reflects average blood glucose levels over approximately the preceding 8–12 weeks. Because the contribution of recent weeks is weighted more heavily, it is not a simple arithmetic average across the full period. When glucose circulates in the bloodstream, it binds irreversibly to haemoglobin — the protein within red blood cells responsible for carrying oxygen. The higher the blood glucose over time, the greater the proportion of haemoglobin that becomes glycated. Because red blood cells have a lifespan of approximately 120 days, HbA1c provides a reliable window into longer-term glycaemic control rather than a single-point-in-time measurement.

This makes HbA1c particularly valuable in clinical practice. Unlike a fasting plasma glucose test, which can fluctuate depending on recent food intake, stress, or illness, HbA1c is less susceptible to short-term variation. It is used both to diagnose type 2 diabetes and prediabetes and to monitor glycaemic control in people already living with diabetes.

The result is expressed in millimoles per mole (mmol/mol) in the UK, following the International Federation of Clinical Chemistry (IFCC) standardisation adopted by the NHS. Older percentage-based units (NGSP/DCCT) may still appear in some international literature, so it is important to be aware of which unit system is being used when interpreting results. For example, an HbA1c of 48 mmol/mol corresponds approximately to 6.5% in the older percentage scale.

Sources: NHS.uk HbA1c test; WHO 2011 'Use of glycated haemoglobin (HbA1c) in the diagnosis of diabetes mellitus'; UK position statement (ACB/RCPath/Diabetes UK).

NICE and NHS Diagnostic Thresholds for Diabetes and Prediabetes

The UK diagnostic cut-off for type 2 diabetes is an HbA1c of 48 mmol/mol (6.5%) or above; 42–47 mmol/mol (6.0–6.4%) indicates prediabetes. A repeat confirmatory test is required in the absence of symptoms before confirming a diabetes diagnosis.

The cut-off value of HbA1c for diagnosing type 2 diabetes in UK practice — as set out in WHO guidance (2011), the UK position statement (ACB/RCPath/Diabetes UK), and summarised in NICE Clinical Knowledge Summaries (CKS) — is 48 mmol/mol (6.5%) or above. A single HbA1c result at or above this threshold is sufficient to confirm a diagnosis of type 2 diabetes in a person who has clear symptoms of hyperglycaemia. In the absence of symptoms, a repeat confirmatory test is required on a separate occasion before a diagnosis is made, to rule out laboratory error or transient elevation.

For prediabetes — also referred to as non-diabetic hyperglycaemia (NDH) — the following range is recognised:

• 42–47 mmol/mol (6.0–6.4%): This range indicates an elevated risk of developing type 2 diabetes and warrants lifestyle intervention and regular monitoring.

Eligible people with results in this range should be offered referral to the NHS Diabetes Prevention Programme (NHS DPP), which provides structured education and support to reduce progression to type 2 diabetes. Eligibility is defined by NHS England/UKHSA criteria and clinicians should confirm eligibility before referring.

When HbA1c should NOT be used for diagnosis

HbA1c is unreliable for diagnosing diabetes in the following circumstances, and plasma glucose measurements (fasting plasma glucose or a two-hour oral glucose tolerance test, OGTT) should be used instead:

• Children and young people

• Pregnancy and within 2 months postpartum

• Acute onset of diabetes symptoms (where type 1 diabetes or diabetic ketoacidosis may be present)

• Conditions affecting red cell turnover or haemoglobin structure (see the following section)

• Certain medicines that affect glycation or red cell survival, such as corticosteroids or some antiretroviral drugs

NICE also advises that HbA1c should only be used for diagnosis when measured in a laboratory using a validated method — point-of-care testing devices are not currently recommended for diagnostic purposes in the UK, though they may be used for monitoring.

Sources: WHO 2011 HbA1c diagnostic guidance; UK position statement (ACB/RCPath/Diabetes UK); NICE CKS: Type 2 diabetes; NHS England/UKHSA NHS Diabetes Prevention Programme eligibility guidance.

