Diagnosing type 2 diabetes with HbA1c is the standard approach recommended across the NHS, offering a reliable, non-fasting blood test that reflects average blood glucose over the preceding 8 to 12 weeks. Unlike a single fasting glucose reading, HbA1c captures longer-term glycaemic patterns, making it a practical and accurate diagnostic tool. This article explains how HbA1c works, the diagnostic thresholds used in UK clinical practice, when the test is unsuitable, how NICE guidelines govern confirmation of diagnosis, and what to expect following a new type 2 diabetes diagnosis.

What Is HbA1c and How Does It Detect Type 2 Diabetes?

HbA1c detects type 2 diabetes by measuring the percentage of glycated haemoglobin in the blood, with a result of 48 mmol/mol or above consistent with diagnosis; it is unreliable in conditions affecting red blood cell turnover.

HbA1c — formally known as glycated haemoglobin — is a blood marker that reflects average blood glucose levels over the preceding 8 to 12 weeks. When glucose circulates in the bloodstream, it binds irreversibly to haemoglobin, the protein found inside red blood cells. Because red blood cells have a lifespan of approximately 120 days, the proportion of haemoglobin that has become glycated provides a reliable indication of longer-term glycaemic control, rather than a single moment in time.

This characteristic makes HbA1c particularly valuable for diagnosing type 2 diabetes. Unlike a fasting plasma glucose test, which captures blood sugar at one specific point, HbA1c is less affected by short-term fluctuations caused by recent meals. However, it is important to note that acute severe illness and any condition that alters red blood cell turnover can affect the reliability of the result.

HbA1c is expressed in millimoles per mole (mmol/mol) in the UK, following the International Federation of Clinical Chemistry (IFCC) standardisation. The key diagnostic thresholds are:

• Below 42 mmol/mol — considered normal

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

• 48 mmol/mol or above — consistent with a diagnosis of type 2 diabetes

HbA1c is not suitable for everyone. Conditions that affect red blood cell turnover or haemoglobin structure can produce falsely low or high results, making the test unreliable for diagnosis. These include:

• Haemoglobinopathies (including sickle cell disease and other haemoglobin variants)

• Haemolytic anaemia or recent significant blood loss

• Iron, vitamin B12, or folate deficiency anaemia

• Recent blood transfusion or erythropoietin therapy

• Chronic kidney disease (CKD) stage 4 or 5

• HIV antiretroviral therapy

• Following splenectomy

• Pregnancy and up to 2–3 months postpartum

In these circumstances, alternative diagnostic tests — such as fasting plasma glucose or an oral glucose tolerance test (OGTT) — should be used instead. When HbA1c is unsuitable, the following laboratory glucose thresholds apply (aligned with WHO and UK practice):

• Fasting plasma glucose ≥7.0 mmol/L — consistent with diabetes

• 2-hour plasma glucose ≥11.1 mmol/L following a 75 g OGTT — consistent with diabetes

How the HbA1c Test Is Carried Out on the NHS

NHS HbA1c testing requires a venous blood sample analysed in an accredited laboratory; no fasting is needed, and point-of-care results must be confirmed by laboratory analysis before a diagnosis is made.

On the NHS, an HbA1c test is a straightforward blood test that can be requested by a GP, practice nurse, or other healthcare professional. It does not require the patient to fast beforehand, which makes it considerably more convenient than fasting glucose tests and helps improve uptake, particularly in people who find fasting difficult due to other health conditions or medications.

For diagnostic purposes, the test should be performed using a venous blood sample analysed in an accredited laboratory. This is important to ensure accuracy and reproducibility. Point-of-care HbA1c analysers — small devices that can provide a result within minutes during a consultation — are primarily intended for monitoring in people already diagnosed with diabetes, rather than for making a new diagnosis. If a point-of-care result suggests diabetes, it should be confirmed by a laboratory-analysed venous sample before a diagnosis is established. Any point-of-care device used must meet rigorous internal quality control standards and participate in external quality assessment schemes.

HbA1c testing is used in several clinical contexts on the NHS:

• Opportunistic screening — offered to individuals identified as being at high risk of type 2 diabetes, for example through the NHS Health Check, the NHS Diabetes Prevention Programme, or following a high score on a validated risk assessment tool such as QDiabetes or the Leicester Risk Assessment

• Symptomatic investigation — requested when a patient presents with classic symptoms of diabetes, including increased thirst, frequent urination, unexplained weight loss, or fatigue

• Routine monitoring — used regularly in people already diagnosed with diabetes to assess how well blood glucose is being managed over time

Patients should inform their clinician of any relevant medical history — particularly conditions affecting red blood cells, recent changes in medication, or recent blood transfusion — before the test is carried out, as these factors may influence the reliability of the result and guide the choice of diagnostic approach.

Confirming a Type 2 Diabetes Diagnosis: NICE Guidelines

NICE (NG28) requires a single HbA1c of 48 mmol/mol or above to confirm diagnosis in symptomatic individuals, or two separate results at or above this threshold in those without symptoms.

NICE guidance (NG28) sets out clear criteria for confirming a diagnosis of type 2 diabetes using HbA1c. A single HbA1c result of 48 mmol/mol or above is sufficient to confirm the diagnosis in a person who has symptomatic hyperglycaemia — that is, someone presenting with classic symptoms such as polydipsia, polyuria, or unexplained weight loss.

