HbA1c and fasting insulin tests together offer a more complete picture of how your body manages blood glucose and insulin than either test alone. The HbA1c test is a well-established NHS diagnostic tool that reflects average blood glucose over the preceding 8–12 weeks, whilst fasting insulin measures circulating insulin after an overnight fast and can help identify insulin resistance at an early stage. Understanding what each test measures, when they are recommended, and how to interpret the results is essential for making informed decisions about your metabolic health and next steps in care.

What Are HbA1c and Fasting Insulin Tests?

HbA1c measures average blood glucose over 8–12 weeks via glycated haemoglobin, whilst fasting insulin measures circulating insulin after an overnight fast to help detect insulin resistance before glucose levels become abnormal.

The HbA1c test and the fasting insulin test are two distinct but complementary blood tests used to assess how the body manages blood glucose. Together, they can provide a more complete picture of metabolic health than either test alone.

HbA1c (glycated haemoglobin) measures the percentage of haemoglobin in red blood cells that has glucose attached to it. Red blood cells have a lifespan of approximately 120 days; because glucose attaches to haemoglobin progressively over this period, HbA1c reflects average blood glucose levels over the preceding 8–12 weeks rather than a single moment in time. It is expressed in millimoles per mole (mmol/mol) in the UK, following standardisation by the International Federation of Clinical Chemistry (IFCC), and is endorsed by the World Health Organization (WHO 2011) as a diagnostic tool for type 2 diabetes. This test is central to the diagnosis and monitoring of type 2 diabetes within NHS clinical practice.

Fasting insulin measures the concentration of insulin in the blood after a period of fasting, typically 8–12 hours. Insulin is a hormone produced by the beta cells of the pancreas; its primary role is to facilitate the uptake of glucose into cells for energy. When cells become resistant to insulin — a condition known as insulin resistance — the pancreas compensates by producing more insulin. Elevated fasting insulin levels may therefore indicate insulin resistance before blood glucose levels become overtly abnormal. However, it is important to note that fasting insulin is not a standard NHS screening or diagnostic test, reference intervals are assay- and laboratory-specific, and results must always be interpreted alongside other clinical findings by a qualified clinician.

Whilst HbA1c is a well-established NHS diagnostic tool, fasting insulin testing is not routinely offered on the NHS and is more commonly requested in specialist or private settings. Understanding what each test measures helps patients and clinicians interpret results in the appropriate clinical context.

When Are These Tests Recommended on the NHS?

HbA1c is the NHS-recommended diagnostic test for type 2 diabetes in adults per NICE NG28 and WHO 2011; fasting insulin is not a standard NHS test and is reserved for specialist scenarios such as unexplained hypoglycaemia or metabolic syndrome assessment.

According to NICE guideline NG28 (Type 2 diabetes in adults: management) and the WHO 2011 report on the use of HbA1c in the diagnosis of diabetes mellitus, HbA1c is the preferred diagnostic test for type 2 diabetes in adults who are not acutely unwell. It is recommended in the following circumstances:

• Diagnosis of type 2 diabetes: An HbA1c of 48 mmol/mol (6.5%) or above on two separate occasions (or once if symptomatic) confirms a diagnosis. For diagnostic purposes, a venous blood sample analysed by an IFCC-aligned laboratory method should be used; point-of-care (finger-prick) HbA1c testing is not recommended for confirming a diagnosis.

• Non-diabetic hyperglycaemia (prediabetes): An HbA1c of 42–47 mmol/mol (6.0–6.4%) indicates increased risk and warrants lifestyle intervention and annual monitoring, as outlined in NICE guideline NG215 (Type 2 diabetes: prevention in people at high risk).

• Routine monitoring: For people already diagnosed with diabetes, HbA1c is typically measured every 3–6 months to assess glycaemic control.

• Cardiovascular risk assessment: HbA1c or fasting plasma glucose may be offered as part of the NHS Health Check for adults aged 40–74, but only when a risk assessment indicates an elevated risk of diabetes — it is not offered routinely to all attendees.

HbA1c is not appropriate for diagnosing diabetes in the following groups, where fasting plasma glucose or an oral glucose tolerance test (OGTT) should be used instead:

• Pregnant women

• Children and young people

• People with suspected type 1 diabetes

• People who are acutely unwell

• Those with symptoms of diabetes present for fewer than two months

• People with haemoglobin variants or haemoglobinopathies (e.g., sickle cell trait)

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

• People with advanced chronic kidney disease

Fasting insulin is not a standard NHS diagnostic test for diabetes. It may be considered by a GP or specialist in specific clinical scenarios, such as:

• Supporting assessment of metabolic syndrome in individuals with central obesity, dyslipidaemia, or hypertension, where it may provide additional context alongside glucose-based tests

• Investigating possible insulin resistance in people with polycystic ovary syndrome (PCOS) — noting that PCOS diagnosis does not require fasting insulin, and NICE CKS recommends OGTT or fasting plasma glucose when assessing glycaemic risk in this group

• Evaluating unexplained hypoglycaemia, where inappropriately elevated insulin levels may suggest insulinoma or another cause of endogenous hyperinsulinism — however, a formal supervised fast measuring insulin, C-peptide, proinsulin, and a sulfonylurea screen is required for this investigation, and endocrinology referral is essential

• Supporting clinical decisions in specialist endocrinology or obesity medicine settings

Patients seeking fasting insulin testing outside these indications are likely to need to access it through private healthcare providers.

