HbA1c fasting or non-fasting — it is one of the most common questions patients ask before attending a diabetes blood test. The HbA1c test (glycated haemoglobin) measures your average blood glucose over the preceding two to three months and, crucially, does not require fasting. Unlike a fasting plasma glucose test, you can eat and drink normally beforehand. This article explains how the HbA1c test works, how it differs from fasting glucose tests, what your result means according to NHS and NICE guidelines, and how to prepare — including important situations where HbA1c may not be the most appropriate test.
Summary: HbA1c is a non-fasting blood test — you can eat and drink normally beforehand, as it measures average blood glucose over the preceding two to three months rather than a single point-in-time reading.
- HbA1c measures the proportion of haemoglobin glycated over approximately 90–120 days, reflecting sustained blood glucose control rather than a momentary level.
- No fasting is required before an HbA1c test; eating or drinking beforehand does not meaningfully affect the result.
- A result of 48 mmol/mol or above indicates diabetes; 42–47 mmol/mol indicates non-diabetic hyperglycaemia (prediabetes) under NICE NG28 thresholds.
- HbA1c is not suitable for diagnosing diabetes in pregnancy, children, suspected type 1 diabetes, or where conditions affect red blood cell turnover — an OGTT or plasma glucose test should be used instead.
- Conditions such as haemolytic anaemia, iron deficiency, haemoglobin variants, and advanced chronic kidney disease can produce falsely low or falsely high HbA1c results.
- Asymptomatic adults with an HbA1c of 48 mmol/mol or above require a confirmatory repeat venous blood sample on a separate day before a diagnosis of diabetes is made.
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What Is an HbA1c Test and What Does It Measure?
The HbA1c test measures the proportion of haemoglobin bonded to glucose over the preceding 2–3 months, providing a sustained picture of blood glucose control used to diagnose and monitor diabetes under NICE NG28 guidance.
The HbA1c test — formally known as the glycated haemoglobin test — is a blood test used to assess average blood glucose (sugar) 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 become chemically bonded to glucose through a process called glycation. Because red blood cells have a lifespan of approximately 90 to 120 days, the HbA1c result reflects a sustained picture of blood glucose control rather than a single point-in-time measurement.
This makes the HbA1c test particularly valuable in both the diagnosis of diabetes and the ongoing monitoring of glycaemic control in people already living with diabetes. The test is widely used across NHS services and is recommended by NICE (National Institute for Health and Care Excellence) as a primary diagnostic tool for diabetes in adults in the UK. It is reported in millimoles per mole (mmol/mol), following IFCC (International Federation of Clinical Chemistry) standardisation.
Unlike some other blood tests, the HbA1c does not capture your glucose at a single moment. Instead, it reflects how well glucose has been managed across weeks and months, making it a more comprehensive indicator of metabolic health. In routine clinical practice, it is generally preferred over fasting glucose tests for diagnosing and monitoring diabetes in adults — however, there are important situations where HbA1c is not appropriate for diagnosis (see below).
When HbA1c should not be used to diagnose diabetes: NICE (NG28) specifies that HbA1c is not suitable as a diagnostic test in the following circumstances:
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Pregnancy (including the postpartum period) — use an oral glucose tolerance test (OGTT) instead
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Children and young people
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Suspected type 1 diabetes or rapidly developing (acute-onset) hyperglycaemia
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Recent acute illness — which can transiently affect results
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Conditions affecting red blood cell turnover (such as haemolytic anaemia, haemoglobin variants, or recent blood transfusion)
In these situations, plasma glucose-based tests — such as fasting plasma glucose, random plasma glucose, or OGTT — should be used instead. If your clinician considers HbA1c unreliable in your case, they will discuss the most appropriate alternative with you.
A result of 48 mmol/mol or above indicates diabetes; the clinical context and further assessment determine the type of diabetes. This is consistent with WHO 2011 guidance on the use of HbA1c in diagnosing diabetes, which has been adopted in the UK.
How HbA1c Differs From Fasting Blood Glucose Tests
HbA1c is a non-fasting test requiring no dietary restriction beforehand, whereas a fasting plasma glucose test requires an 8-hour fast and reflects only a single point-in-time glucose level.
A fasting blood glucose test measures the concentration of glucose in your blood at a specific moment, typically after you have not eaten or drunk anything (other than water) for at least eight hours. It provides an immediate snapshot of your blood sugar level and is sensitive to recent dietary intake, physical activity, and stress. In contrast, the HbA1c test reflects a longer-term average and is not influenced by what you have eaten in the hours before the test.
This is the key practical distinction: HbA1c is a non-fasting test. You do not need to fast before having your HbA1c measured. You can eat and drink normally beforehand, which makes it considerably more convenient for patients and reduces the logistical burden of scheduling early-morning appointments. This flexibility also means there is less risk of results being affected by short-term dietary changes made in anticipation of the test.
Fasting glucose tests remain clinically useful in certain situations — for example, when HbA1c results are unreliable due to conditions affecting red blood cell turnover (discussed further below), or when assessing for impaired fasting glucose as part of a broader metabolic screen.
