ADA HbA1c reference ranges provide internationally recognised thresholds for diagnosing diabetes and prediabetes, expressed as a percentage on the NGSP scale. HbA1c — glycated haemoglobin — reflects average blood glucose over the preceding two to three months, making it a cornerstone of diabetes diagnosis and management. This article explains the ADA's key reference ranges, how they compare with UK NICE and WHO criteria, what your result means in a clinical context, and which factors can affect test accuracy. Whether you are newly screened or monitoring an established diagnosis, understanding these values helps you engage meaningfully with your healthcare team.

What Is HbA1c and Why Is It Measured?

HbA1c measures the proportion of glycated haemoglobin in red blood cells, reflecting average blood glucose over two to three months. It is the primary diagnostic and monitoring tool for type 2 diabetes under NHS and NICE guidance, reported in mmol/mol in the UK.

HbA1c, or glycated haemoglobin, is a blood test that reflects average blood glucose levels over the preceding two to three months. When glucose circulates in the bloodstream, it binds irreversibly to haemoglobin — the protein inside 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, the HbA1c result provides a reliable window into longer-term glycaemic control, rather than a single snapshot in time.

This makes HbA1c particularly valuable in the diagnosis and ongoing management of type 2 diabetes, as well as in monitoring individuals with type 1 diabetes. In the UK, the NHS and NICE (National Institute for Health and Care Excellence) recommend HbA1c testing as a primary diagnostic tool for type 2 diabetes and as a routine monitoring measure for people already living with the condition. Results are reported in millimoles per mole (mmol/mol) in the UK, following the IFCC (International Federation of Clinical Chemistry) standardisation. Some international sources — including the American Diabetes Association (ADA) — also express values as a percentage using the NGSP (National Glycohaemoglobin Standardisation Program) scale.

Importantly, for diagnostic purposes in the UK, HbA1c must be measured from a venous blood sample analysed in a laboratory using an IFCC-standardised assay. Point-of-care (finger-prick) HbA1c devices should not be used to diagnose diabetes. In addition, HbA1c is not appropriate for diagnosing diabetes in the following situations, and alternative diagnostic methods should be used instead:

• Pregnancy (including screening for gestational diabetes)

• Children and young people

• Suspected type 1 diabetes, or where symptoms have developed rapidly (within the preceding two months)

• Conditions that alter red blood cell turnover (such as haemolytic anaemia, haemoglobin variants, or recent blood transfusion)

Understanding your HbA1c result is an important step in managing your health. Whether you are being screened for non-diabetic hyperglycaemia (prediabetes), recently diagnosed, or monitoring an established condition, this test gives both patients and clinicians meaningful information to guide treatment decisions, lifestyle adjustments, and medication reviews.

Key references: NICE NG28; WHO 2011 report on use of HbA1c for diagnosis of diabetes; NHS 'HbA1c test' page.

ADA HbA1c Reference Ranges Explained

The ADA classifies HbA1c below 5.7% (39 mmol/mol) as normal, 5.7%–6.4% (39–47 mmol/mol) as prediabetes, and 6.5% (48 mmol/mol) or above as diabetes. UK NICE guidance defines non-diabetic hyperglycaemia from 42 mmol/mol, making the prediabetes range narrower than the ADA's.

The American Diabetes Association (ADA) publishes widely referenced HbA1c thresholds that are used globally, including as a benchmark in many clinical and educational contexts. Although the ADA uses percentage-based (NGSP) values, these can be converted to mmol/mol for use in UK practice using the IFCC–NGSP conversion formula (available at NGSP.org). The key ADA HbA1c reference ranges are as follows:

• Normal (no diabetes): Below 5.7% (below 39 mmol/mol)

• Prediabetes: 5.7% to 6.4% (39 to 47 mmol/mol)

• Diabetes: 6.5% or above (48 mmol/mol or above)

For individuals already diagnosed with diabetes, the ADA recommends an individualised target, but generally suggests aiming for an HbA1c of below 7.0% (below 53 mmol/mol) for most non-pregnant adults, balancing glycaemic control against the risk of hypoglycaemia and other individual factors.

While the ADA thresholds are broadly consistent with those used in the UK, there are important differences in how results are categorised clinically. For UK diagnosis and risk stratification, clinicians follow NICE and WHO guidance rather than ADA categories. Specifically:

• Non-diabetic hyperglycaemia (NDH) — the UK term for prediabetes — is defined as 42 to 47 mmol/mol (6.0% to 6.4%), which is a narrower range than the ADA's prediabetes category starting at 39 mmol/mol (5.7%).