How to Prepare and What to Expect

HbA1c requires no fasting and can be taken at any time, whilst fasting insulin requires an 8–12 hour fast, avoidance of strenuous exercise for 24 hours prior, and a morning venous blood draw.

Preparing correctly for these blood tests is essential to ensure accurate and clinically meaningful results.

For HbA1c, no fasting is required. The test can be taken at any time of day, which makes it particularly convenient for patients. For diagnostic purposes, a venous blood sample is drawn from a vein in the arm (venepuncture) and sent to an accredited laboratory for analysis. In some monitoring settings, a finger-prick capillary sample may be used, but this is not suitable for confirming a new diagnosis of diabetes. Results are typically available within a few days, though turnaround times may vary between NHS trusts.

For fasting insulin, preparation is more specific:

• Fast for 8–12 hours before the test — water is permitted, but food, sugary drinks, tea, and coffee should be avoided

• Avoid strenuous exercise in the 24 hours prior, as physical activity can temporarily affect insulin sensitivity

• Do not stop any prescribed medicines before testing unless specifically advised to do so by your clinician

• Inform your clinician of all medications you are taking, particularly corticosteroids, antipsychotics, or hormonal therapies, as these can influence insulin and glucose levels

• The blood sample is ideally taken in the morning to account for natural diurnal variation in insulin secretion; your local laboratory will advise on the preferred timing

• Specific pre-analytical instructions may vary between laboratories; always follow the guidance provided by your healthcare provider or local pathology service

Both tests involve a routine blood draw, which carries minimal risk. Some patients may experience mild bruising, light-headedness, or discomfort at the venepuncture site. If you feel faint, inform the healthcare professional immediately. If you experience any unexpected reaction to a medicine or medical device, you can report it via the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk.

If both tests are being conducted simultaneously — as is sometimes the case in private metabolic health assessments — a single fasting blood draw can often provide samples for both analyses, minimising inconvenience for the patient.

Understanding Your Results and Reference Ranges

An HbA1c below 42 mmol/mol is normal, 42–47 mmol/mol indicates prediabetes, and 48 mmol/mol or above confirms type 2 diabetes; fasting insulin reference ranges are not standardised and must be interpreted using your laboratory's specific intervals.

Interpreting your results requires understanding the reference ranges used in UK clinical practice, as well as the context of your individual health history.

HbA1c reference ranges (IFCC, UK standard):

• Below 42 mmol/mol: Normal — no evidence of diabetes or prediabetes

• 42–47 mmol/mol: Non-diabetic hyperglycaemia (prediabetes) — increased risk of developing type 2 diabetes

• 48 mmol/mol or above: Diagnostic of type 2 diabetes (when confirmed on a second venous laboratory sample, or on a single sample in the presence of symptoms)

HbA1c results can be affected by a number of conditions. Iron deficiency anaemia generally causes HbA1c to be falsely elevated; correction of iron deficiency may lower the result. Haemolytic anaemia, acute blood loss, and recent blood transfusion can cause HbA1c to be falsely low or unreliable, as red blood cell turnover is accelerated. In advanced chronic kidney disease, HbA1c is often less reliable and may be lower than expected due to reduced red blood cell lifespan, use of erythropoiesis-stimulating agents, and transfusions. Your clinician will consider these factors when interpreting your result, and may request alternative glucose-based tests if HbA1c is considered unreliable.

Fasting insulin reference ranges are not universally standardised and vary between laboratories and assay methods. There are no broadly accepted UK-wide diagnostic thresholds for fasting insulin. Your result should always be interpreted using the reference interval provided by the reporting laboratory, alongside your fasting glucose level, clinical history, and other relevant investigations. Providing specific numerical thresholds here would be misleading, as these differ between assays.

The HOMA-IR (Homeostatic Model Assessment of Insulin Resistance) is a calculated index derived from fasting glucose and fasting insulin, sometimes used in research and specialist settings to estimate insulin resistance. It is not a validated NHS diagnostic tool, thresholds vary by assay, laboratory, and ethnicity, and it is not routinely recommended by NICE for clinical diagnosis. If your clinician reports a HOMA-IR value, they will interpret it using their laboratory's specific reference values and in the context of your overall clinical picture.

Always discuss your results with a qualified clinician rather than interpreting them in isolation.

Next Steps If Your Results Are Abnormal

An HbA1c of 42–47 mmol/mol warrants referral to the NHS Diabetes Prevention Programme and annual monitoring per NICE NG215; confirmed type 2 diabetes triggers a structured care plan with metformin as first-line therapy per NICE NG28.

Receiving an abnormal result can feel concerning, but early detection provides an opportunity for meaningful intervention before more serious complications develop.