The oral glucose tolerance test (OGTT), which does require fasting, is another important diagnostic tool. Notably, HbA1c must not be used to diagnose diabetes in pregnancy; NICE (NG3) recommends the OGTT as the standard test for screening for gestational diabetes. Similarly, plasma glucose-based tests (fasting plasma glucose, random plasma glucose, or OGTT) should be used — rather than HbA1c — when type 1 diabetes or rapidly developing hyperglycaemia is suspected, or in children and young people.
In summary:
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HbA1c: No fasting required; reflects 2–3 months of glucose control; not suitable for diagnosis in pregnancy, children/young people, suspected type 1 diabetes, or acute-onset hyperglycaemia
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Fasting plasma glucose: Requires an 8-hour fast; reflects current glucose level
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OGTT: Requires fasting; used in specific clinical contexts, including gestational diabetes screening and where HbA1c is unreliable
Factors That Can Affect Your HbA1c Result
Conditions altering red blood cell lifespan — including haemolytic anaemia, iron deficiency, haemoglobin variants, and advanced CKD — can produce falsely low or falsely high HbA1c results, making the test unreliable in these situations.
Although the HbA1c test is generally reliable and reproducible, several medical conditions and physiological factors can influence the result, potentially leading to falsely high, falsely low, or otherwise unreliable readings. Clinicians should be aware of these when interpreting results.
Conditions that may cause falsely low HbA1c results include:
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Haemolytic anaemia — increased red blood cell destruction shortens red cell lifespan, reducing the time available for glycation
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Recent blood transfusion — introduces donor red blood cells that have not been exposed to the patient's glucose levels
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Haemoglobin variants (such as sickle cell trait or haemoglobin C disease) — may interfere with certain laboratory assay methods
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Advanced chronic kidney disease (CKD) and dialysis — shortened red cell survival in advanced CKD often produces lower-than-expected HbA1c values; some assay methods may also be affected. HbA1c should generally be regarded as unreliable in advanced CKD and in patients on haemodialysis
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Erythropoietin (EPO) therapy — stimulates production of new red blood cells, reducing average red cell age and glycation time
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Acute blood loss
Conditions that may cause falsely high HbA1c results include:
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Iron deficiency anaemia (untreated) — reduced red cell turnover prolongs glycation time
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Vitamin B12 or folate deficiency — can reduce red cell turnover
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Splenectomy — removal of the spleen prolongs red blood cell lifespan
Settings where HbA1c is not recommended for diagnosis:
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Pregnancy and the postpartum period — physiological changes in red cell turnover make HbA1c unreliable; OGTT is the recommended diagnostic test (NICE NG3)
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Children and young people
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Suspected type 1 diabetes or rapidly developing hyperglycaemia
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Recent acute illness
Ethnicity and HbA1c: Some studies suggest that HbA1c levels may differ between ethnic groups at equivalent blood glucose concentrations. This is an area of ongoing clinical discussion and research. Clinicians should be aware of this when interpreting results, particularly in individuals from certain ethnic backgrounds, and may consider complementary glucose-based testing where appropriate.
If your clinician suspects any of these factors may be affecting your result, they will discuss alternative testing options — such as fasting plasma glucose or OGTT — with you. This is consistent with NICE NG28 guidance on the limitations of HbA1c for diagnosis.
| Feature | HbA1c (Non-Fasting) | Fasting Plasma Glucose |
|---|---|---|
| Fasting required | No — eat and drink normally beforehand | Yes — minimum 8-hour fast required |
| What it measures | Average blood glucose over preceding 2–3 months via glycated haemoglobin | Blood glucose concentration at a single point in time |
| Diagnostic threshold (UK) | ≥48 mmol/mol indicates diabetes (NICE NG28); 42–47 mmol/mol = prediabetes | ≥7.0 mmol/L indicates diabetes; 6.1–6.9 mmol/L = impaired fasting glucose |
| Affected by recent food/drink | No — short-term dietary changes do not meaningfully alter result | Yes — highly sensitive to recent dietary intake and activity |
| Key limitations / when not to use | Unreliable in pregnancy, children, suspected type 1 diabetes, haemolytic anaemia, haemoglobin variants, advanced CKD, recent transfusion | Requires scheduling; affected by stress, illness, and acute physiological changes |
| Primary clinical use | Diagnosis and ongoing monitoring of type 2 diabetes in adults; NHS Diabetes Prevention Programme referral | Diagnosis when HbA1c is unreliable; screening for impaired fasting glucose |
| NICE / NHS guidance | Recommended as primary diagnostic tool for adults (NICE NG28); confirmatory repeat test required in asymptomatic individuals | Used when HbA1c is contraindicated; OGTT preferred in pregnancy (NICE NG3) |
What Your HbA1c Result Means According to NHS Guidelines
Under NICE NG28, an HbA1c below 42 mmol/mol is normal, 42–47 mmol/mol indicates prediabetes, and 48 mmol/mol or above indicates diabetes, requiring confirmatory repeat testing in asymptomatic adults.
