The best time to take an HbA1c test is when you are in your usual state of health, with stable medication and no recent major illness — but unlike a fasting glucose test, no fasting is required beforehand. The HbA1c test measures average blood glucose over the preceding 8 to 12 weeks and is used by NHS clinicians to diagnose type 2 diabetes and monitor ongoing glucose control. Understanding the optimal timing, what can affect your result, and how to prepare ensures your reading accurately reflects your glucose management and supports well-informed treatment decisions.
Summary: The best time to take an HbA1c test is when you are clinically stable, your medication has been unchanged for at least 8 to 12 weeks, and no major illness or physiological disruption has occurred recently.
- The HbA1c test measures average blood glucose over the preceding 8 to 12 weeks by assessing glycated haemoglobin in red blood cells.
- No fasting is required before an HbA1c test — you can eat, drink, and take usual medications as normal on the day.
- Testing should be deferred for approximately 8 to 12 weeks after a significant illness, surgery, or medication change to ensure results reflect usual glucose control.
- Conditions such as haemolytic anaemia, iron deficiency, haemoglobin variants, kidney disease, and pregnancy can falsely alter HbA1c results.
- NICE NG28 recommends HbA1c monitoring every 6 months for stable type 2 diabetes, or every 3 months following a treatment change.
- A result of 48 mmol/mol or above is diagnostic of type 2 diabetes in the appropriate clinical context; 42–47 mmol/mol indicates non-diabetic hyperglycaemia.
Table of Contents
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
What Is an HbA1c Test and Why Does Timing Matter?
The HbA1c test measures average blood glucose over 8 to 12 weeks by quantifying glycated haemoglobin; timing matters because illness, medication changes, and certain conditions can distort results and mislead clinical decisions.
The HbA1c test — formally known as the glycated haemoglobin test — measures the average blood glucose level over the preceding two to three months. When glucose circulates in the bloodstream, it binds to haemoglobin (the protein inside red blood cells that carries oxygen). The higher your blood glucose levels have been, the more glucose attaches to haemoglobin, producing a higher HbA1c reading. Because red blood cells live for approximately 120 days, the test reflects average glycaemia over roughly the preceding 8 to 12 weeks, rather than a single-moment snapshot.
This distinction is clinically important. Unlike a fasting blood glucose test, which reflects your glucose level at one specific point in time, the HbA1c captures patterns across weeks and months. It is used by NHS clinicians both to diagnose type 2 diabetes and to monitor how well blood glucose is being managed in people already living with diabetes. NICE guideline NG28 (Type 2 diabetes in adults: management) recommends HbA1c as the primary monitoring tool for type 2 diabetes in most adults, and the WHO 2011 guidance on HbA1c endorses a threshold of 48 mmol/mol for diagnosis.
However, HbA1c is not appropriate for diagnosing diabetes in all situations. It should not be used as a diagnostic test in the following circumstances:
-
Children and young people
-
Pregnant women (including for the diagnosis of gestational diabetes)
-
People with suspected type 1 diabetes
-
People with conditions affecting red blood cell lifespan or haemoglobin structure (such as haemolytic anaemia or haemoglobin variants)
-
People who have had a recent blood transfusion
-
People with acute illness or medication-induced hyperglycaemia (for example, following high-dose corticosteroid or antipsychotic therapy)
In these situations, a fasting plasma glucose (FPG) test or an oral glucose tolerance test (OGTT) should be used instead. Your GP will advise which test is most appropriate for your circumstances.
Timing matters because certain physiological states, recent illnesses, changes in medication, and other factors can influence the result. Understanding when and how to have the test performed helps ensure the reading accurately reflects your true glucose control — and supports your clinical team in making well-informed treatment decisions.
When Is the Best Time to Have an HbA1c Test?
The best time is when you are clinically stable, with medication unchanged for at least 8 to 12 weeks and no recent major illness; no fasting is required and the test can be taken at any time of day.
