You ate a croissant on the morning of your blood draw and now you doubt whether your result still holds. With an ordinary glucose measurement that would matter. With HbA1c it does not, and that is exactly its strength. HbA1c is your average blood sugar over the past two to three months, expressed in mmol/mol. Unlike a single glucose measurement, HbA1c shows a trend, not just the moment of the blood draw.
Honestly, I think HbA1c is one of the most useful values for following your sugar metabolism. A single measurement tells you something about weeks, and it is insensitive to what you happened to eat that morning.
What exactly is HbA1c?
HbA1c stands for glycated haemoglobin: haemoglobin with glucose attached. The higher your blood sugar was on average, the more glucose sticks to your red blood cells. Because red blood cells last about three months, the value reflects that period. You do not need to fast for HbA1c, which makes the test practical. You can have the value measured through your HbA1c blood value.
What is a good HbA1c value?
A value below 42 mmol/mol is generally seen as normal. Above that, doctors distinguish two zones. The table below summarises the limits doctors often use. These are guides, not a hard diagnosis on their own.
| HbA1c (mmol/mol) | Interpretation doctors often use | What it means |
|---|---|---|
| Below 42 | Generally normal | Your average blood sugar is in a healthy range |
| 42 to 47 | Raised risk | Sometimes called prediabetes, a reason to adjust |
| 48 or higher | May point to diabetes | Needs confirmation and consultation with your GP |
A single abnormal result is not a diagnosis. Your GP looks at the whole picture, compares it with your glucose, and repeats the measurement if needed.
When is it worthwhile to measure HbA1c?
HbA1c can be informative with symptoms like a lot of thirst, frequent urination or unexplained fatigue, or with risk factors such as excess weight, little movement and diabetes in the family. Some people follow their value periodically to see a trend. If you want to look at your broader sugar metabolism, a diabetes blood test that measures glucose and HbA1c together fits, or a complete metabolic panel for a broader picture.
Read more about the context in our pillar on blood sugar and preventing type 2 diabetes, on fasting glucose and, if your value falls in the in-between zone, on prediabetes and how to reverse it.
The WHO recognises HbA1c as a diagnostic tool for diabetes, with 48 mmol/mol as a commonly used threshold. The Diabetes Fonds stresses that a raised HbA1c can often be favourably influenced by lifestyle, precisely because the value is an average over weeks and therefore moves with sustained change, not with a single good day. That makes HbA1c an honest mirror of your habits.
When does HbA1c differ from your glucose?
In most cases HbA1c and glucose move nicely in step, but there are situations where HbA1c gives a distorted picture. Because the value depends on your red blood cells, it can be less reliable with conditions that affect the lifespan of those cells. With anaemia or after substantial blood loss, for example, the result can turn out lower or higher than your actual average blood sugar. Pregnancy and certain rare haemoglobin variants can also disturb the measurement.
That is why a doctor, faced with a striking HbA1c, does not look only at that one number but combines it with your fasting glucose and your symptoms. If the two values look very different, that can itself be a reason to look further. If you suspect your red blood cells play a role, read also our explainer on fasting glucose and discuss the combination of your values with your GP.
Frequently asked questions
Can HbA1c be too low?
A low HbA1c is usually not a problem, but in rare cases it can relate to anaemia or a short lifespan of red blood cells. Discuss a striking result with your GP.
How quickly does HbA1c change?
Because it is an average over weeks, HbA1c changes slowly. You usually see an effect of lifestyle only after a few months, which is precisely what makes it honest. If you want to measure a change, do not repeat the value sooner than about three months, otherwise you are partly comparing the same period.
Do I need to fast for HbA1c?
No. You do not need to fast for HbA1c, because the value reflects an average over weeks and not the moment of the blood draw.
My advice: use HbA1c as a trend meter, not a snapshot. If you have symptoms or risk factors, discuss your value with your GP. Every blood test result at Vitalcheck includes a professional assessment by a BIG-registered doctor. A blood value is not a diagnosis: always discuss treatment decisions with your GP.
References
- NHG guideline Diabetes mellitus type 2. Dutch College of General Practitioners. 2021.
- World Health Organization. Use of glycated haemoglobin (HbA1c) in the diagnosis of diabetes mellitus. 2011.
- Diabetes Fonds. HbA1c and lifestyle in type 2 diabetes. Accessed 2026.
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