Fasting blood glucose and HbA1c are both used to assess blood sugar, but they are not the same test. Fasting blood glucose measures the concentration of glucose in your blood at a single point in time after an overnight fast, whilst HbA1c reflects your average blood glucose levels over the preceding two to three months. Understanding the distinction between these two tests is essential for interpreting your results accurately, knowing which test is appropriate for your circumstances, and taking the right steps if your results fall outside the normal range. This article explains what each test measures, how they compare, and what UK clinical guidance recommends.
Summary: Fasting blood glucose and HbA1c are not the same test — fasting blood glucose measures glucose at a single point in time after an overnight fast, whilst HbA1c reflects average blood glucose levels over the preceding two to three months.
- Fasting plasma glucose (FPG) requires an 8–10 hour fast and provides an immediate snapshot of blood sugar; a result of 7.0 mmol/L or above on two occasions is diagnostic of diabetes.
- HbA1c measures the proportion of haemoglobin glycated by glucose over approximately 120 days, expressed in mmol/mol; 48 mmol/mol (6.5%) or above is diagnostic of type 2 diabetes in UK practice.
- HbA1c is unreliable in certain conditions including haemolytic anaemia, haemoglobin variants, pregnancy, recent blood transfusion, and advanced chronic kidney disease — fasting plasma glucose or OGTT should be used instead.
- NICE guidance (NG28) recommends HbA1c as the preferred first-line diagnostic test for type 2 diabetes in most adults, as it requires no fasting and reflects longer-term glycaemic control.
- Non-diabetic hyperglycaemia (prediabetes) is indicated by HbA1c 42–47 mmol/mol or FPG 6.1–6.9 mmol/L; lifestyle intervention can significantly reduce progression to type 2 diabetes.
- Discordant results between the two tests are not uncommon and may reflect postprandial hyperglycaemia or conditions affecting HbA1c reliability; clinical context guides which test to repeat or prioritise.
Table of Contents
What Fasting Blood Glucose and HbA1c Actually Measure
Fasting plasma glucose measures blood sugar at a single point in time after an 8–10 hour fast, whilst HbA1c measures average blood glucose over the preceding 2–3 months by assessing glycated haemoglobin in red blood cells.
Have any more questions about this? Message our pharmaceutical team to get more info →
To understand whether fasting blood glucose is the same as HbA1c, it helps to first appreciate what each test is actually measuring — because they assess blood sugar in fundamentally different ways.
Fasting plasma glucose (FPG) measures the concentration of glucose circulating in your blood at a single point in time, specifically after you have not eaten or drunk anything (other than water) for at least 8–10 hours. It is measured from a venous blood sample analysed in a laboratory and reflects your body's baseline ability to regulate blood sugar in the absence of recent food intake. The result is expressed in millimoles per litre (mmol/L) and provides an immediate snapshot of glycaemic status.
HbA1c (glycated haemoglobin), by contrast, does not measure glucose directly. Instead, it measures the proportion of haemoglobin — the protein in red blood cells — that has become permanently bound to glucose through a process called glycation. Because red blood cells survive for around 120 days, HbA1c reflects your average blood glucose levels over the preceding 2–3 months. It is expressed either as a percentage or in millimoles per mole (mmol/mol), with the latter being the standard unit used in UK laboratories.
In essence, fasting plasma glucose is a real-time measurement, whilst HbA1c is a longer-term indicator. Neither test is superior in isolation; rather, they provide complementary information about how the body is managing glucose. Understanding this distinction is the first step in interpreting your results accurately and knowing what action, if any, may be needed.
Am I eligible for weight loss injections?
Find out whether you might be eligible!
Answer a few quick questions to see whether you may be suitable for prescription weight loss injections (like Wegovy® or Mounjaro®).
- No commitment — just a quick suitability check
- Takes about 1 minute to complete
Key Differences Between the Two Tests
HbA1c requires no fasting and is more stable day-to-day, but is unreliable in conditions such as haemolytic anaemia, haemoglobin variants, pregnancy, and advanced CKD, where fasting plasma glucose or OGTT is preferred.
Although both tests are used to assess blood glucose control, there are several important practical and clinical differences between them that influence when and why each is chosen.
Preparation requirements differ significantly:
-
Fasting plasma glucose requires an 8–10 hour fast before the venous blood sample is taken, usually first thing in the morning.
-
HbA1c requires no fasting whatsoever and can be taken at any time of day, making it considerably more convenient for patients.
