HbA1c is also referred to as haemoglobin A1c or simply A1c. 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'. 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 identifies average plasma glucose concentration.
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 generally 12 weeks or 3 months.
For people with diabetes this is important as the higher the HbA1c, the greater the risk of developing diabetes-related complications.
The HbA1c target for people with diabetes to aim for is:
48 mmol/mol (6.5%)
Note: - this is a general target and people with diabetes should be given an individual target to aim towards by their health team
An individual HbA1c should take into account your ability to achieve the target based on your day to day life and whether you are at risk of having regular or severe hypos.
Use of HbA1c in diagnosis
HbA1c can indicate people with prediabetes or diabetes as follows:
- HbA1c mmol/mol %
- Normal Below 42 mmol/mol Below 6.0%
- Prediabetes 42 to 47 mmol/mol 6.0% to 6.4%
- Diabetes 48 mmol/mol or over 6.5% or over
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:
- Diabetic nephropathy (kidney disease)
Research has also shown that people with type 2 diabetes who reduce their HbA1c level by 1% are:
- 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
Difference between HbA1c and blood glucose level test
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.
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.
When to test HbA1c levels?
Everyone with diabetes mellitus should undergo 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.
Although HbA1c level alone does not predict diabetes complications, good control is known to lower the risk of complications.
Comparison of blood glucose levels with HbA1c readings
HbA1c readings are translated into average blood sugar levels in mmol/L and vice versa. However, 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.
The World Health Organisation (WHO) suggests the following diagnostic guidelines for diabetes:
- HbA1c below 42 mmol/mol (6.0%): Non-diabetic
- HbA1c between 42 and 47 mmol/mol (6.0–6.4%): Impaired glucose regulation (IGR) or Prediabetes
- HbA1c of 48 mmol/mol (6.5%) or over: Type 2 diabetes
If your HbA1c test returns a reading of 6.0–6.4%, that indicates prediabetes. Your doctor should work with you to suggest appropriate lifestyle changes that could reduce your risk of developing type 2 diabetes.
Note: - HbA1c is not used to diagnose gestational diabetes. Instead, an oral glucose tolerance test is used.
A random blood glucose test will usually be used to diagnose type 1 diabetes. However, in some cases, an HbA1c test may be used to support a diagnosis of type 1 diabetes.
Importance of HbA1c
People with diabetes who reduced their HbA1c by less than 1% can cut their risk of dying within 5 years by 50%, according to Swedish research presented at the annual meeting of the European Association for the Study of Diabetes, Sept. 2012 (EASD).
This is how HbA1c tested
To measure a person's HbA1c level, a blood sample is taken from the patient's arm, and used to produce a reading. In some cases, such as with HbA1c testing for children, a single droplet of blood may only be required to find out how much haemoglobin A1c is present.
This is how HbA1c test
HbA1c (glycated haemoglobin, haemoglobin A1c) occurs when haemoglobin, the oxygen-carrying protein in red blood cells, becomes bonded with glucose in the bloodstream. The bonding with glucose is called glycation.
The higher a person’s blood glucose levels have been, the higher the number of red blood cells that will have become glycated, and therefore the higher HbA1c level they will have.
Note that red blood cells exist in the body for around 3 months, therefore an HbA1c levels generally reflects a person’s blood glucose levels over the previous 8-12 weeks.
Limitations of HbA1c tests
While HbA1c tests are usually reliable, there are some limitations to the accuracy of the test. For example, people with forms of anaemia may not have sufficient haemoglobin for the test to be accurate and may need to have a fructosamine test instead.
Being pregnant or having an uncommon form of haemoglobin (known as a haemoglobin variant) can also return an inaccurate HbA1c, while readings can also be affected by short term issues such as illness as they can cause a temporary rise in blood glucose.
Because of the way the HbA1c test measures blood sugar, if you have higher blood sugar levels in the weeks leading up to your HbA1c test, this will have a greater impact on your test result than your glucose levels 2 to 3 months before the test.