FPG and HbA1c in diabetics are the two cornerstone blood tests used to monitor and diagnose diabetes in the UK. The fasting plasma glucose (FPG) test measures blood sugar after an overnight fast, reflecting the body's baseline glucose regulation, while HbA1c reveals average blood glucose control over the preceding two to three months. Together, they give clinicians and patients a fuller picture of glycaemic health. This article explains how both tests work, what your results mean, NHS diagnostic thresholds, target ranges, and the factors that can affect accuracy — all aligned with current NICE and NHS guidance.

Understanding FPG and HbA1c as Diabetes Monitoring Tools

FPG measures baseline fasting blood glucose, while HbA1c reflects average glucose control over two to three months; together they provide a comprehensive assessment of glycaemic regulation in people with diabetes.

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For people living with diabetes, two blood tests form the cornerstone of glucose monitoring: the fasting plasma glucose (FPG) test and the HbA1c (glycated haemoglobin) test. Although both measure aspects of blood glucose control, they provide distinctly different types of information and are used in complementary ways.

The FPG test measures the concentration of glucose in the blood after a period of fasting — typically at least eight hours without food or caloric drink. It reflects the body's baseline ability to regulate blood sugar in the absence of recent dietary intake. Elevated fasting glucose indicates impaired insulin secretion, reduced insulin sensitivity (particularly hepatic insulin resistance), or both — resulting in excess glucose release from the liver overnight and inadequate suppression of hepatic glucose output.

The HbA1c test, by contrast, does not require fasting. It measures the percentage of haemoglobin molecules in red blood cells that have become glycated — that is, chemically bonded to glucose over time. Because red blood cells have a lifespan of approximately 90–120 days, HbA1c provides a reliable retrospective picture of average blood glucose control over the preceding two to three months. This makes it particularly valuable for assessing long-term glycaemic management rather than a single moment in time.

Together, FPG and HbA1c give clinicians a more complete picture of a patient's glucose regulation. While HbA1c reflects sustained trends, FPG can highlight acute fasting hyperglycaemia that may not be fully captured by the longer-term average. Understanding both tests helps patients engage more meaningfully with their diabetes care and treatment decisions.

Sources: NHS.uk — Diabetes diagnosis; NICE NG28 (Type 2 diabetes in adults).

How FPG and HbA1c Tests Are Used in NHS Diagnosis

The NHS diagnoses type 2 diabetes when FPG is ≥7.0 mmol/L or HbA1c is ≥48 mmol/mol on two separate occasions; a single abnormal result suffices if symptoms are present.

In the United Kingdom, both FPG and HbA1c are recognised diagnostic tools for type 2 diabetes and non-diabetic hyperglycaemia (NDH), in line with guidance from NICE (National Institute for Health and Care Excellence) and the World Health Organization (WHO). The NHS uses these tests not only to confirm a diagnosis but also to identify individuals at high risk who may benefit from early intervention.

For a diagnosis of type 2 diabetes, the following thresholds apply in asymptomatic individuals (two separate tests are required to confirm the diagnosis):

• FPG ≥ 7.0 mmol/L on two occasions

• HbA1c ≥ 48 mmol/mol (6.5%) on two occasions

• Random plasma glucose ≥ 11.1 mmol/L or 2-hour OGTT plasma glucose ≥ 11.1 mmol/L are also diagnostic

In symptomatic individuals (e.g., polyuria, polydipsia, unexplained weight loss), a single abnormal result is sufficient for diagnosis.

If FPG and HbA1c results are discordant, the test should be repeated and results interpreted in clinical context, taking into account factors such as acute illness, anaemia, or assay limitations.

Important: HbA1c should not be used to diagnose type 1 diabetes or diabetes in children and young people. If type 1 diabetes is suspected at any age, plasma glucose-based tests should be used and same-day specialist assessment arranged urgently. HbA1c is also not appropriate for diagnosis during pregnancy or within two months postpartum.

For non-diabetic hyperglycaemia (NDH/prediabetes), the NHS recognises the following ranges:

• FPG between 6.1 and 6.9 mmol/L — this reflects the WHO definition of impaired fasting glucose (IFG)

• HbA1c between 42 and 47 mmol/mol (6.0–6.4%)

However, the NHS Diabetes Prevention Programme (NHS DPP) uses broader eligibility criteria: referral is appropriate for adults with an HbA1c of 42–47 mmol/mol or an FPG of 5.5–6.9 mmol/L. This means some individuals with fasting glucose between 5.5 and 6.0 mmol/L may be eligible for the programme even though they fall below the WHO IFG threshold. The NHS DPP offers structured lifestyle interventions to reduce the risk of progression to type 2 diabetes. Your GP or diabetes care team will advise on the most appropriate test and referral pathway for your individual circumstances.

Sources: NICE NG28 (Type 2 diabetes in adults); NHS England — NHS Diabetes Prevention Programme service specification; WHO 2011 guidance on HbA1c for diagnosis; NHS.uk — Diabetes diagnosis.

