HbA1c: what it is, what it tells you and why we test it

HbA1c, or glycated haemoglobin, shows how much sugar you have had in your blood, on average, over the last two to three months. It does not depend on what you ate yesterday, and you do not need to fast for it. It is one of the most widely used tests to find prediabetes and diabetes early.

Why we test it

Sugar in your blood slowly sticks to haemoglobin, the protein in red blood cells, and stays there for the cell’s whole life, about four months. The more sugar there is, the more sticks.

That is why HbA1c is an average: it shows how the last two to three months went, not this morning.

Prediabetes and diabetes can stay hidden for years, with no symptoms. HbA1c finds them early, when there is more time to act.

What it tells you, and what it does not

If it is high, your blood sugar has usually been high for weeks. It often means your body finds it harder to handle sugar.

It does not show what you ate yesterday: one meal or one night barely counts.

And it is not always about sugar: anaemia, blood loss and some haemoglobin variants can skew it.

That is why it is read together with other tests:

• Glucose: Your blood sugar this morning, fasting
• Insulin: How hard your pancreas works to keep sugar in check
• Haemoglobin: Whether anaemia may skew the number
• Ferritin: Whether iron deficiency may shift the number

Normal HbA1c levels

The lab decides what is normal. It is usually 4 to 5.6%. Your own report may print different limits, and often gives the number in mmol/mol as well.

From 5.7% the number is a little high: many call it prediabetes. Some reports put the limit a little higher, so they may still mark it as normal.

From 6.5%, doctors speak of diabetes, but only if a second test confirms it. Age does not change the limits on your report. People who already have diabetes have a personal target, set by their doctor.

In the Rosa report you also see your optimal range and what to do to reach it or stay in it.

What your result means

Find the row for your result, in %. If your report gives it in mmol/mol, the conversion is in the questions below. 6.5% or above should always be seen by a doctor.

• Below your report’s limit, Usually below 4: Low. It is rare. Red blood cells, not sugar, are usually behind it. Check your blood count too, and talk to your doctor.
• Within your report’s limits, Usually 4 to 5.6: Normal. Usually fine. Check your glucose too: the two are read together. In the Rosa report you also see whether you are in your optimal range.
• 5.7 to 6.4: A little high. Some lab reports still call this normal, but many call it prediabetes. It is a signal you can often turn around. Move more, eat less sugar and white flour, and check again after about three months.
• 6.5 or above: High. From here, doctors speak of diabetes, but only if a second test confirms it. Talk to a doctor: they usually repeat the test or check your glucose.

If your HbA1c is high or low

• High HbA1c: Your blood sugar has usually been high for weeks. The causes and what to do are below.
• Low HbA1c: It is rare. Red blood cells are usually behind it, for example after blood loss or with some anaemias.

High HbA1c: the causes

Usually your blood sugar is behind it, rising little by little. More rarely the number is skewed, for example by iron deficiency.

• Insulin resistance: Your body responds less to insulin, the hormone that lets sugar into your cells. It often comes years before diabetes, and it can be improved.
• Sugar and refined flour: Weeks of sweets, sugary drinks and white bread and pasta show up in this number.
• Extra kilos and little movement: Gaining weight and moving little make sugar harder to handle.
• Stress and poor sleep: Weeks of stress or short nights can raise your glucose, and over time your HbA1c too.
• Diabetes: From 6.5%, doctors speak of diabetes, but only if a second test confirms it.

Normal glucose but high HbA1c

This happens often. Glucose is your blood sugar at one moment, in the morning while fasting. HbA1c is the average of day and night, meals included. If it is high while your glucose is normal, it often means your blood sugar rises too much after meals.

Your doctor can check this with a glucose tolerance test, which measures your blood sugar before and after a sugary drink. They can also check your iron, which sometimes shifts the number.

The opposite can happen too, high glucose and a normal HbA1c: a night of poor sleep or a stressful stretch raises glucose for a short while. But if your glucose stays high, talk to your doctor: some anaemias make HbA1c look lower than it is.

How to lower your HbA1c

It comes down when your blood sugar stays lower for months. Where to start:

• Move every day: even a walk after meals helps your muscles use sugar.
• If you carry a few extra kilos, losing even a few helps.
• What to eat: fewer sweets, sugary drinks and white flour; more wholegrain bread and pasta, legumes, vegetables and extra virgin olive oil.
• More fibre: oats, barley and legumes slow down how fast sugar reaches your blood.
• Sleep enough: poor sleep makes sugar harder to handle.
• Be patient: HbA1c changes at the pace your red blood cells renew, so check it again after about three months.

What can skew the result

Some conditions change the link between HbA1c and sugar. It helps to know them before you read it.

• Anaemia: Some anaemias make HbA1c look lower than it is. That is why it is read with the blood count.
• Iron deficiency: It can shift the number even when your sugar does not change. It is read together with ferritin.
• Thalassaemia carriers: Being a healthy carrier of thalassaemia, which is fairly common, or having another haemoglobin variant can skew the number. It also depends on the lab’s method.
• Blood loss and transfusions: After a major blood loss or a transfusion, the number can be skewed for a few months.
• Pregnancy: In pregnancy the number changes. Other tests are used to look for diabetes in pregnancy.
• Kidney disease: With advanced kidney disease, or on dialysis, the number is less reliable.

When to worry

Talk to a doctor if:

• Your HbA1c is 6.5% or above.
• It is high and your ferritin is low: the number needs a second look.
• It stays high after three months of changes.
• You are very thirsty, pass urine often or lose weight for no reason: see a doctor the same day. If you also have vomiting, belly pain, fast breathing, confusion or severe drowsiness, call 112 or go to the emergency department.

With Rosa, an HbA1c of 6.5% or above is always flagged to a doctor.

Frequently asked questions

What is HbA1c? It is the part of your haemoglobin, the protein in red blood cells, that has sugar stuck to it. It is measured as a percentage: the higher it is, the more sugar you have had in your blood over the last two to three months.

What are normal HbA1c levels? Usually 4 to 5.6%, but the lab decides. From 5.7% the number is a little high, and from 6.5% doctors speak of diabetes if a second test confirms it.

My report shows mmol/mol: how do I read it? It is the same measurement in another unit, and many labs print both. 5.6% is about 38 mmol/mol. From 39 to 47 mmol/mol the number is a little high, and from 48 up doctors speak of diabetes, if a second test confirms it.

Do I need to fast for an HbA1c test? No: it is an average over months, so one meal barely counts. It is usually done with other tests that do need fasting, like glucose, so it is best to arrive fasted.

How often should it be checked? For people who feel well there is no fixed rule: Rosa measures it once a year. If it is a little high, it is usually checked again after about three months, or when your doctor says so.

What is the difference between HbA1c and glucose? Glucose is the sugar in your blood at that moment, and it changes from day to day. HbA1c is the average of the last two to three months, and it changes much less. Together they tell you much more.

Sources

• World Health Organization, use of glycated haemoglobin (HbA1c) in the diagnosis of diabetes mellitus. Geneva, 2011. PMID 26158184
• American Diabetes Association, diagnosis and classification of diabetes: Standards of Care in Diabetes 2025. Diabetes Care, 2025. PMID 39651986
• Echouffo-Tcheugui and colleagues, diagnosis and management of prediabetes: a review. JAMA, 2023. PMID 37039787
• Hoelzel and colleagues, the comparison of the international reference method with national methods, the source of the HbA1c conversion equation. Clinical Chemistry, 2004. PMID 14709644
• English and colleagues, the effect of anaemia and red blood cell changes on HbA1c: a systematic review. Diabetologia, 2015. PMID 25994072