Guide to HbA1c

HbA1c forms when haemoglobin joins with glucose in the blood
HbA1c forms when haemoglobin joins with glucose in the blood

HbA1c is a term commonly used in relation to diabetes. This guide explains what HbA1c is, how it differs from blood glucose levels and how it's used for diagnosing diabetes.

What is HbA1c?

The term HbA1c refers to glycated haemoglobin. It develops when haemoglobin, a protein within red blood cells that carries oxygen throughout your body, joins with glucose in the blood, becoming 'glycated'.

By measuring glycated haemoglobin (HbA1c), clinicians are able to get an overall picture of what our average blood sugar levels have been over a period of weeks/months.

For people with diabetes this is important as the higher the HbA1c, the greater the risk of developing diabetes-related complications. 

HbA1c is also referred to as haemoglobin A1c or simply A1c.

Definition of HbA1c
HbA1c refers to glycated haemoglobin (A1c), which identifies average plasma glucose concentration.

How does HBA1c return an accurate average measurement of average blood glucose?

When the body processes sugar, glucose in the bloodstream naturally attaches to haemoglobin.

The amount of glucose that combines with this protein is directly proportional to the total amount of sugar that is in your system at that time.

Because red blood cells in the human body survive for 8-12 weeks before renewal, measuring glycated haemoglobin (or HbA1c) can be used to reflect average blood glucose levels over that duration, providing a useful longer-term gauge of blood glucose control.

If your blood sugar levels have been high in recent weeks, your HbA1c will also be greater.

HbA1c targets

Targets for HbA1c are as follows:

  • For people without diabetes, the range is 20-41 mmol/mol
  • For people with diabetes, an HbA1c level of 48 mmol/mol
    is considered good control, although some people may prefer their numbers to be closer to that of non-diabetics
  • For people at greater risk of hypoglycemia (lower than normal blood sugar), a target HbA1c of 59 mmol/mol (7.5%) to reduce the risk of hypos
HbA1c targets mmol/mol %
Non-diabetics 20 - 41 mmol/mol 4% - 5.9%
Diabetics 48 mmol/mol 6.5%
Diabetics at higher risk of hypoglycemia 59 mmol/mol 7.5%

HbA1c levels between 5.7% and 6.4% indicate increased risk of diabetes (prediabetes).

HbA1c chart

What are the benefits of lowering HbA1c?

Two large-scale studies - the UK Prospective Diabetes Study (UKPDS) and the Diabetes Control and Complications Trial (DCCT) - demonstrated that improving HbA1c by 1% (or 11 mmol/mol) for people with type 1 diabetes or type 2 diabetes cuts the risk of microvascular complications by 25%.

Microvascular complications include:

Research has also shown that people with type 2 diabetes who reduce their HbA1c level by 1% are: [1]

  • 19% less likely to suffer cataracts
  • 16% less likely to suffer heart failure
  • 43% less likely to suffer amputation or death due to peripheral vascular disease

How does HbA1c differ from a blood glucose level?

HbA1c provides a longer-term trend, similar to an average, of how high your blood sugar levels have been over a period of time.

An HbA1c reading can be taken from blood from a finger but is often taken from a blood sample that is taken from your arm.

Blood glucose level is the concentration of glucose in your blood at a single point in time, i.e. the very moment of the test.

This is measured using a fasting plasma glucose test, which can be carried out using blood taken from a finger or can be taken from a blood sample from the arm.

However, fasting glucose tests provide an indication of your current glucose levels only, whereas the HbA1c test serves as an overall marker of what your average levels are over a period of 2-3 months.

HbA1c can be expressed as a percentage (DCCT unit) or as a value in mmol/mol (IFCC unit). Since 2009, mmol/mol has been the default unit to use in the UK.

Note that the HbA1c value, which is measured in mmol/mol, should not be confused with a blood glucose level which is measured in mmol/l. Use our HbA1c conversion tool to help with switching between the two measurement units.


One of the things you will become familiar with your diabetic diagnosis is the phrase HbA1c. It’s not very user friendly, it is a medical term. It’s to do with your haemoglobin in your blood. Haemoglobin actually gives the red colour to the blood cells, making your blood red.

One of the things that happens in the body’s metabolism is that the glucose actually sticks to haemoglobin. Haemoglobin molecules usually live for 8-12 weeks. So within that life cycle they just give an average of the whole 2-3 months of your control.