Interpreting Your Results: Target Ranges for People with Diabetes

NICE recommends an HbA1c target of 48 mmol/mol for type 2 diabetes managed by lifestyle or a single non-hypoglycaemic drug, rising to 53 mmol/mol for those on hypoglycaemia-risk medications.

Once a diagnosis of diabetes has been established, ongoing monitoring of FPG and HbA1c helps assess how well blood glucose is being managed and whether treatment adjustments are needed. Understanding your personal target ranges is an important part of self-management.

HbA1c targets for people with type 2 diabetes, as recommended by NICE (NG28), are generally:

• 48 mmol/mol (6.5%) for those managed by lifestyle or a single non-hypoglycaemic drug

• 53 mmol/mol (7.0%) for those on drugs that carry a risk of hypoglycaemia (e.g., sulphonylureas or insulin)

For people with type 1 diabetes, NICE (NG17) recommends an HbA1c target of 48 mmol/mol (6.5%) where this can be achieved safely without problematic hypoglycaemia.

Capillary blood glucose targets differ between diabetes types. The following are typical UK targets, though individual targets should always be agreed with your care team:

Type 1 diabetes (NICE NG17):

• Waking (fasting): 5–7 mmol/L

• Before meals: 4–7 mmol/L

• Two hours after meals: 5–9 mmol/L

Type 2 diabetes (NICE NG28 / Diabetes UK guidance):

• Before meals (fasting or pre-prandial): 4–7 mmol/L

• Two hours after meals: typically below 8.5 mmol/L, though this may vary by treatment regimen

It is important to recognise that targets should be individualised. Older adults, those with significant comorbidities, or individuals with a history of severe hypoglycaemia may have less stringent targets agreed with their care team. Conversely, younger patients or those planning pregnancy may aim for tighter control. Always discuss your personal targets with your GP, practice nurse, or diabetes specialist, rather than comparing results with others.

Sources: NICE NG17 (Type 1 diabetes in adults); NICE NG28 (Type 2 diabetes in adults); Diabetes UK — Blood sugar level targets.

Factors That Can Affect FPG and HbA1c Readings

Anaemia, haemoglobin variants, pregnancy, and chronic kidney disease can distort HbA1c results, while illness, certain medications, and inadequate fasting can falsely elevate FPG.

Both FPG and HbA1c results can be influenced by a range of physiological, medical, and lifestyle factors. Being aware of these helps patients and clinicians interpret results accurately and avoid unnecessary concern or misdiagnosis.

Factors affecting FPG:

• Recent illness or infection can temporarily raise fasting glucose due to stress hormones such as cortisol and adrenaline

• Inadequate fasting — consuming food or sugary drinks within eight hours of the test will falsely elevate the result

• Certain medications can raise fasting glucose levels, including corticosteroids (e.g., prednisolone), thiazide diuretics, and atypical antipsychotics (e.g., olanzapine, clozapine); always inform your healthcare team of all medicines you are taking

• Physical inactivity and poor sleep have been shown to impair insulin sensitivity and raise fasting glucose

Factors affecting HbA1c:

• Anaemia (particularly iron-deficiency anaemia) can falsely elevate HbA1c, while haemolytic anaemia may falsely lower it

• Haemoglobin variants such as HbS or HbC can interfere with certain HbA1c assay methods, producing unreliable results; laboratories will usually flag potential interference

• Pregnancy causes increased red blood cell turnover, which can lower HbA1c independently of glucose levels

• Chronic kidney disease (CKD) and liver disease may also affect HbA1c reliability; in CKD, the effect varies depending on the assay method used and whether erythropoietin therapy is prescribed

• Recent blood transfusion introduces donor red blood cells, diluting the proportion of glycated haemoglobin

If your clinician suspects that a result may be unreliable due to any of these factors, they may request an alternative test or repeat the measurement under different conditions. Always inform your healthcare team of any recent illness, new medications, or significant lifestyle changes before undergoing testing.

If you believe a medicine has caused an unexpected change in your blood glucose or HbA1c, this can be reported to the MHRA via the Yellow Card Scheme at yellowcard.mhra.gov.uk or through the Yellow Card app.

Sources: NICE NG28 (Type 2 diabetes in adults) — when HbA1c is inappropriate; NHS.uk — HbA1c test.

When Clinicians Use FPG Alongside HbA1c for Monitoring

FPG is used alongside HbA1c when HbA1c is unreliable, when assessing the dawn phenomenon, or in gestational diabetes, where a fasting plasma glucose ≥5.6 mmol/L is diagnostic.

While HbA1c remains the primary monitoring tool for long-term glycaemic control in most people with diabetes, there are specific clinical situations where FPG testing provides additional or complementary value.