If your HbA1c indicates non-diabetic hyperglycaemia (42–47 mmol/mol), NICE guideline NG215 recommends:

• Referral to the NHS Diabetes Prevention Programme (NHS DPP), a structured lifestyle intervention recommended by NICE and associated with improvements in weight and HbA1c in people at high risk of type 2 diabetes

• Dietary modification, including reducing refined carbohydrates and ultra-processed foods

• Increasing physical activity to at least 150 minutes of moderate-intensity exercise per week, in line with UK Chief Medical Officers' guidelines

• Annual HbA1c monitoring to track progression or improvement

• Weight management support if BMI is elevated

If your HbA1c confirms type 2 diabetes (48 mmol/mol or above), your GP will initiate a structured care plan, which may include lifestyle advice, self-monitoring guidance, and consideration of pharmacological therapy. Metformin remains the first-line medication recommended by NICE (NG28) for most people with type 2 diabetes, though treatment is individualised and renal function (eGFR) must be assessed before initiation, as per NICE NG28 and BNF guidance.

If you have symptoms suggesting possible type 1 diabetes — such as rapid onset of thirst, frequent urination, unexplained weight loss, or ketones in your urine or blood — seek urgent medical assessment. Treatment should not be delayed whilst awaiting further tests. When the type of diabetes is uncertain or atypical, your clinician may arrange additional tests including C-peptide levels and diabetes-specific autoantibodies (such as GAD antibodies) to help clarify the diagnosis.

If your fasting insulin is elevated, management will depend on the underlying cause identified by your clinician. Common next steps may include:

• Further investigation with an OGTT or fasting plasma glucose test where clinically indicated

• Assessment for PCOS, metabolic syndrome, or metabolic dysfunction-associated steatotic liver disease (MASLD, formerly NAFLD), including cardiometabolic risk stratification

• If insulinoma or another cause of endogenous hyperinsulinism is suspected, endocrinology referral is essential; investigation requires a supervised fast with measurement of insulin, C-peptide, proinsulin, and a sulfonylurea screen — fasting insulin alone is insufficient for this diagnosis

• Lifestyle interventions targeting insulin sensitivity, including a low-glycaemic-index diet and regular physical activity including resistance exercise

Contact your GP promptly if you experience symptoms such as unexplained weight loss, excessive thirst, frequent urination, or recurrent hypoglycaemic episodes, as these may require urgent assessment regardless of your test results.

Frequently Asked Questions About These Blood Tests

Fasting insulin is not routinely available on the NHS and is more commonly accessed privately; medications including corticosteroids, certain antipsychotics, and erythropoiesis-stimulating agents can affect both HbA1c and fasting insulin results.

Can I have an HbA1c and fasting insulin test done on the NHS at the same time? HbA1c is routinely available on the NHS for appropriate indications. Fasting insulin is not a standard NHS test for most patients and is more commonly accessed through private providers or specialist referral. If your GP believes fasting insulin testing is clinically indicated — for example, in the investigation of unexplained hypoglycaemia or as part of a specialist metabolic assessment — they may be able to arrange it, though availability varies by NHS trust and local laboratory repertoire. Interpretation should always be guided by a specialist familiar with the assay used.

Does HbA1c diagnose type 1 diabetes? HbA1c alone is not used to diagnose type 1 diabetes. Type 1 diabetes often presents acutely with symptoms, and treatment should not be delayed whilst awaiting further tests. When the type of diabetes is uncertain or atypical, additional investigations — including C-peptide levels and diabetes-specific autoantibodies (e.g., GAD antibodies) — may be arranged to help clarify the diagnosis. Your clinician will advise on the most appropriate pathway.

Can medications affect my HbA1c or fasting insulin results? Yes. Some medicines affect HbA1c by altering red blood cell production or turnover: erythropoiesis-stimulating agents (such as erythropoietin) and iron supplementation can increase red blood cell production and may lower HbA1c, whilst medicines causing haemolysis can also affect the result. Other medicines — including corticosteroids, certain antipsychotics (particularly olanzapine and clozapine), and some hormonal therapies — can raise blood glucose and insulin levels, affecting both HbA1c and fasting insulin results. Always inform your clinician of all current medications before testing, and do not stop any prescribed medicine without medical advice.

Is there a link between high fasting insulin and weight gain? Insulin promotes fat storage, and chronically elevated insulin levels — as seen in insulin resistance — are associated with difficulty losing weight and increased adiposity, particularly around the abdomen. However, the relationship is complex and bidirectional; excess body weight itself drives insulin resistance. This is an area of ongoing research, and individual circumstances vary considerably.

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How often should these tests be repeated? For people with confirmed type 2 diabetes, HbA1c is typically monitored every 3–6 months, as recommended by NICE NG28. For those with non-diabetic hyperglycaemia, annual testing is recommended per NICE NG215. Fasting insulin retesting frequency is not standardised and should be guided by your specialist or GP based on your clinical circumstances and local laboratory guidance.

Frequently Asked Questions