Fasting blood sugar and HbA1c are the two principal blood tests used in the UK to diagnose diabetes and assess glucose control. Although both measure aspects of blood glucose, they work in fundamentally different ways: fasting blood sugar captures a single-point glucose reading after an overnight fast, while HbA1c reflects average glucose levels over the preceding two to three months. Understanding the difference between these tests — including when each is appropriate, what your results mean, and what to do next — is essential for anyone navigating a diabetes diagnosis or monitoring their metabolic health through the NHS.

What Are Fasting Blood Sugar and HbA1c Tests?

Fasting blood sugar measures glucose at a single point after an eight-hour fast, while HbA1c reflects average blood glucose over the preceding two to three months by measuring glycated haemoglobin in red blood cells.

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Fasting blood sugar (also called fasting plasma glucose, or FPG) and HbA1c (glycated haemoglobin) are two of the most widely used blood tests for assessing glucose metabolism and diagnosing diabetes. Both are available through the NHS and are recommended by NICE (NG28) as primary diagnostic tools, yet they measure different aspects of blood glucose control.

A fasting blood sugar test measures the concentration of glucose in your blood after a period of at least eight hours without eating or drinking anything other than water. It provides a snapshot of your blood glucose level at a single point in time, reflecting how well your body manages glucose in a fasted state.

The HbA1c test measures the percentage of haemoglobin molecules in your red blood cells that have glucose attached to them. Because red blood cells survive for approximately 120 days (around four months), HbA1c reflects your average blood glucose level over the preceding two to three months — though it is weighted more heavily towards the most recent four to six weeks. This makes it a particularly useful indicator of longer-term glucose control rather than a momentary reading.

Both tests are performed using a blood sample — typically taken from a vein in the arm by a healthcare professional; venous sampling is the standard method. The fasting blood sugar test requires specific preparation (fasting overnight), whereas the HbA1c test can generally be taken at any time of day without fasting, making it more convenient for many patients. In some settings, a validated point-of-care HbA1c device may be used where quality assurance standards are met, though laboratory analysis remains the reference standard.

How Each Test Is Used to Diagnose Diabetes in the UK

NICE (NG28) accepts both HbA1c and fasting plasma glucose as first-line diagnostic tests for type 2 diabetes; two abnormal results are required in asymptomatic individuals, but HbA1c must not be used in pregnancy, children, or suspected type 1 diabetes.

In the UK, NICE guidelines (NG28) and NHS diagnostic criteria align with World Health Organisation (WHO) recommendations for diagnosing type 2 diabetes and identifying prediabetes (also referred to as non-diabetic hyperglycaemia, or NDH). Both fasting blood sugar and HbA1c are accepted diagnostic tests, and either can be used depending on clinical circumstances.

For a diagnosis of type 2 diabetes in a person without symptoms, NICE recommends that two separate abnormal test results are required — these can be two HbA1c readings, two fasting plasma glucose readings, or one of each. Where results are discordant, the test that was above the diagnostic threshold should be repeated. In a person with classic symptoms of diabetes (such as polyuria, polydipsia, and unexplained weight loss), a single abnormal result is sufficient to confirm the diagnosis.

A random plasma glucose of 11.1 mmol/L or above, taken at any time of day regardless of when the person last ate, is also diagnostic of diabetes in the presence of classic symptoms. This is an important pathway that avoids the need for fasting or a repeat appointment.

The HbA1c test has become the preferred first-line diagnostic tool in many UK primary care settings because it does not require fasting, is less susceptible to acute fluctuations, and can be performed at any time. However, HbA1c should not be used for diagnosis in the following groups, as results may be unreliable or inappropriate:

• Pregnant women (gestational diabetes requires an oral glucose tolerance test, OGTT)

• Children and young people

• People with suspected type 1 diabetes (who should be referred for same-day specialist assessment)

• People with symptoms of acute onset or short duration

• People with conditions affecting red blood cell turnover (e.g., haemolytic anaemia, haemoglobin variants, recent blood transfusion)

In these situations, fasting plasma glucose or OGTT should be used instead.

HbA1c is also used to monitor glycaemic control in people already diagnosed with diabetes, helping clinicians and patients assess how well blood glucose is being managed over time.

The fasting plasma glucose test remains important where HbA1c may be unreliable (see above) and is used alongside HbA1c within the NHS Health Check programme, which screens adults aged 40–74 for cardiovascular and metabolic risk factors. In practice, HbA1c is commonly used within the NHS Health Check because it does not require fasting; FPG is an acceptable alternative where HbA1c is unsuitable or unavailable. Both tests play a complementary role in the broader framework of diabetes prevention and management in the UK.