Factors That Can Affect HbA1c Accuracy and Interpretation

Several conditions can cause falsely low or falsely high HbA1c results, including haemolytic anaemia, iron deficiency, haemoglobin variants, pregnancy, and advanced chronic kidney disease. In these circumstances, fasting plasma glucose or an OGTT should be used instead.

While HbA1c is a robust and widely used marker, several clinical conditions and physiological factors can affect its accuracy, potentially leading to falsely high or falsely low results. Clinicians and patients should be aware of these limitations when interpreting HbA1c values.

Conditions that may cause falsely LOW HbA1c:

• Haemolytic anaemia (increased red cell turnover reduces glycation time)

• Haemoglobin variants such as sickle cell trait or haemoglobin C disease

• Pregnancy (particularly the second and third trimesters)

• Erythropoiesis-stimulating agent (ESA) therapy or advanced chronic kidney disease with associated anaemia (which increases red cell turnover)

Conditions that may cause falsely HIGH HbA1c:

• Iron deficiency anaemia (untreated; reduced red cell turnover prolongs glycation time)

• Vitamin B12 or folate deficiency

• Splenectomy (prolonged red cell survival)

• Certain haemoglobin variants

• Some assay methods in the context of advanced chronic kidney disease (carbamylation of haemoglobin can interfere with certain assays, potentially raising the measured result)

A note on iron deficiency anaemia and iron treatment: Untreated iron deficiency anaemia typically raises HbA1c. After iron supplementation, HbA1c levels may fall as red cell turnover normalises; this represents a return towards a more accurate value rather than a genuinely 'falsely low' result. Clinicians should interpret HbA1c with caution during and shortly after iron therapy.

A note on recent blood transfusion: Transfusion introduces donor red blood cells of unknown age and glycation status, rendering HbA1c unreliable in either direction. Plasma glucose-based tests should be used instead in this situation.

A note on chronic kidney disease (CKD): The effect of CKD on HbA1c depends on the stage of disease, the presence of anaemia, ESA use, and the assay method used. In advanced CKD, HbA1c may underestimate true glycaemic exposure. Alternative markers such as fructosamine or glycated albumin, or plasma glucose testing, may be more appropriate.

In all of the above circumstances, alternative diagnostic methods — fasting plasma glucose or a two-hour OGTT — should be used. NICE guidance explicitly states that HbA1c is unreliable for diagnosis in people with haemoglobinopathies, haemolytic conditions, or during pregnancy. HbA1c should also not be used to diagnose diabetes in children and young people or within 2 months postpartum.

Ethnicity may also influence HbA1c interpretation. Some studies suggest that HbA1c levels can differ between ethnic groups at equivalent glucose concentrations, though there is no official NICE recommendation to apply ethnicity-specific cut-off values at present. Clinicians should exercise clinical judgement and consider the full clinical picture when results appear inconsistent with a patient's symptoms or other investigations.

Sources: NGSP: Factors that interfere with HbA1c; ACB/RCPath UK position statement and pitfalls in HbA1c measurement; NICE CKS: Type 2 diabetes (diagnosis limitations).

What Happens After an Abnormal HbA1c Result?

A prediabetes result (42–47 mmol/mol) prompts lifestyle advice, possible NHS Diabetes Prevention Programme referral, and repeat testing in 6–12 months. A confirmed diabetes result (48 mmol/mol or above) triggers comprehensive assessment including cardiovascular risk review, retinal screening, and structured diabetes education.

Receiving an abnormal HbA1c result — whether in the prediabetes or diabetes range — should prompt a structured clinical response. Patients are encouraged not to panic, as a single result is rarely acted upon in isolation without further assessment.