One of the practical advantages of the HbA1c test is that, unlike a fasting glucose test, it does not require you to fast beforehand. You can eat and drink normally before attending your appointment, which makes it considerably more convenient for most patients. There is no strict requirement to book a morning appointment, and the test can be performed at any time of day.
That said, there are some timing considerations worth bearing in mind:
-
Defer testing until you are clinically stable after a significant illness. Acute infections, surgery, or hospitalisation can temporarily alter red blood cell turnover and affect results. For routine monitoring purposes, it is reasonable to allow approximately 8 to 12 weeks after a major physiological disruption before testing, so that the result reflects your usual glucose control rather than the acute episode. Your GP will advise if earlier testing is clinically necessary.
-
Allow time after a medication change. If your GP has recently adjusted your diabetes medication — for example, starting or increasing a dose of metformin or an SGLT2 inhibitor — waiting at least 8 to 12 weeks before testing is appropriate. This is because HbA1c integrates glycaemia over that same 8 to 12 week period, so testing sooner will not capture the full effect of the new regimen.
-
Be aware of potential seasonal patterns. Some studies suggest HbA1c levels may be marginally higher in winter months, possibly due to reduced physical activity and dietary changes. Any such effect appears to be small and is not usually a reason to delay testing, but it may be worth noting when interpreting borderline results.
-
Routine monitoring intervals. In line with NICE NG28, people with well-controlled type 2 diabetes should have HbA1c checked every 6 months. Those with less stable control, or within 3 to 6 months of a treatment change, should be tested every 3 months until stable.
In short, the best time to take an HbA1c test is when you are in your usual state of health, your medication has been stable for at least 8 to 12 weeks, and no major physiological disruption has occurred recently.
Factors That Can Affect Your HbA1c Result
Haemolytic anaemia, iron deficiency, haemoglobin variants, chronic kidney disease, recent blood transfusion, and pregnancy can all falsely raise or lower HbA1c, making alternative glucose tests more appropriate in these situations.
Several biological and clinical factors can cause the HbA1c result to appear falsely high or falsely low, independent of actual blood glucose control. Being aware of these helps both patients and clinicians interpret results accurately.
Conditions that may falsely lower HbA1c:
-
Haemolytic anaemia — increased red blood cell destruction shortens red blood cell lifespan, meaning less time for glucose to bind to haemoglobin, which can produce an artificially low reading.
-
Iron deficiency anaemia treated with iron supplementation — as new red blood cells are produced more rapidly, the proportion of glycated cells may fall transiently.
-
Recent blood transfusion — donor red blood cells can dilute the patient's own glycated haemoglobin. The effect on HbA1c is unpredictable and depends on the donor's own HbA1c and the timing of the transfusion; the result may be unreliable for several weeks to months afterwards. Alternative glucose tests should be considered during this period.
-
Pregnancy — particularly in the second and third trimesters, increased red blood cell turnover can lower HbA1c, making it unreliable for monitoring or diagnosing diabetes in pregnancy. Fasting plasma glucose or OGTT should be used instead (see NICE NG3).
Conditions that may falsely raise HbA1c:
-
Iron deficiency anaemia (untreated) — older red blood cells accumulate, increasing the proportion of glycated haemoglobin.
-
Vitamin B12 or folate deficiency — reduced red blood cell production can prolong cell lifespan and elevate readings.
-
Chronic kidney disease (CKD) — the effect of CKD on HbA1c is variable. Depending on the degree of anaemia, use of erythropoiesis-stimulating agents (such as EPO therapy), and assay interference from uraemia, HbA1c may be falsely low or falsely high. In people with advanced CKD, HbA1c should be interpreted alongside glucose profiles; continuous glucose monitoring (CGM), self-monitored blood glucose (SMBG), or alternative markers such as glycated albumin may be more informative.
