The HbA1c test, also known as the haemoglobin A1c or glycated haemoglobin test, is an important blood test that gives a good indication of how well your diabetes is being controlled
Together with the fasting plasma glucose test, the HbA1c test is one of the main ways in which type 2 diabetes is diagnosed.
HbA1c tests are not the primary diagnostic test for type 1 diabetes but may sometimes be used together with other tests.
For HbA1c guidelines for monitoring diabetes control, see our HbA1c targets page
HbA1c testing in diagnosing diabetes
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.
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.
How is 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.
How does the HBA1c test work?
HbA1c (glycated haemoglobi, haemoglobin A1c) occurs when haemoglobi, 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 bee, 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
Whilst 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.