Red blood cells contain a protein called hemoglobin. It helps carry oxygen to cells of the body. Hemoglobin is constantly in contact with glucose in the blood and can become attached to it. Glucose that binds to hemoglobin is called glycated hemoglobin. The more glucose in the blood, the higher the amount of glycated hemoglobin.
Glucose stays attached to the hemoglobin until the red blood cell dies. The life span of a red blood cell is about 120 days. So, a glycated hemoglobin (A1C) test measures average blood glucose control in the last two to three months. To do this laboratory test, a blood sample is required.
A1C reflects control of blood glucose over many days. For this reason it reflects each and every blood glucose value, 24 hours a day over two to three months. However, the A1C number is only an average. It does not give information about each individual blood glucose reading. You must still monitor your blood glucose to see what it is doing each day. Your fasting plasma glucose and two-hour postprandial glucose must be within target for your A1C to stay at less than seven per cent.
If your fasting plasma glucose and two-hour postprandial glucose levels are often higher than your targets, your A1C will likely be higher than the target (see table below).
|Relationship between A1C and average blood glucose level|
|A1C (per cent)||Average blood glucose (mmol/L)|
A1C is a valuable test. It predicts the risk of developing complications like heart attack, stroke, and nerve, kidney and eye damage. A1C results show how well your diabetes treatment plan is working. The A1C target for those with diabetes is less than seven per cent.
If your A1C is above seven per cent, changes should be made to your treatment plan until you reach an acceptable level. A higher A1C result puts you at increased risk for complications. A1C can be lowered by monitoring blood glucose daily to keep blood glucose within recommended targets.
By lowering your A1C just one per cent, you reduce your risk for microvascular (tiny blood vessel) complications to the nerve, eye, and kidney by 37 per cent. The A1C test should be done every three months if A1C is not at target or if changes are being made to diabetes treatment. Once targets are met and maintained, A1C can be done every six months.
In Canada, testing for diabetes usually involves measuring fasting plasma (blood) glucose, two-hour plasma glucose levels, or both. Often, a second test is required to confirm diagnosis.
Recent evidence shows that if you have higher A1C levels, you are at more risk of developing diabetes.
If we can identify people with a high A1C who do not yet have diabetes, we have a better chance to slow or stop its onset. A treatment plan, including lifestyle changes and possibly medications, may prevent some people from developing diabetes.
The A1C test also has benefits over fasting blood glucose tests. People do not need to fast before having the A1C test. It also varies less from day to day, and levels do not change as much if you are stressed or ill.
Although the Canadian Diabetes Guidelines do not currently recommend the A1C test for diagnosis of diabetes, American Diabetes Guidelines do. In January 2010, they were changed to confirm:
A1C tests currently help Canadians with diabetes to monitor overall blood glucose control. Eventually, they may also be used here to identify those at higher risk of developing diabetes.