In the UK, HbA1c results are expressed in mmol/mol and interpreted according to thresholds established by NICE and adopted across NHS services. Understanding these thresholds can help patients engage more meaningfully with their healthcare team.
NHS and NICE HbA1c reference ranges (NICE NG28):
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Below 42 mmol/mol: Normal range — diabetes is unlikely
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42–47 mmol/mol: Non-diabetic hyperglycaemia (also called prediabetes or impaired glucose regulation) — indicates an elevated risk of developing diabetes
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48 mmol/mol or above: Indicates diabetes (in asymptomatic adults, this should be confirmed by a repeat venous blood sample on a separate day)
It is important to note that an HbA1c of 48 mmol/mol or above indicates diabetes; the type of diabetes (for example, type 1 or type 2) is determined by clinical assessment and further investigation, not by the HbA1c result alone.
In individuals who are symptomatic (for example, experiencing excessive thirst, frequent urination, or unexplained weight loss), a single elevated result may be sufficient for diagnosis without the need for a confirmatory repeat test.
Monitoring targets for people already diagnosed with diabetes: For people living with diabetes, HbA1c is used to monitor how well blood glucose is being managed over time. NICE recommends individualised targets, but the following are commonly used benchmarks:
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48 mmol/mol — target for many adults with type 2 diabetes who are not taking medications that carry a risk of hypoglycaemia (such as insulin or sulfonylureas)
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53 mmol/mol — a less stringent target that may be appropriate for those on insulin or sulfonylureas, where a lower target would increase the risk of hypoglycaemia
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58 mmol/mol or above — NICE recommends considering intensification of treatment if HbA1c reaches or exceeds this level
These targets should always be agreed between the patient and their healthcare team, taking into account individual circumstances, preferences, and any other health conditions.
If your result falls in the non-diabetic hyperglycaemia range (42–47 mmol/mol), your GP may refer you to the NHS Diabetes Prevention Programme, a structured lifestyle intervention shown to reduce the risk of progression to diabetes. Regular monitoring — typically every 12 months — is recommended for those in this category.
Preparing for Your HbA1c Test: Practical Advice
No special preparation is needed for an HbA1c test — continue normal eating, drinking, and medications — but inform your clinician of recent illness, pregnancy, blood transfusion, or conditions that may affect red blood cell turnover.
One of the most reassuring aspects of the HbA1c test is that no special preparation is required. Because the test measures an average over several months rather than your blood glucose at a single moment, eating or drinking before the test will not meaningfully affect the result. You can attend your appointment having had a normal breakfast, a cup of tea, or your usual morning routine without concern.
However, there are a few practical points worth bearing in mind:
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Continue taking your regular medications as prescribed unless your GP or nurse specifically advises otherwise. Stopping diabetes medications before the test is not recommended and could be harmful.
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Inform your healthcare team of any recent illnesses, significant changes in diet, new medications, recent blood transfusions, or if you are pregnant or have recently given birth — as these may influence how your result is interpreted or whether an alternative test is more appropriate.
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Let your clinician know if you have a known haemoglobin variant (such as sickle cell trait), advanced kidney disease, or are receiving dialysis or erythropoietin therapy, as HbA1c may be unreliable in these situations and an alternative test may be needed.
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Be honest about lifestyle factors such as diet, physical activity, and alcohol intake. Your HbA1c result is a clinical tool to support your care, not a judgement — accurate information helps your team provide the best advice.
The test itself involves a simple blood draw, usually from a vein in your arm, and takes only a few minutes. Results are typically available within a few days, depending on your GP surgery or clinic. Some services use point-of-care HbA1c testing, which can provide a rapid result from a finger-prick sample; however, confirmatory venous laboratory testing may still be required in certain circumstances.
When to contact your GP:
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If you receive a result in the non-diabetic hyperglycaemia or diabetes range and have not yet been contacted to discuss next steps
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If you experience symptoms such as increased thirst, frequent urination, unexplained fatigue, or blurred vision before your scheduled test
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If you are unsure whether any medical condition you have might affect your HbA1c result
Regular HbA1c monitoring is a cornerstone of diabetes care in the UK. Understanding what the test involves — and what it does not require — can help reduce anxiety and encourage timely engagement with NHS services.
Frequently Asked Questions
Do I need to fast before an HbA1c blood test?
No, fasting is not required before an HbA1c test. Because it measures average blood glucose over the preceding two to three months, eating or drinking normally beforehand will not affect the result.
Can HbA1c be used to diagnose diabetes in pregnancy?
No. NICE (NG3) advises that HbA1c should not be used to diagnose diabetes or gestational diabetes in pregnancy, as physiological changes in red blood cell turnover make the result unreliable. An oral glucose tolerance test (OGTT) is the recommended alternative.
What HbA1c level indicates diabetes according to NHS guidelines?
An HbA1c of 48 mmol/mol or above indicates diabetes under NICE NG28 guidelines. In asymptomatic adults, a confirmatory repeat venous blood sample on a separate day is required before a formal diagnosis is made.
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