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.
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.
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.
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.