Variability and reliability also differ: Fasting plasma glucose can fluctuate considerably from day to day depending on factors such as recent illness, stress, physical activity, sleep quality, and the timing of the last meal. HbA1c is generally more stable and reproducible, as it averages glucose exposure over months rather than capturing a single moment.
Situations where HbA1c should not be used for diagnosis: NICE and the WHO advise that HbA1c is unreliable or inappropriate for diagnosis in a number of clinical circumstances, including:
-
Children and young people (FPG or OGTT is preferred)
-
Suspected type 1 diabetes or acute hyperglycaemia (short duration of symptoms; FPG or random plasma glucose should be used)
-
Pregnancy — HbA1c must not be used to diagnose gestational diabetes at any stage; an oral glucose tolerance test (OGTT) is used instead
-
Haemolytic anaemia or other conditions that shorten red blood cell lifespan
-
Haemoglobin variants (e.g., sickle cell trait, haemoglobin C disease)
-
Iron deficiency anaemia, which may falsely elevate HbA1c
-
Recent blood transfusion or acute blood loss
-
Advanced chronic kidney disease (CKD)
-
Medications that may cause rapid glycaemic change, such as corticosteroids or certain antipsychotics
-
Erythropoietin therapy, HIV antiretroviral therapy, or vitamin B12 deficiency
In these circumstances, fasting plasma glucose or an OGTT is preferred. Conversely, fasting plasma glucose may be less informative in people whose glucose levels fluctuate widely throughout the day, as it captures only one data point. Recognising these limitations helps clinicians select the most appropriate test for each individual patient.
When Each Test Is Used in NHS Diabetes Diagnosis
HbA1c of 48 mmol/mol or above is the preferred first-line diagnostic test for type 2 diabetes in UK adults; fasting plasma glucose of 7.0 mmol/L or above on two occasions is used when HbA1c is inappropriate or to confirm borderline results.
In the UK, both fasting plasma glucose and HbA1c are recognised diagnostic tools for type 2 diabetes and non-diabetic hyperglycaemia (prediabetes), as outlined in NICE guidance (NG28) and endorsed by NHS England. However, their use is not entirely interchangeable, and specific criteria apply.
HbA1c is now the preferred first-line diagnostic test for type 2 diabetes in most adults in the UK, largely because it does not require fasting and is less susceptible to acute fluctuations. For diagnostic purposes, a quality-assured venous laboratory HbA1c is required; point-of-care (POC) HbA1c devices are not acceptable for diagnosis unless they meet the relevant quality assurance standards. A single HbA1c result of 48 mmol/mol (6.5%) or above is diagnostic of type 2 diabetes in a person without symptoms, provided the result is confirmed by a repeat test. In symptomatic individuals, a single result may be sufficient.
Fasting plasma glucose is used when HbA1c testing is inappropriate or unreliable (as outlined above), or to confirm a borderline result. A fasting plasma glucose of 7.0 mmol/L or above on two separate occasions is diagnostic of diabetes.
Random plasma glucose is a further diagnostic option: a result of 11.1 mmol/L or above in a person with classic symptoms of hyperglycaemia (such as excessive thirst, frequent urination, or unexplained weight loss) is diagnostic of diabetes without the need for a fasting sample.
For non-diabetic hyperglycaemia (prediabetes), the NHS recognises:
-
HbA1c of 42–47 mmol/mol (6.0–6.4%) as indicating increased risk
-
Fasting plasma glucose of 6.1–6.9 mmol/L as impaired fasting glucose (WHO definition)
It is worth noting that NHS Diabetes Prevention Programme (NHS DPP) referral criteria in England may use a broader FPG range of 5.5–6.9 mmol/L alongside HbA1c 42–47 mmol/mol; local referral thresholds may vary, and your GP or practice nurse can advise on eligibility in your area. Similar prevention programmes exist across the devolved nations, though criteria may differ.
The oral glucose tolerance test (OGTT), which measures blood glucose before and two hours after a glucose drink (a 2-hour value of ≥11.1 mmol/L is diagnostic of diabetes), remains the gold standard in certain situations, including gestational diabetes screening. Your GP or practice nurse will advise which test is most appropriate based on your individual circumstances.
How Results From Both Tests Compare and Relate
Results broadly correlate — both elevated FPG and raised HbA1c indicate poor glycaemic control — but discordant results occur, for example when postprandial hyperglycaemia elevates HbA1c despite a normal fasting glucose.
Whilst fasting plasma glucose and HbA1c measure different aspects of glycaemic control, their results do broadly correlate — higher average blood glucose levels over time will generally produce a higher HbA1c, and a raised fasting glucose often accompanies an elevated HbA1c. However, the relationship is not perfectly linear, and discordant results are not uncommon.