-
Haemoglobin variants — certain haemoglobin variants (such as HbS or HbC, more prevalent in people of African or Caribbean heritage) can interfere with some laboratory assays, producing unreliable results. UK laboratories may flag potential interference and, where necessary, use an alternative assay method not affected by the specific variant. Clinicians should be aware that even with variant-insensitive methods, results may require careful interpretation.
When HbA1c is unreliable for diagnosis, a fasting plasma glucose (FPG) test or oral glucose tolerance test (OGTT) should be used instead. For monitoring in situations where HbA1c is unreliable, your diabetes team may recommend SMBG, CGM, or alternative biochemical markers such as fructosamine or glycated albumin.
Always inform your clinical team of any relevant medical history — including anaemia, kidney disease, haemoglobin variants, recent transfusion, or pregnancy — before the test is performed, so that results can be interpreted appropriately.
| Consideration | Recommendation | Rationale | Notes |
|---|---|---|---|
| Fasting requirement | No fasting needed; eat and drink normally | HbA1c reflects 8–12 week average, not a single-moment glucose level | Unlike fasting plasma glucose or OGTT |
| Time of day | Any time of day is acceptable | No diurnal variation affects HbA1c reliability | Book at whatever time is convenient |
| After illness or surgery | Wait approximately 8–12 weeks after major illness | Acute illness can alter red blood cell turnover and skew results | GP may advise earlier testing if clinically necessary |
| After medication change | Wait at least 8–12 weeks after starting or adjusting diabetes medication | HbA1c integrates glycaemia over 8–12 weeks; earlier testing misses full effect | Applies to metformin, SGLT2 inhibitors, insulin, etc. |
| Routine monitoring frequency | Every 6 months if well controlled; every 3 months if unstable or post-treatment change | NICE NG28 recommendation for type 2 diabetes management | Frequency individualised by clinical team |
| Seasonal variation | No need to delay testing; be cautious with borderline results in winter | Some studies suggest marginally higher HbA1c in winter months | Effect is small; not routinely clinically significant |
| When HbA1c is inappropriate | Use fasting plasma glucose or OGTT instead | HbA1c unreliable in pregnancy, haemolytic anaemia, haemoglobin variants, recent transfusion, suspected type 1 diabetes | Consult GP; see NICE NG28 and WHO 2011 guidance |
How to Prepare for an HbA1c Test on the NHS
No fasting is needed; eat and drink normally, take usual medications, stay well hydrated, and inform the phlebotomist of any relevant conditions such as anaemia, kidney disease, or haemoglobin variants.
Preparing for an HbA1c test on the NHS is straightforward, and the process is largely the same whether you are attending your GP surgery, a community phlebotomy clinic, or a hospital outpatient department.
Key preparation steps:
-
No fasting required. You may eat and drink as normal before the test. There is no need to avoid food, water, tea, or coffee.
-
Stay well hydrated. Drinking plenty of water before your appointment can make it easier to obtain a blood sample.
-
Take your usual medications. Unless your GP has specifically advised otherwise, continue taking all prescribed medicines — including diabetes medications — as normal on the day of the test.
-
Inform the phlebotomist or nurse of relevant conditions. Let the person taking your blood know if you have a haemoglobin variant, anaemia, kidney disease, or if you are pregnant, as this may affect how the result is interpreted or which test is most appropriate.
-
Wear comfortable, loose-fitting clothing. This makes it easier to access a vein in the arm for the blood draw.
The test itself involves a simple venous blood sample, usually taken from the inside of the elbow. Results are typically available within a few days and will be communicated by your GP surgery — either via an online patient portal such as the NHS App, by letter, or at a follow-up appointment.
If you are being tested for the diagnosis of diabetes (rather than routine monitoring), NICE NG28 and WHO guidance state that a single raised HbA1c result of 48 mmol/mol (6.5%) or above is sufficient for diagnosis in a person with typical symptoms of diabetes. In the absence of symptoms, a second confirmatory test is required; this should ideally be a repeat HbA1c (using the same method), or a fasting plasma glucose test if urgent confirmation is needed.