• Type 2 diabetes is diagnosed at 48 mmol/mol (6.5%) or above, consistent with both NICE/WHO and ADA thresholds.

Patients reviewing ADA materials should be aware of this distinction. A result that falls within the ADA's prediabetes range (39–41 mmol/mol) would be considered normal under UK NICE/WHO criteria. Always discuss your results with a UK-based clinician for contextually appropriate interpretation.

Key references: ADA Standards of Care (current year); NICE NG28; NGSP–IFCC conversion resource (NGSP.org).

Interpreting Your HbA1c Result in a UK Clinical Setting

In UK practice, a laboratory HbA1c of 48 mmol/mol or above diagnoses type 2 diabetes; 42–47 mmol/mol indicates non-diabetic hyperglycaemia requiring lifestyle intervention. NICE treatment targets range from 48 to 53 mmol/mol depending on medication type and individual risk.

In UK clinical practice, HbA1c results are interpreted within the framework set out by NICE and NHS England. For diagnostic purposes, a single HbA1c result of 48 mmol/mol or above is sufficient to diagnose type 2 diabetes in a symptomatic individual. In the absence of symptoms, a confirmatory repeat test is recommended. Diagnosis must be made using an IFCC-standardised laboratory assay; point-of-care HbA1c tests should not be used for this purpose.

HbA1c should not be used to diagnose diabetes in pregnancy, in children and young people, in suspected type 1 diabetes, or where conditions affecting red blood cell turnover are present (see the first section for the full list of exclusions).

A result between 42 and 47 mmol/mol indicates non-diabetic hyperglycaemia (NDH) — sometimes referred to as prediabetes — which warrants lifestyle intervention and regular monitoring (typically annually) to prevent progression to type 2 diabetes. NHS Diabetes Prevention Programme referrals are available for eligible individuals with NDH.

For people already living with diabetes, NICE recommends the following individualised HbA1c targets:

• Type 2 diabetes managed by lifestyle and/or glucose-lowering medicines not associated with hypoglycaemia (e.g., metformin, SGLT2 inhibitors, DPP-4 inhibitors, GLP-1 receptor agonists): 48 mmol/mol (6.5%)

• Type 2 diabetes managed with medicines that can cause hypoglycaemia (e.g., sulfonylureas or insulin): 53 mmol/mol (7.0%)

• Type 1 diabetes: 48 mmol/mol (6.5%) or lower, if safely achievable without problematic hypoglycaemia, though this is adjusted based on individual circumstances, hypoglycaemia risk, and patient preference

It is important to emphasise that HbA1c targets should always be personalised. Older adults, those with significant comorbidities, or individuals at high risk of hypoglycaemia may have less stringent targets agreed with their diabetes team. Conversely, younger patients or those planning a pregnancy may have tighter goals. The result should never be viewed in isolation — it forms part of a broader clinical picture that includes blood pressure, cholesterol, kidney function, and lifestyle factors.

If your HbA1c is rising despite treatment, or if it falls unexpectedly, this should prompt a review with your GP or diabetes specialist to reassess your management plan.

Key references: NICE NG28; NICE NG17; NHS 'Type 2 diabetes' pages.

Factors That Can Affect HbA1c Accuracy

Haemolytic anaemia, haemoglobin variants, chronic kidney disease, pregnancy, and certain medicines can cause falsely low or high HbA1c results. In these situations, alternative measures such as fructosamine, glycated albumin, or continuous glucose monitoring may be more appropriate.

While HbA1c is a robust and widely validated test, several physiological and pathological factors can affect its accuracy, potentially leading to falsely elevated or falsely low results. Clinicians and patients should be aware of these variables, particularly when results appear inconsistent with other clinical findings or self-monitored blood glucose readings.

Conditions that may cause falsely low HbA1c:

• Haemolytic anaemia — increased red blood cell turnover reduces the time available for glycation

• Iron deficiency anaemia treated with iron supplementation — HbA1c may fall after iron therapy independently of any change in blood glucose, as new red cells are produced

• Haemoglobin variants (e.g., sickle cell trait, HbS, HbC) — may interfere with certain laboratory assay methods; many UK laboratories flag these variants, but alternative monitoring may still be needed

• Recent blood transfusion — introduces donor red blood cells, diluting the glycated fraction

• Pregnancy — particularly in the second and third trimesters, HbA1c may underestimate true glycaemic exposure

• Recent erythropoietin (EPO) therapy — stimulates new red cell production, reducing glycation time

Conditions that may cause falsely high HbA1c:

• Iron deficiency anaemia (untreated) — reduced red cell turnover prolongs glycation time; note that once iron therapy is started, HbA1c may fall independently of glycaemic change

• Vitamin B12 or folate deficiency — similarly slows red cell replacement

• Splenectomy — prolongs red cell survival, increasing glycation time

Conditions where HbA1c reliability is generally reduced:

• Chronic kidney disease (CKD) — HbA1c reliability is reduced across all stages of CKD. In advanced CKD and in patients on dialysis, HbA1c is often lower than expected due to shortened red cell lifespan and other factors, meaning it may underestimate true glycaemic exposure. The direction and degree of bias varies by CKD stage and assay method.