One key scenario is in the assessment of fasting hyperglycaemia, which may be disproportionately elevated compared to post-meal glucose. This pattern — sometimes associated with the dawn phenomenon (a natural rise in blood glucose in the early morning hours due to hormonal changes) — may not be fully reflected in the HbA1c result. Identifying elevated fasting glucose in this context can prompt targeted adjustments to evening medication or insulin regimens.

FPG is also particularly useful when HbA1c is unreliable due to the conditions described in the previous section. In patients with haemoglobin variants, haemolytic conditions, or during pregnancy, fasting glucose measurements (or an OGTT) provide a more accurate assessment of glycaemic status.

In gestational diabetes, FPG is a central component of both diagnosis and monitoring throughout pregnancy, as HbA1c is not recommended for diagnostic use in this context. According to NICE NG3 (Diabetes in pregnancy), gestational diabetes is diagnosed if:

• Fasting plasma glucose ≥ 5.6 mmol/L, or

• 2-hour OGTT plasma glucose ≥ 7.8 mmol/L

Typical capillary blood glucose targets during pregnancy (NICE NG3) are:

• Fasting: ≤ 5.3 mmol/L

• 1 hour after meals: ≤ 7.8 mmol/L

• 2 hours after meals: ≤ 6.4 mmol/L

Women with gestational diabetes should be under the care of a joint obstetric and diabetes team and should contact their maternity diabetes team promptly if glucose readings fall outside agreed targets.

For people using continuous glucose monitoring (CGM) or self-monitoring of blood glucose (SMBG), fasting readings taken at home can complement HbA1c results reviewed in clinic, helping to identify patterns such as nocturnal hypoglycaemia or early morning hyperglycaemia. NICE (NG17) recommends that CGM should be offered to all adults with type 1 diabetes. For CGM users, time in range (TIR) — commonly defined as the proportion of time spent with glucose between 3.9 and 10.0 mmol/L, with a target of ≥ 70% — is increasingly used alongside HbA1c to give a fuller picture of glycaemic control. Clinicians may review both sets of data together to make more informed, personalised treatment decisions.

Sources: NICE NG3 (Diabetes in pregnancy); NICE NG17 (Type 1 diabetes in adults) — CGM use.

NICE Guidelines on Blood Glucose Testing in Diabetes Management

NICE recommends HbA1c every three to six months when treatment is being adjusted and every six months once stable; CGM should be offered to all adults with type 1 diabetes.

NICE provides detailed, evidence-based guidance on the frequency and use of blood glucose testing in diabetes management, which forms the basis of NHS clinical practice across England and Wales.

For type 2 diabetes, NICE guidance (NG28) recommends that HbA1c is measured:

• Every 3–6 months when treatment is being adjusted or when glycaemic control is suboptimal

• Every 6 months once the patient is stable on treatment and targets are being met

For type 1 diabetes, NICE guidance (NG17) recommends HbA1c testing at least every 3–6 months, with more frequent monitoring during periods of change in treatment or lifestyle. NICE also recommends that CGM should be offered to all adults with type 1 diabetes, and that TIR should be considered alongside HbA1c when reviewing glycaemic control.

Regarding self-monitoring of blood glucose, NICE recommends that structured SMBG — which includes fasting readings — should be offered to people with type 2 diabetes who are on insulin, or where self-monitoring is clinically indicated (e.g., during illness, when driving, or when adjusting medication). Routine SMBG is not recommended for all people with type 2 diabetes managed by diet or non-insulin medication alone, unless there is a specific clinical reason.

When to contact your GP or diabetes team:

• HbA1c consistently above your agreed target despite adherence to treatment

• Fasting glucose readings persistently above 7.0 mmol/L

• Frequent hypoglycaemic episodes (blood glucose below 4.0 mmol/L)

• Symptoms of hyperglycaemia such as excessive thirst, frequent urination, or unexplained fatigue

• If you are pregnant and glucose readings fall outside your agreed targets, contact your maternity diabetes team promptly

When to seek urgent or emergency care: If you experience symptoms that may indicate diabetic ketoacidosis (DKA) — such as abdominal pain, vomiting, rapid or deep breathing, drowsiness, confusion, or a fruity smell on the breath — or if you have very high blood glucose with ketones present, seek emergency medical care immediately (call 999 or go to A&E). Similarly, symptoms of hyperosmolar hyperglycaemic state (HHS) — including extreme thirst, confusion, or reduced consciousness in the context of very high glucose — require urgent emergency assessment.

If you are unwell (sick-day rules): check your blood glucose and ketones more frequently, maintain hydration, and contact your diabetes team or GP for guidance on medication management during illness.

Regular review with your GP, practice nurse, or diabetes specialist is essential. NICE emphasises a person-centred approach to diabetes management, meaning that testing frequency and targets should always be tailored to the individual's circumstances, preferences, and overall health goals.

Sources: NICE NG17 (Type 1 diabetes in adults); NICE NG28 (Type 2 diabetes in adults); NHS.uk — Diabetic ketoacidosis (DKA).

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