Understanding Your Results: NHS Reference Ranges Explained

An HbA1c of 48 mmol/mol or above, or a fasting plasma glucose of 7.0 mmol/L or above, is diagnostic of diabetes; values in the 42–47 mmol/mol or 5.5–6.9 mmol/L range indicate prediabetes (non-diabetic hyperglycaemia).

Understanding what your results mean is essential for making informed decisions about your health. The NHS and NICE use the following reference ranges:

Fasting Blood Sugar (Fasting Plasma Glucose):

• Below 5.5 mmol/L — Normal range; no evidence of impaired glucose regulation

• 5.5–6.9 mmol/L — Non-diabetic hyperglycaemia (NDH/prediabetes); indicates increased risk of developing type 2 diabetes and may qualify for NHS Diabetes Prevention Programme referral. Note: the WHO defines impaired fasting glucose (IFG) as 6.1–6.9 mmol/L, but UK practice (including NHS DPP eligibility) uses the lower threshold of 5.5 mmol/L

• 7.0 mmol/L or above — Diagnostic of diabetes (if confirmed on a second test or accompanied by symptoms)

HbA1c:

• Below 42 mmol/mol (6.0%) — Normal; no evidence of diabetes or prediabetes

• 42–47 mmol/mol (6.0–6.4%) — Non-diabetic hyperglycaemia (NDH/prediabetes); increased risk of type 2 diabetes

• 48 mmol/mol (6.5%) or above — Diagnostic of type 2 diabetes (if confirmed on a second test or accompanied by symptoms)

Random Plasma Glucose:

• 11.1 mmol/L or above with classic symptoms — Diagnostic of diabetes without the need for a fasting test

Oral Glucose Tolerance Test (OGTT) — 2-hour plasma glucose:

• Below 7.8 mmol/L — Normal

• 7.8–11.0 mmol/L — Impaired glucose tolerance (IGT)

• 11.1 mmol/L or above — Diagnostic of diabetes

These thresholds apply to adults and are used for diagnostic purposes. Different criteria apply in children and young people, and in pregnancy, where gestational diabetes is diagnosed using an OGTT rather than HbA1c or fasting glucose alone. HbA1c should not be used for diagnosis in these groups.

If your results fall in the prediabetes range, this does not mean you have diabetes — but it is a significant warning sign. With appropriate lifestyle changes, many people are able to return their blood glucose levels to the normal range and substantially reduce their risk of progressing to type 2 diabetes.

Which Test Is More Appropriate for Your Situation?

HbA1c is generally preferred for routine screening and monitoring as it requires no fasting, but fasting plasma glucose or OGTT should be used when HbA1c may be unreliable or when type 1 diabetes is suspected.

The choice between fasting blood sugar and HbA1c is not always straightforward and depends on individual clinical circumstances. Your GP or healthcare team will consider several factors when deciding which test — or combination of tests — is most appropriate for you.

HbA1c is generally preferred when:

• Fasting is difficult or impractical (e.g., in elderly patients or those with complex medication regimens)

• A longer-term picture of glucose control is needed

• Monitoring the effectiveness of diabetes treatment over time

• Screening is being conducted as part of a routine health check

Fasting plasma glucose may be more appropriate when:

• HbA1c results may be unreliable due to haematological conditions (see next section)

• Rapid diagnosis is needed in symptomatic patients (alongside or instead of random plasma glucose)

• Assessing acute changes in glucose regulation

Random plasma glucose (≥11.1 mmol/L with classic symptoms) is a valid and convenient diagnostic route that avoids the need for fasting and is particularly useful when a patient presents acutely unwell.

Important: If type 1 diabetes is suspected — for example, in a younger person with rapid onset of symptoms, significant weight loss, or ketonuria — this should prompt same-day urgent assessment by a specialist. Do not wait for routine repeat testing.

In some cases, neither FPG nor HbA1c alone is sufficient, and an oral glucose tolerance test (OGTT) — which measures blood glucose before and two hours after consuming a standardised glucose drink — may be recommended. This is particularly relevant in pregnancy, or when results from other tests are borderline or inconsistent.

Note that fasting plasma glucose is not an appropriate investigation for suspected hypoglycaemia. Evaluation of hypoglycaemia requires glucose measurement during symptoms (Whipple's triad) and, where needed, specialist-supervised testing. If you experience recurrent episodes of low blood sugar, speak to your GP, who can arrange appropriate assessment or referral.

No single test should be interpreted in isolation. Your GP will consider your full clinical picture, including symptoms, risk factors (such as family history, BMI, ethnicity, and age), and any relevant medical history, before reaching a diagnosis or recommending further investigation.

Factors That Can Affect the Accuracy of Each Test

HbA1c can be falsely raised by iron deficiency anaemia and falsely lowered by haemolytic anaemia or recent blood transfusion; fasting blood sugar is invalidated by failure to fast and can be temporarily elevated by illness, stress, or certain medicines.