In individuals who are asymptomatic, NICE recommends that the diagnosis should be confirmed by a second HbA1c test, ideally performed on a different day. This two-sample approach reduces the risk of a false-positive diagnosis resulting from laboratory error or transient elevation. Both results must be 48 mmol/mol or above to confirm type 2 diabetes in the absence of symptoms. If the two results are discordant — for example, one above and one below the threshold — clinical judgement is required, and further investigation may be warranted.

NICE distinguishes between type 2 diabetes and non-diabetic hyperglycaemia, defined as an HbA1c of 42–47 mmol/mol. People in this range do not have diabetes but are at significantly increased risk of developing it. NICE recommends that these individuals be referred to a structured lifestyle intervention programme, such as the NHS Diabetes Prevention Programme, and have their HbA1c rechecked annually.

When HbA1c is not appropriate, laboratory plasma glucose testing should be used instead (see thresholds above). Situations requiring particular care include:

• Pregnancy — HbA1c is not recommended for diagnosing gestational diabetes; an OGTT is used instead (see NICE NG3)

• Children and young people — type 1 diabetes is more common in this group, and clinical presentation should guide the diagnostic approach

• Acute severe illness — HbA1c may be unreliable if measured during or shortly after a serious illness

• CKD stage 4–5, haemoglobinopathies, iron or B12/folate deficiency, recent transfusion or erythropoietin therapy, HIV antiretroviral therapy, splenectomy, and within 2–3 months postpartum — alternative glucose-based tests should be used

Urgent referral red flags: If a person presents with features suggesting type 1 diabetes (rapid onset of symptoms, significant unintentional weight loss, ketosis) or signs of a hyperglycaemic emergency such as diabetic ketoacidosis (DKA) or hyperosmolar hyperglycaemic state (HHS) — including abdominal pain, vomiting, drowsiness or confusion, deep or laboured breathing, or severe dehydration — same-day emergency assessment should be arranged. Do not wait for HbA1c results in these situations.

Healthcare professionals should always interpret HbA1c results within the full clinical context, rather than in isolation.

Next Steps After a Type 2 Diabetes Diagnosis

Following diagnosis, initial assessment includes renal function, lipid profile, blood pressure, foot examination, and retinal screening referral, with lifestyle modification and metformin as first-line management per NICE NG28.

Receiving a diagnosis of type 2 diabetes can feel overwhelming, but the condition is manageable, and early intervention significantly reduces the risk of long-term complications. Following confirmation of the diagnosis, the GP or diabetes care team will typically arrange a structured initial assessment, which forms the foundation of an individualised management plan.

This initial assessment usually includes:

• Further blood tests — including renal function (eGFR and urine albumin-to-creatinine ratio), lipid profile, liver function, and a repeat HbA1c to establish a baseline

• Blood pressure measurement — hypertension is common in people with type 2 diabetes and significantly increases cardiovascular risk

• BMI and weight assessment — given the strong association between excess weight and insulin resistance

• Foot examination — to assess for early signs of peripheral neuropathy or vascular disease

• Referral for retinal screening — the NHS Diabetic Eye Screening Programme offers annual screening to detect diabetic retinopathy at an early, treatable stage

In terms of treatment, NICE guidance (NG28) recommends that lifestyle modification — including dietary changes, increased physical activity, and weight management — should be the cornerstone of initial management for most people with type 2 diabetes. Where pharmacological treatment is indicated, metformin remains the first-line medication of choice in the absence of contraindications. It works primarily by reducing hepatic glucose production and improving insulin sensitivity. Key safety considerations include:

• Renal function should be checked before starting metformin and monitored during treatment

• The dose should be reviewed if eGFR falls below 45 mL/min/1.73 m², and metformin should be avoided if eGFR is below 30 mL/min/1.73 m²

• Long-term use is associated with reduced vitamin B12 absorption; periodic monitoring may be appropriate

• Gastrointestinal side effects (nausea, diarrhoea) are common when starting treatment and usually improve over time

• There is a rare risk of lactic acidosis, particularly in the context of renal impairment, dehydration, or contrast media use

For people with established atherosclerotic cardiovascular disease, heart failure, or CKD, NICE recommends considering an SGLT2 inhibitor or GLP-1 receptor agonist as part of the treatment regimen, in addition to or instead of other agents, based on individual clinical circumstances.

Patients are encouraged to attend a structured diabetes education programme, such as DESMOND (Diabetes Education and Self-Management for Ongoing and Newly Diagnosed) or X-PERT, which equips individuals with the knowledge and skills to manage their condition effectively.

When to seek urgent help: Patients should seek same-day medical attention if they experience severe hyperglycaemia with dehydration, vomiting, drowsiness or confusion, or if they detect ketones in their urine or blood. They should also contact their GP or diabetes nurse if they experience symptoms of hypoglycaemia, significant changes in their general health, or if blood glucose monitoring results are consistently outside the agreed target range.

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If you experience any suspected side effects from your diabetes medication, you can report these to the MHRA via the Yellow Card Scheme at yellowcard.mhra.gov.uk.

Regular review — typically every three to six months initially — ensures that management remains appropriate as the condition evolves.

Frequently Asked Questions