If the result falls in the prediabetes range (42–47 mmol/mol), the GP will typically:

• Provide lifestyle advice around diet, physical activity, and weight management

• Offer referral to the NHS Diabetes Prevention Programme if eligible

• Arrange a repeat HbA1c in 6–12 months to monitor for progression (the interval may be shorter if values are rising or risk factors are present)

If the result is 48 mmol/mol or above, and a diagnosis of type 2 diabetes is confirmed (either through symptoms or a repeat test), the GP will initiate a more comprehensive assessment. In line with UK practice, this includes:

• Reviewing cardiovascular risk factors, kidney function (eGFR and urine albumin-to-creatinine ratio), blood pressure, and cholesterol

• Referral for diabetic eye (retinal) screening, ideally within 3 months of diagnosis

• Foot risk stratification as part of an initial diabetes foot assessment

• Review of vaccination status (for example, influenza and pneumococcal vaccines)

• Offering a structured diabetes education programme, such as DESMOND

Patients should contact their GP promptly if they experience symptoms suggestive of significantly elevated blood glucose, including:

• Excessive thirst or urination

• Unexplained weight loss

• Blurred vision

• Persistent fatigue

These symptoms alongside a raised HbA1c may indicate a need for more urgent assessment. In cases where type 1 diabetes is suspected — for example in younger patients, those with rapid symptom onset, or those with weight loss — same-day capillary blood glucose and ketone testing should be performed. A positive result for ketones alongside hyperglycaemia requires urgent or emergency referral, as diabetic ketoacidosis (DKA) is a medical emergency.

Sources: NICE NG28: Type 2 diabetes in adults; NHS England/UKHSA NHS DPP referral guidance; NHS.uk: Diabetes eye screening.

Monitoring HbA1c Over Time and Treatment Targets

NICE recommends an HbA1c target of 48 mmol/mol (6.5%) for most adults with type 2 diabetes on lifestyle or metformin, rising to 53 mmol/mol (7.0%) for those on hypoglycaemia-risk medicines. Monitoring frequency is every 3–6 months when adjusting treatment, and every 6–12 months once stable.

Once a diagnosis of diabetes is established, HbA1c becomes an essential tool for monitoring how well blood glucose is being managed over time. NICE provides specific treatment targets depending on the individual's circumstances and the medicines they are taking.

For most adults with type 2 diabetes managed by lifestyle and diet alone, or with metformin, NICE (NG28) recommends an HbA1c target of 48 mmol/mol (6.5%). For those on medicines that carry a risk of hypoglycaemia — such as sulphonylureas or insulin — the target is slightly higher at 53 mmol/mol (7.0%), to reduce the risk of low blood sugar episodes.

HbA1c is typically measured:

• Every 3–6 months when treatment is being adjusted or newly initiated

• Every 6–12 months once stable control is achieved

For people with type 1 diabetes, NICE (NG17) recommends aiming for an HbA1c of 48 mmol/mol (6.5%) or below if achievable without problematic hypoglycaemia, acknowledging that individual targets should be agreed collaboratively between the patient and their clinical team.

It is important to understand that HbA1c targets are not one-size-fits-all. Factors such as age, frailty, comorbidities, risk of hypoglycaemia, and patient preference all influence what constitutes an appropriate target. For older or frailer individuals, less stringent targets may be appropriate to avoid the harms associated with hypoglycaemia.

Self-monitoring of blood glucose (SMBG) is not routinely recommended for all people with type 2 diabetes. NICE advises that SMBG may be appropriate for those on medicines associated with hypoglycaemia risk (such as sulphonylureas or insulin), those who are pregnant or planning pregnancy, those with intercurrent illness, and those with specific occupational requirements such as driving. The decision should be made on an individual basis in discussion with the clinical team.

Regular HbA1c monitoring, combined with SMBG where clinically indicated, empowers patients to understand their condition and supports shared decision-making with their healthcare team. If HbA1c remains persistently above target despite treatment, a medication review or referral to a specialist diabetes team may be warranted.

Sources: NICE NG28: Type 2 diabetes in adults: management; NICE NG17: Type 1 diabetes in adults: diagnosis and management.

Frequently Asked Questions