For example, a person may have a normal fasting glucose but a mildly elevated HbA1c if their blood sugar rises significantly after meals (postprandial hyperglycaemia) but returns to normal by morning. Conversely, someone with impaired fasting glucose may have a near-normal HbA1c if their post-meal glucose control is relatively preserved.
The comparison below provides a general overview of how results align diagnostically across the main tests used in UK practice:
-
Normal: FPG below 6.1 mmol/L | HbA1c below 42 mmol/mol (6.0%)
-
Non-diabetic hyperglycaemia/increased risk: FPG 6.1–6.9 mmol/L | HbA1c 42–47 mmol/mol (6.0–6.4%)
-
Diabetes: FPG 7.0 mmol/L or above | HbA1c 48 mmol/mol (6.5%) or above | Random plasma glucose 11.1 mmol/L or above with symptoms | 2-hour OGTT glucose 11.1 mmol/L or above
It is important to understand that these thresholds are diagnostic cut-offs rather than absolute biological boundaries. Risk exists on a continuum, and someone with results just below the diagnostic threshold may still benefit from lifestyle intervention.
When results from two tests are discordant, clinicians will typically repeat the discordant test or use an alternative WHO-accepted diagnostic method, taking into account the clinical context and any factors that may affect HbA1c reliability (such as those listed in the previous section), before making a diagnosis.
| Feature | Fasting Plasma Glucose (FPG) | HbA1c (Glycated Haemoglobin) |
|---|---|---|
| What it measures | Blood glucose concentration at a single point in time | Proportion of haemoglobin bound to glucose; reflects average over 2–3 months |
| Fasting required | Yes — 8–10 hours, nothing except water | No — can be taken at any time of day |
| Unit of measurement | Millimoles per litre (mmol/L) | Millimoles per mole (mmol/mol) or percentage (%) |
| Normal range (UK) | Below 6.1 mmol/L | Below 42 mmol/mol (6.0%) |
| Non-diabetic hyperglycaemia (prediabetes) | 6.1–6.9 mmol/L (impaired fasting glucose) | 42–47 mmol/mol (6.0–6.4%) |
| Diagnostic threshold for diabetes | 7.0 mmol/L or above on two occasions | 48 mmol/mol (6.5%) or above; confirmed by repeat test if asymptomatic |
| When HbA1c is unreliable / FPG preferred | Used when HbA1c is inappropriate (e.g., pregnancy, haemolytic anaemia, haemoglobin variants, advanced CKD, recent transfusion, suspected type 1 diabetes) | Not suitable for gestational diabetes diagnosis; unreliable in haematological conditions or with certain medications |
Which Test Is Right for You: Guidance From NICE
NICE recommends HbA1c as the preferred test for most adults, but fasting plasma glucose is more appropriate when haematological conditions, pregnancy, suspected type 1 diabetes, or certain medications may affect HbA1c reliability.
Deciding which test is most appropriate depends on your individual health circumstances, and this decision is best made in partnership with your GP or practice nurse. NICE guidance (NG28) provides a clear framework to support this.
HbA1c is generally preferred for routine risk assessment and testing in high-risk groups, and for diagnosis of type 2 diabetes in adults, because it is convenient, does not require fasting, and reflects longer-term glucose exposure. A quality-assured venous laboratory HbA1c is required for diagnostic purposes. It is particularly useful in:
-
Routine health checks, including the NHS Health Check (offered to adults aged 40–74 in England)
-
Monitoring glycaemic control in people already diagnosed with diabetes
-
Testing individuals at high risk, such as those with obesity, a family history of type 2 diabetes, or polycystic ovary syndrome (PCOS)
Fasting plasma glucose may be more appropriate in the following situations:
-
When HbA1c results may be unreliable due to haematological conditions, haemoglobin variants, recent transfusion, advanced CKD, or certain medications
-
In children and young people, or when type 1 diabetes or acute hyperglycaemia is suspected
-
During pregnancy or when gestational diabetes is suspected (OGTT is used; HbA1c must not be used to diagnose gestational diabetes)
-
When confirming a borderline or discordant HbA1c result
NICE also recommends that people identified with non-diabetic hyperglycaemia (HbA1c 42–47 mmol/mol or impaired fasting glucose) should be offered structured lifestyle advice. In England, eligible individuals may be referred to the NHS Diabetes Prevention Programme (NHS DPP), a free evidence-based programme. Referral criteria in England commonly include FPG 5.5–6.9 mmol/L or HbA1c 42–47 mmol/mol, though local criteria may vary. Equivalent prevention services are available in Scotland, Wales, and Northern Ireland. Regular monitoring — typically annually — is advised for this group, as a proportion will progress to type 2 diabetes without intervention.