You should be getting HbA1c reading every time you go to the clinic. Type 1 diabetics should go to the clinic once every 3-4 months if possible. Not all GPs do HbA1c tests. You might end up being sent to either to a phlebotomist at your GP’s or to the local hospital to get the blood drawn to get this done. It’s not usually something that can get measured on an everyday meter.

Although there are some meters coming out being designed for GPs surgery so that they can do an HbA1c test on the spot.

Like any number it needs to be taken in context.  It’s presented as a percentage. For example my HbA1c is normally 8.2-8.3%, it’s not bad but it’s not great. Ideally it should be under 6%. Normal people who are not diabetic it’s usually under 5%.

Now, because the diabetic is dealing with the fact that their blood sugars can rise quite a lot we can’t literally be expected to have the same reading as a non diabetic. Infact there is some evidence to point that trying to achieve that level actually reduces the chances of living a really long time if you do try to achieve that with diabetes, it means such tight control. Having said that the HbA1c is the best guide we have.

One of the things happening today is that the HbA1c test is actually being used to diagnose diabetes.  It has been for a long time but the fact is that people do a one off test. If your doctor does a one-off test on you, you might actually be at that stage of having a normal or low blood sugar, so you won’t show as having an abnormal blood sugar reading.

But if you have blood tests done over the space of a couple of weeks you might find that there are peaks and troughs that are not normal. Normal non diabetics simply don’t go out of this range, if the range is like this people stay within it. Diabetics tend to do this. You go over and above the norm.

The HbA1c test is a stake in the ground, it’s literally something you can hang your hat on. You can’t judge all your diabetic control from it because of the fact that it’s average. If your HbA1c is 6.5% it might actually be hiding quite badly controlled diabetes, so you might be spending an awful lot of time having hypos which wouldn’t be very desirable in terms of quality of life.

Having said that it is very important because it’s the clearest indicator we have as to your long term ability to live with diabetes and not end up with what they call diabetic complications which is damage to the body.

There was a test called DCCT, done in America. It wasn’t a test but a study. It went on for 10 years from 1983 to 1993 and they took more than a 1000 people with diabetes, who had only had it between 1-15 years and who had no signs of either nerve damage or eye damage or kidney damage and they put them into two groups.

One had normal control as per the time frame, the history, how it was looked at being controlled back then which is 25 years ago. Versus what they call tight control which is really carb counting, multiple blood testing, multiple injections and they were able to show that the multiple, tight control helped stave off any complications.

It helped to have a much better HbA1c results and good HbA1c lead to a much better long term prognosis. So the HbA1c is an important number and you should be getting it looked at least every 3-6 months. If the numbers are always good then there is no need to worry but if it’s always bad then you need to keep trying with new medications and new tools until you can get the balance right in your life.

Just a point about using the HbA1c test to diagnose diabetes: if it comes up higher than or greater than 6.5% you’re pretty much defined as having diabetes. If it comes up between 6-6.5% you are probably going to be diagnosed with pre diabetes or what was for a while called the 1.5 diabetes, as opposed to type 1 or type 2.

If your reading is less than 6% then you are not considered to be pre diabetic or diabetic.

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When should HbA1c levels be tested?

Everyone with diabetes mellitus in the UK should be offered an HbA1c test at least once a year.

Some people may have an HbA1c test more often. This may be more likely if you have recently had your medication changed or your health team are otherwise wishing to monitor your diabetes control more than once a year.

Fig 2: HbA1c & Glucose Blood Levels
Ave. Blood Glucose
13 119 18 mmol/L
12 108 17 mmol/L
11 97 15 mmol/L
10 86 13 mmol/L
9 75 12 mmol/L
8 64 10 mmol/L
7 53 8 mmol/L
6 42 7 mmol/L
5 31 5 mmol/L

Although HbA1c level alone does not predict diabetes complications, good control is known to lower the risk of complications.

How do blood glucose levels compare with HbA1c readings?

The table on the right (figure 2) shows how average blood sugar levels in mmol/L would be translated into HbA1c readings, and vice versa.

It is important to note that because blood glucose levels fluctuate constantly, literally on a minute by minute basis, regular blood glucose testing is required to understand how your levels are changing through the day and learning how different meals affect your glucose levels.

Read more about HbA1c:

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