Your GP might have mentioned your HbA1c in passing, or you might have seen it listed on a blood test form and wondered what it actually checks. Here is the direct answer: an HbA1c blood test measures the percentage of your red blood cells carrying sugar-coated haemoglobin, which gives you an average picture of your blood sugar over the past two to three months. It is one of the main markers used to assess diabetes risk, and it is different from the finger-prick or fasting glucose reading most people picture when they think of a "blood sugar test."
Why "average" is the whole point
A regular fasting glucose test tells you what your blood sugar is doing right now, at the moment the sample is taken. That number moves constantly, depending on what you ate the night before, how you slept, or whether you exercised that morning.
HbA1c works differently. Glucose in your blood attaches to haemoglobin, the protein in red blood cells that carries oxygen, and it stays attached for the roughly three-month lifespan of the red blood cell. The more sugar has been circulating in your blood over that period, the more of your haemoglobin ends up coated in it. So instead of a single snapshot, HbA1c gives you a rolling average, which is why it is used to track blood sugar control over time rather than to catch a one-off spike.
This is also why HbA1c and a fasting glucose or oral glucose tolerance test (OGTT) answer different questions. Fasting glucose and OGTT (usually done at your GP surgery or a hospital, since it involves several blood draws over a couple of hours) are more sensitive to what is happening in the days around the test. HbA1c is more useful for the bigger trend. Many people track both, because they can tell different parts of the story.
What the result ranges generally mean
Blood test reports typically categorise HbA1c results into three bands, based on the diagnostic criteria set out by NICE [1]. These are population-level reference ranges used by UK labs and GPs, not a personal diagnosis, and your GP will always interpret your result alongside your history and any other tests.
| Category | HbA1c (%) | HbA1c (mmol/mol) | | --- | --- | --- | | Within the normal range | Below 6.0% | Below 42 mmol/mol | | Increased risk range (sometimes called non-diabetic hyperglycaemia, or prediabetes) | 6.0% to 6.4% | 42 to 47 mmol/mol | | Diabetes range | 6.5% or above | 48 mmol/mol or above |
A result in the increased risk range does not mean a diagnosis. It means a marker your GP will usually want to discuss further, often alongside a fasting glucose or OGTT, before deciding on next steps. A result in the normal range is reassuring, but it is a point-in-time picture of the last few months, not a guarantee about the months ahead, particularly if other risk factors like weight change, family history or insulin resistance are in play.
Where HbA1c can be misleading
HbA1c is a reliable, well-established test, but it has known limitations worth knowing about before you interpret a result on your own:
- Iron-deficiency anaemia can raise HbA1c artificially, independent of your actual blood sugar level. This effect reverses once the iron deficiency is treated [5][6].
- Haemolytic anaemia and recent blood loss can lower HbA1c artificially, because red blood cells are being replaced faster than usual [7].
- Kidney disease can affect how HbA1c is interpreted.
- Pregnancy changes red blood cell turnover, so HbA1c is not the preferred test for gestational diabetes screening.
- Some haemoglobin variants, including thalassaemia trait, sickle cell trait and other inherited variants more common in people of Mediterranean, African or South Asian background, can interfere with how some lab methods measure the test [7].
None of this means the test is unreliable for most people. It means that if your HbA1c and how you actually feel, or what a home glucose reading shows, seem to disagree, that is a reason to raise it with your GP rather than a reason to dismiss either result.
Why this matters beyond diabetes risk
HbA1c sits inside a bigger picture of metabolic health. Blood sugar that runs a little high for a long time, even within the "increased risk" band, is often connected to insulin resistance, the process where your cells stop responding properly to insulin and your body has to produce more of it to keep blood sugar in range. Insulin resistance can show up years before HbA1c or fasting glucose move outside the normal range, which is why people trying to understand stubborn weight gain, fatigue or sugar cravings often look at insulin resistance testing alongside HbA1c rather than instead of it.
If you want the fuller metabolic picture in one order, rather than piecing together separate tests, a broader panel such as the Metabolic Reset Test covers blood sugar alongside insulin, inflammation, hormones and organ function. For a simpler annual baseline that includes a diabetes screen alongside your cholesterol, liver and kidney markers, the Lifestyle Check is the more accessible starting point.
How often should you test
There is no single answer that fits everyone, which is exactly why this is a conversation for your GP rather than a fixed rule. In the UK, if your result falls in the increased risk range, your GP will advise on retesting, often around a year later, in line with how non-diabetic hyperglycaemia is monitored in NHS practice [3]. Separately, the NHS Health Check offers a broader diabetes risk assessment to adults aged 40 to 74 every five years [4]. For people with a confirmed type 2 diabetes diagnosis, NICE guidance recommends testing every three to six months while treatment is being adjusted, moving to six-monthly once blood sugar and treatment are stable [2].
What to do with your result
If your HbA1c comes back in the normal range, that is a good baseline to have on record, and testing again in a year or two gives you a trend rather than a single point. If it comes back in the increased risk or diabetes range, the right next step is a conversation with your GP, not a lifestyle overhaul based on one number. Your GP may want to repeat the test, add a fasting glucose or OGTT, or look at insulin resistance markers to build a fuller picture before deciding what, if anything, needs to change.
For a full view of the tests that sit alongside HbA1c, including insulin, thyroid and metabolic hormone testing, see Support Healthy Weight, Metabolism & Blood Sugar.
You do not need a GP referral to order an HbA1c test with i-screen. Your finger-prick test kit is posted directly to you, so there is no need to visit a clinic or collection centre, and you can view your result in your dashboard, ready to bring to a conversation with your GP if it needs one.
This article is general information, not personal medical advice. It does not diagnose any condition. Always discuss your results with a qualified healthcare professional.