Importantly, HbA1c should not be used to diagnose diabetes in children and young people, during pregnancy, in people with suspected type 1 diabetes, or in those with conditions affecting red blood cell lifespan or haemoglobin structure. In these situations, fasting plasma glucose or OGTT is the appropriate diagnostic test. Your GP will guide you through this process and explain what the result means for your care.
If you experience any side effects from diabetes medicines, you can report these to the Medicines and Healthcare products Regulatory Agency (MHRA) via the Yellow Card Scheme at yellowcard.mhra.gov.uk.
Understanding Your HbA1c Results and Next Steps
UK NHS results below 42 mmol/mol are normal, 42–47 mmol/mol indicates non-diabetic hyperglycaemia, and 48 mmol/mol or above is diagnostic of type 2 diabetes, with individualised targets set through shared decision-making.
HbA1c results in the UK are reported in millimoles per mole (mmol/mol), following the IFCC (International Federation of Clinical Chemistry) standardisation adopted by the NHS. Understanding where your result sits within the recognised ranges helps you engage meaningfully with your healthcare team.
NHS HbA1c reference ranges:
-
Below 42 mmol/mol — Normal range; diabetes is unlikely.
-
42–47 mmol/mol — Non-diabetic hyperglycaemia (sometimes referred to as prediabetes); indicates an elevated risk of developing type 2 diabetes.
-
48 mmol/mol or above — Indicative of type 2 diabetes (in the appropriate clinical context, and confirmed by a second test in the absence of symptoms).
For people already diagnosed with type 2 diabetes, NICE NG28 recommends an individualised target, agreed through shared decision-making with your clinical team. Typical targets are:
-
48 mmol/mol for those managed by lifestyle and diet alone, or on metformin.
-
53 mmol/mol for those on medications that carry a risk of hypoglycaemia (such as sulphonylureas or insulin).
Targets may be appropriately relaxed in older adults, those with significant comorbidities, or where tight control poses a risk of hypoglycaemia. Your clinical team will discuss what target is right for you.
If your result falls in the non-diabetic hyperglycaemia range, your GP may refer you to the NHS Diabetes Prevention Programme, a structured lifestyle intervention shown to reduce the risk of progression to type 2 diabetes. If your result confirms diabetes, your clinical team will discuss a personalised management plan, which may include dietary advice, physical activity guidance, and medication.
When to contact your GP:
-
Your result has changed significantly since your last test.
-
You are experiencing symptoms such as increased thirst, frequent urination, unexplained weight loss, or fatigue.
-
You have concerns about your medication or side effects.
Urgent safety advice: If you feel severely unwell and are experiencing symptoms such as vomiting, abdominal pain, drowsiness, rapid or laboured breathing, or confusion alongside very high blood glucose, seek same-day urgent medical advice. Call 999 or go to your nearest emergency department if you are severely unwell, as these symptoms may indicate a serious complication such as diabetic ketoacidosis (DKA) or hyperosmolar hyperglycaemic state (HHS).
Regular HbA1c monitoring is a cornerstone of diabetes management. Keeping your result within your agreed target range significantly reduces the long-term risk of complications, including cardiovascular disease, kidney disease, and diabetic retinopathy.
Frequently Asked Questions
Do I need to fast before an HbA1c test?
No fasting is required before an HbA1c test. You can eat, drink, and take your usual medications as normal before attending your appointment, as the test measures average blood glucose over the preceding 8 to 12 weeks rather than a single-moment reading.
How soon after a medication change should I have an HbA1c test?
You should wait at least 8 to 12 weeks after a diabetes medication change before having an HbA1c test, as the result integrates blood glucose control over that same period and testing sooner will not capture the full effect of the new regimen.
Can illness affect my HbA1c result?
Yes, acute infections, surgery, or hospitalisation can temporarily alter red blood cell turnover and affect HbA1c results. For routine monitoring, it is advisable to wait approximately 8 to 12 weeks after a significant illness before testing, unless your GP advises otherwise.
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
