HbA1c explained: The 3-Month Blood Sugar Picture

An HbA1c test is a blood test that shows your average blood sugar level over the past two to three months. It's one of the most useful checks for spotting type 2 diabetes and prediabetes early, and for keeping an eye on blood sugar if you already live with diabetes. Here's what the test measures, what your results mean, and when it's worth checking.

What is an HbA1c test?

An HbA1c test measures how much sugar (glucose) has attached to the haemoglobin in your red blood cells. Haemoglobin is the protein that carries oxygen around your body, and when there's glucose in your blood, some of it sticks to that protein. The more glucose in your blood over time, the more builds up.

Because red blood cells live for around two to three months, the amount of glycated haemoglobin gives a reliable read on your average blood sugar across that whole period. That's where the name comes from: HbA1c is shorthand for glycated haemoglobin.

HbA1c vs a standard glucose test: snapshot vs average

A finger-prick glucose test is a snapshot; it tells you what your blood sugar is doing at one single moment. If you eat a meal, feel stressed, or skip breakfast, the number shifts.

An HbA1c test works differently. Rather than one moment, it reflects the trend across two to three months, so a single high or low reading won't skew it. That longer view is why clinicians rely on it to diagnose diabetes and track how well blood sugar is being managed.

You don't need to fast before an HbA1c test. You can eat and drink normally beforehand, which makes it easier to fit around your day than tests that require an empty stomach.

What your HbA1c results mean

In the UK, HbA1c results are reported in mmol/mol (you may also see a percentage). Levels fall into three broad ranges:

These thresholds follow NICE and NHS guidance. A result of 48 mmol/mol or above may suggest type 2 diabetes, though if you don't have symptoms, a second confirmatory test is usually needed before a diagnosis is made.

A result in the 42–47 range doesn't mean you have diabetes, but it's a signal that blood sugar is higher than it should be, and a prompt to look at diet, activity and weight.

If you already have diabetes, your healthcare team may set you a personal target rather than a single fixed number, based on your circumstances.

Why the 3-month window matters

Blood sugar that creeps up tends to do so quietly. Prediabetes in particular often has no symptoms at all, so it's easy to miss until it progresses. A one-off glucose reading can look fine on the day and still miss a gradual rise. The two-to-three-month average is what catches that pattern early, while there's still time to act.

Our own health check data shows how often this goes unnoticed. Around 1 in 8 men we test are prediabetic or diabetic, and many have no idea. More broadly, around 1 in 3 completed blood profiles come back with at least one result outside the healthy range. Checking your HbA1c is one of the simpler ways to check where you stand.

What can affect your HbA1c reading

HbA1c is reliable for most people, but a few things can skew it. Conditions affecting your red blood cells, such as iron-deficiency anaemia, haemolytic anaemia, and inherited haemoglobin variants like sickle cell or thalassaemia, can raise or lower results. Advanced kidney disease and a recent blood transfusion can have a similar effect.

HbA1c also isn't the right test for diagnosing diabetes in pregnancy, in children, or where type 1 diabetes is suspected. In those situations, a different test is used. If any of these apply to you, mention it when you book, as another type of test may give a clearer answer.

How often should you check your HbA1c?

If you live with diabetes, HbA1c is usually checked every three to six months, more often if your treatment has recently changed or your levels have been hard to manage. If you're at higher risk but don't have diabetes, periodic checks help you track whether your levels are holding steady or starting to climb. The two-to-three-month biology is a natural guide here: testing much more frequently than that tells you little extra.

Checking your HbA1c with Bluecrest

An HbA1c blood test is included in all of our full body health checks, so you can understand your blood sugar alongside a wider view of your health. There's no need to fast, appointments take around 20 to 30 minutes, and there's likely to be a clinic within 20 minutes of your home.

Every result comes with a clear green, amber or red flag, plus follow-up advice on what to do next, whether that's a simple lifestyle change, ongoing monitoring, or a conversation with your GP. You can explore our health check packages or find your nearest clinic to book.

Frequently asked questions

Is an HbA1c test the same as a blood sugar test?

Not quite. A standard glucose test measures your blood sugar at one moment in time. An HbA1c test measures your average blood sugar over the past two to three months, giving a longer-term view.

Do I need to fast before an HbA1c test?

No. You can eat and drink normally before an HbA1c test, which is one of the reasons it's such a convenient check.

What is a normal HbA1c level in the UK?

A normal HbA1c is below 42 mmol/mol (below 6.0%). A result of 42–47 mmol/mol suggests prediabetes, and 48 mmol/mol or above may suggest type 2 diabetes.

Can I check my HbA1c at home?

Home HbA1c kits exist, but they don't always give the most complete results. A test carried out at a clinic, analysed by an accredited laboratory, gives you a more reliable reading and a personalised results report.