Both fasting blood sugar and HbA1c tests can be influenced by a range of physiological, medical, and lifestyle factors, which may lead to falsely elevated or falsely lowered results. Being aware of these factors helps ensure that results are interpreted correctly.

Factors affecting fasting blood sugar accuracy:

• Recent illness or stress can temporarily raise blood glucose levels due to the release of counter-regulatory hormones such as cortisol and adrenaline

• Certain medicines, including corticosteroids, antipsychotics, and some antihypertensives, can elevate fasting glucose

• Failure to fast properly — consuming food or caloric drinks within eight hours of the test — will invalidate the result

• Timing of the test relative to recent physical activity or alcohol consumption may also influence readings

Factors affecting HbA1c accuracy:

• Haemoglobin variants (e.g., sickle cell trait, haemoglobin C or E) can interfere with certain HbA1c assay methods, producing unreliable results. If interference is suspected, consult your local laboratory about the assay method used

• Iron deficiency anaemia may falsely raise HbA1c, whereas haemolytic anaemia or recent blood transfusion typically lower it by increasing red blood cell turnover

• Chronic kidney disease and liver disease may also affect HbA1c reliability

• Ethnicity — some studies suggest HbA1c may slightly overestimate average glucose in people of African or Afro-Caribbean descent; however, UK diagnostic thresholds are not adjusted by ethnicity, and the same reference ranges apply to all adults

Where HbA1c is considered unreliable, fasting plasma glucose or OGTT should be used instead. In some specialist settings, fructosamine (which reflects glucose control over the preceding two to three weeks) may be considered as an alternative. If your clinician suspects that either test may be unreliable in your case, they may request an alternative or additional test to ensure an accurate diagnosis.

Next Steps After Abnormal Results: NHS Guidance

Prediabetes results should prompt GP referral to the NHS Diabetes Prevention Programme; a confirmed diabetes diagnosis triggers structured education, a management plan, and regular monitoring of HbA1c, kidney function, eyes, and feet.

Receiving an abnormal fasting blood sugar or HbA1c result can feel unsettling, but it is important to understand that a single result is rarely the end of the story. The NHS and NICE have clear pathways to guide what happens next, depending on the level of abnormality detected.

If your results suggest non-diabetic hyperglycaemia (prediabetes):

• Eligibility for the NHS Diabetes Prevention Programme (NHS DPP) is based on an HbA1c of 42–47 mmol/mol or a fasting plasma glucose of 5.5–6.9 mmol/L recorded within the past 12 months

• Your GP should refer you to the NHS DPP, a free, evidence-based behavioural intervention programme available across England

• The programme focuses on structured education around healthy eating, physical activity, and weight management

• Blood glucose should typically be monitored annually to check whether levels are improving, stable, or progressing

• Lifestyle changes — including losing 5–10% of body weight if overweight, increasing physical activity to at least 150 minutes per week, and reducing refined carbohydrate intake — have been shown to significantly reduce the risk of progression to type 2 diabetes

If your results are diagnostic of type 2 diabetes:

• A second confirmatory test will usually be arranged (unless you have clear symptoms or a random plasma glucose ≥11.1 mmol/L)

• You will be referred for structured diabetes education, such as the DESMOND or X-PERT programme

• Your GP will initiate a management plan, which may include lifestyle advice, monitoring, and potentially medicine such as metformin

• Regular reviews will be scheduled to monitor HbA1c, blood pressure, kidney function (eGFR and urine albumin:creatinine ratio, ACR), and cholesterol

• You will be referred for NHS Diabetic Eye Screening and will receive an annual foot examination

• Relevant vaccinations (including influenza and pneumococcal) will be offered

When to seek urgent medical attention:

• If you experience symptoms such as excessive thirst, frequent urination, unexplained weight loss, blurred vision, or extreme fatigue, contact your GP promptly

• If you feel unwell and have a known diagnosis of diabetes, particularly if you are vomiting, have abdominal pain, feel drowsy or confused, are breathing rapidly, or are severely dehydrated, seek emergency care immediately (call 999 or go to A&E) — these may be signs of diabetic ketoacidosis (DKA) or hyperosmolar hyperglycaemic state (HHS), both of which are medical emergencies

• If you are at risk of DKA (e.g., type 1 diabetes or insulin-treated type 2 diabetes) and feel unwell, check your blood ketone levels if you have a ketone meter and follow your sick-day rules

• If a home glucose monitor reading is consistently above 15 mmol/L, contact your GP or diabetes team

If you believe any medicine has caused an unexpected reaction, you can report this to the MHRA via the Yellow Card scheme (yellowcard.mhra.gov.uk).

Early intervention is key. Whether your results indicate prediabetes or diabetes, acting promptly and engaging with NHS support services can make a meaningful difference to your long-term health outcomes.

Frequently Asked Questions