HbA1c explained: what the number means, its units and how often it is checked

What HbA1c actually measures

Haemoglobin is the protein inside red blood cells that carries oxygen. When glucose is present in the blood, a small share of it sticks to haemoglobin. The more glucose there is over time, the larger the share that becomes “glycated” haemoglobin, written HbA1c or simply A1c.

Red blood cells live for about three months, so the test works like a long-exposure photograph rather than a snapshot. It reflects average glucose over the previous two to three months, with the most recent weeks counting for somewhat more than the oldest ones.

This is why HbA1c does not need fasting. A sample taken after breakfast gives the same answer as one taken before it, because a single meal barely changes the share of glycated haemoglobin.

What the test does not show is how your glucose moved during that period. Two people can have the same HbA1c with very different patterns, one with steady values and the other with high peaks and frequent lows. Home glucose readings or a continuous glucose monitor show that detail; HbA1c does not.

Reading the number: per cent and mmol/mol

Indian laboratories usually report HbA1c as a percentage, for example 6.8 %. Many other countries, including the UK, report it in millimoles of glycated haemoglobin per mole of total haemoglobin (mmol/mol). Both describe the same measurement, and a result in one unit can be converted to the other.

The conversion used internationally is: mmol/mol = (HbA1c % − 2.15) × 10.929. Laboratories do this for you, so you rarely need to calculate it, but the table below shows how the two scales line up. Values are rounded.

The last column is the estimated average glucose (eAG), calculated from HbA1c with the formula eAG (mg/dL) = 28.7 × HbA1c − 46.7. It is an estimate drawn from large studies, not a measured average, and your own average may differ from it.

Always check which unit your report uses. A value of 53 on an HbA1c report is almost certainly mmol/mol, while 5.3 is a percentage, and mixing them up is an easy way to be needlessly alarmed or falsely reassured.

What the usual ranges mean

For diagnosis, the ranges commonly used by the American Diabetes Association are listed below. The World Health Organization also accepts an HbA1c of 6.5 % (48 mmol/mol) or higher as a diagnostic value for diabetes.

• Below 5.7 % (39 mmol/mol): in the non-diabetic range.
• 5.7 % to 6.4 % (39 to 46 mmol/mol): often called prediabetes, meaning a higher chance of developing diabetes later.
• 6.5 % (48 mmol/mol) or above: in the range used to diagnose diabetes, usually confirmed with a repeat test or a second kind of test unless the person has clear symptoms.

For people who already have diabetes, the question is different: what target is right for this person? Many guidelines suggest a goal of around 7 % (53 mmol/mol) for many adults, but doctors set looser or tighter goals depending on age, other illnesses, the risk of low glucose, pregnancy and personal circumstances. A target your friend has been given may not be the right one for you.

How often is HbA1c checked?

There is no single schedule. Common practice, reflected in the American Diabetes Association’s Standards of Care, looks like this:

• At least twice a year for people with diabetes whose treatment is stable and who are meeting their goals.
• About every three months when treatment has changed or goals are not being met.
• Less often, as advised by a doctor, for people screened for prediabetes or at risk of diabetes.

Because the test reflects about three months, repeating it more often than every few weeks rarely adds useful information. If your result seems to jump around from one test to the next, mention it to your doctor, because the cause may be the laboratory, a medical condition or the timing of treatment changes.

Why one value is not a verdict

A single HbA1c is a data point. It becomes meaningful when you see it beside your earlier values, your home readings, your medicines and how you have been feeling.

A change of about 0.3 to 0.5 percentage points between two tests can come from ordinary laboratory variation, so a small rise or fall does not necessarily mean your control has changed. A steady drift over three or four tests, in either direction, says much more.

For the cleanest comparison, try to use the same laboratory and keep your reports in date order. Different laboratories should use methods that are standardised to the same international reference, but small differences between instruments do exist. Our guide on reading a laboratory report explains why the same person can get slightly different numbers from two laboratories.

When HbA1c can mislead

Because HbA1c depends on red blood cells and haemoglobin, anything that changes them can distort the result. Tell your doctor if any of these apply to you:

• Anaemia, including iron deficiency, which can push HbA1c up, and anaemia from blood loss or haemolysis, which can push it down.
• Recent blood loss or a blood transfusion.
• Inherited haemoglobin variants or conditions such as sickle cell trait or thalassaemia, which are not uncommon in India and can affect some laboratory methods.
• Advanced kidney disease, and some liver conditions.
• Pregnancy, when red-cell turnover changes and glucose targets are set differently.
• Recent treatment for vitamin B12 or iron deficiency, which changes red-cell production.

In these situations a doctor may rely more on fasting or post-meal glucose, home readings or another marker, and may pick a laboratory method that is not affected by the variant.

How HbA1c compares with other glucose tests

HbA1c is one of several ways of looking at glucose, and each answers a different question.

• Fasting plasma glucose measures glucose after at least eight hours without food. It shows where your glucose sits at that moment and is sensitive to a poor night’s sleep, illness or stress.
• Post-meal or random glucose shows how the body copes after eating or at an unplanned time. A two-hour post-meal value is useful for seeing the effect of food and medicines.
• An oral glucose tolerance test measures glucose before and two hours after a measured sugary drink. It is used in pregnancy screening and in some uncertain cases.
• Home glucose meters and continuous glucose monitors record many readings and show the highs and lows that an average hides.

Doctors often combine them. If your HbA1c and your fasting glucose seem to disagree, that is not necessarily an error: they measure different things over different time spans, and conditions such as anaemia can separate them. Ask your doctor which test they are relying on for decisions about your care.

What can and cannot move the number

Sustained changes in eating, activity, body weight, sleep and medicines can all move HbA1c over a few months, and doctors discuss these with patients as part of a treatment plan. What does not help is a short crash effort just before a test. Because the result reflects about three months, a week of unusually strict eating changes it very little, while a rise in glucose after a few weeks of illness or missed doses will show up only gradually. This is also why a good result is best treated as a sign that your routine is working, rather than as permission to stop it.

A worked example (example values)

Suppose a person has these example HbA1c results over eighteen months: 7.4 %, 7.1 %, 6.9 % and 7.0 %.

Converted, these are about 57, 54, 52 and 53 mmol/mol. The first value is a little above the common 7 % goal, and the next three sit close to it. The differences between the last three are within normal laboratory variation, so the sensible reading is “stable and near goal”, not “getting better, then worse”.

If the next value were 8.4 %, that would be a different story, a rise of well over a percentage point. The doctor would then ask what had changed: medicines, missed doses, illness, diet, a new medicine for another condition, or a possible problem with the sample. The number starts the conversation; it does not end it.

Questions worth asking your doctor

• What HbA1c goal is right for me, and why?
• Is my result affected by anaemia or any other condition I have?
• When should I have the next test, and should I fast for it?
• Should I also check glucose at home, and how do I record it?
• Does a change in my medicines or routine mean I should test sooner?

Sources

• MedlinePlus: Hemoglobin A1C (HbA1c) Test — plain-language overview of the test and how results are used
• NIDDK: The A1C Test and Diabetes — US National Institute of Diabetes and Digestive and Kidney Diseases
• American Diabetes Association: Understanding A1C — ranges, goals and the estimated average glucose
• WHO: Diabetes fact sheet — World Health Organization overview
• Testing.com: Hemoglobin A1c test — what can affect the result

A note on this guide

This guide is general information about HbA1c reports. It is not medical advice and cannot tell you whether your own result is good or bad. Confirm what your result means, and what to do about it, with your doctor.