• Severe liver disease — can affect red cell lifespan and haemoglobin metabolism

• Certain medicines — including dapsone, ribavirin, and some antiretroviral therapies used in HIV, can affect red cell lifespan or haemoglobin and may interfere with HbA1c results

In these situations, alternative measures of glycaemic control — such as fructosamine, glycated albumin, continuous glucose monitoring (CGM), or self-monitored blood glucose profiles — may be more appropriate. Your GP or diabetes team can advise on the most suitable monitoring approach if HbA1c is considered unreliable in your specific circumstances.

Always inform your healthcare provider of any recent illnesses, changes in medication, or new diagnoses that might influence the interpretation of your result. If you suspect that a medicine is causing an unexpected change in your HbA1c or other side effects, you can report this to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk or through the Yellow Card app.

Key references: NICE NG28; NGSP 'Factors that Interfere with HbA1c' resource; NHS laboratory guidance on HbA1c assay interferences.

When to Seek Further Advice From Your GP or Diabetes Team

Contact your GP promptly if your HbA1c reaches 48 mmol/mol for the first time, rises significantly, or if you experience frequent hypoglycaemia. Seek emergency care immediately for symptoms of diabetic ketoacidosis or hyperosmolar hyperglycaemic state.

Knowing when to seek further advice is an important aspect of diabetes self-management. While routine HbA1c monitoring is typically arranged by your GP surgery or diabetes clinic, there are specific circumstances in which you should proactively contact your healthcare team rather than waiting for your next scheduled appointment.

Seek urgent emergency care (call 999 or go to A&E immediately) if you or someone else develops any of the following:

• Vomiting, severe abdominal pain, or nausea that prevents you from keeping fluids down

• Deep, laboured, or rapid breathing

• Drowsiness, confusion, or loss of consciousness

• Signs of severe dehydration

• High blood ketone levels (if you are able to test)

These may be symptoms of diabetic ketoacidosis (DKA) or hyperosmolar hyperglycaemic state (HHS), both of which are medical emergencies requiring immediate hospital assessment. Do not wait for a GP appointment in these circumstances.

Contact your GP or diabetes team promptly if:

• Your HbA1c result is 48 mmol/mol or above for the first time, particularly if you have symptoms such as increased thirst, frequent urination, unexplained weight loss, or fatigue

• Your HbA1c has risen significantly since your last test, suggesting deteriorating glycaemic control

• You are experiencing frequent hypoglycaemic episodes (low blood sugar), which may indicate your treatment target or medication needs to be revised

• You are pregnant or planning a pregnancy, as tighter glycaemic control is essential and targets differ from standard recommendations

• You have been told your result may be unreliable due to a haemoglobin variant, anaemia, or kidney disease

If you cannot reach your GP, NHS 111 (online at 111.nhs.uk or by telephone) can provide out-of-hours advice and direct you to the appropriate service.

It is also advisable to seek a review if you have made significant lifestyle changes — such as sustained weight loss, dietary modification, or increased physical activity — and wish to reassess whether your current medication regimen remains appropriate. Some individuals with type 2 diabetes achieve remission, defined in the UK as an HbA1c below 48 mmol/mol sustained for at least six months without glucose-lowering medication. If you think this may apply to you, discuss it with your clinician — do not stop or reduce any diabetes medicines without medical advice.

In the UK, your GP, practice nurse, or diabetes specialist nurse are all appropriate first points of contact. NHS Diabetes Prevention Programme referrals are available for those identified with non-diabetic hyperglycaemia (NDH). For complex cases, referral to a consultant diabetologist or specialist diabetes team may be arranged. Never hesitate to ask questions about your results — understanding your HbA1c is a key part of taking an active role in your own health.

Key references: NICE NG28; NHS pages on diabetic ketoacidosis and hyperosmolar hyperglycaemic state; NHS England – NHS Diabetes Prevention Programme.

Frequently Asked Questions