Understanding Your Results and Next Steps
Normal results require ongoing healthy lifestyle habits; non-diabetic hyperglycaemia warrants lifestyle intervention and annual monitoring; diabetic-range results require confirmatory testing and a personalised management plan from your GP or diabetes care team.
Receiving a blood test result related to blood glucose can feel concerning, but understanding what the numbers mean — and what to do next — can help you take informed, constructive action.
If your results are within the normal range, this is reassuring, but it does not mean risk is absent, particularly if you have other risk factors such as excess weight, physical inactivity, or a family history of diabetes. Maintaining a healthy lifestyle remains important, and your GP may recommend periodic re-testing.
If your results suggest non-diabetic hyperglycaemia (prediabetes), this is an important opportunity rather than a cause for alarm. Research consistently shows that lifestyle changes — including a balanced diet, regular physical activity, and modest weight loss — can significantly reduce the risk of progression to type 2 diabetes. You may be referred to the NHS Diabetes Prevention Programme (in England), and your GP will likely arrange annual monitoring.
If your results are in the diabetic range, a confirmatory test will usually be arranged before a formal diagnosis is made (unless you have clear symptoms such as excessive thirst, frequent urination, or unexplained weight loss). Following diagnosis, your GP or diabetes care team will discuss a personalised management plan, which may include:
-
Structured diabetes education (e.g., the DESMOND programme)
-
Dietary and lifestyle advice
-
Medication such as metformin, if appropriate
-
Regular monitoring of HbA1c, blood pressure, kidney function, and cholesterol
-
Referral to the NHS Diabetic Eye Screening Programme
-
A foot risk assessment to check circulation and nerve function
When to contact your GP:
-
If you experience symptoms of high blood sugar (excessive thirst, frequent urination, fatigue, or blurred vision)
-
If a home capillary glucose monitor reading is persistently 11 mmol/L or above, particularly with symptoms
Seek urgent same-day medical attention (call 111 or go to A&E) if you or someone with diabetes experiences:
-
Very high blood glucose (e.g., consistently above 20–25 mmol/L)
-
Ketones in the urine or blood
-
Vomiting, severe abdominal pain, or rapid breathing
-
Drowsiness, confusion, or difficulty staying awake
-
Signs of severe dehydration
These may be signs of diabetic ketoacidosis (DKA) or hyperosmolar hyperglycaemic state (HHS), both of which require emergency assessment.
Both fasting plasma glucose and HbA1c are valuable tools in understanding your metabolic health. Used together, they provide a fuller picture — and acting on the results early can make a meaningful difference to long-term health outcomes.
Frequently Asked Questions
Can I use fasting blood glucose instead of HbA1c to diagnose diabetes?
Yes, fasting plasma glucose is a recognised alternative to HbA1c for diagnosing diabetes in the UK. A fasting plasma glucose of 7.0 mmol/L or above on two separate occasions is diagnostic of diabetes, and it is the preferred test when HbA1c results may be unreliable, such as in pregnancy, haemolytic anaemia, or certain haemoglobin variants.
Why might my fasting glucose be normal but my HbA1c be raised?
This can occur when blood glucose rises significantly after meals (postprandial hyperglycaemia) but returns to normal by the following morning, meaning fasting glucose appears normal whilst HbA1c — which reflects average glucose over 2–3 months — remains elevated.
Do I need to fast before an HbA1c blood test?
No, fasting is not required before an HbA1c test. Unlike fasting plasma glucose, HbA1c can be measured at any time of day regardless of when you last ate, making it considerably more convenient for routine testing and diagnosis.
The health-related content published on this site is based on credible scientific sources and is periodically reviewed to ensure accuracy and relevance. Although we aim to reflect the most current medical knowledge, the material is meant for general education and awareness only.
The information on this site is not a substitute for professional medical advice. For any health concerns, please speak with a qualified medical professional. By using this information, you acknowledge responsibility for any decisions made and understand we are not liable for any consequences that may result.
Any third-party brands or services referenced on this site are included for informational purposes only; we are entirely independent and have no affiliation, partnership, or collaboration with any companies mentioned.
Heading 1
Heading 2
Heading 3
Heading 4
Heading 5
Heading 6
Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur.
Block quote
Ordered list
- Item 1
- Item 2
- Item 3
Unordered list
- Item A
- Item B
- Item C
Bold text
Emphasis
Superscript
Subscript
