Glycosylated haemoglobin or HbA1c is the name for the test used to measure average glucose control over a longer period of time. HbA1c is named after a subgroup of adult haemoglobin, the red pigment in blood cells, which binds and transports oxygen in the red blood cells. Glucose molecules become attached to the haemoglobin molecules in the red blood cells, HbA1c have a lifespan of 120 days. During the life of the red blood cell, the amount of glucose bound to haemoglobin in the red cell depends on the level of glucose in the blood at any one time. The percentage of haemoglobin that is bound to glucose provides an estimate of the average blood glucose level over the life of the red cell. Since red cells last for 120 days, the average age is 60 days or 2 months.
Haemoglobin in the red blood cells takes up oxygen in the lungs and transports it to the cells. The red blood cells also take carbon dioxide from the cells back to the lungs. During their lifetime in the blood circulation, glucose also sticks to haemoglobin, which can be measured by HbA1c.
HbA1c is a measure of the percentage of the haemoglobin in the red blood cells that has glucose bound to it. HbA1c reflects an average measurement of the blood glucose levels during the last 2–3 months. The blood glucose levels from the week prior to testing will not be included in the reading as this fraction of HbA1c is not stable. If HbA1c is monitored at regular intervals (at least every 3 months) at the diabetes clinic, the results will provide a good summary of how
your glucose control has been throughout the year.
It is important to remember that HbA1c reflects an average of your blood glucose levels. You can get an acceptable HbA1c reading with a combination of high and low blood glucose values. More often than not, you will feel better when your blood glucose level is relatively even. However, there is no scientific evidence that you will have more complications as a result of your diabetes if your blood glucose level is unstable than if your blood glucose readings are all the same, assuming that HbA1c is unchanged too. There is some evidence that HbA1c goes up with age, due to an increase in glucose bound to proteins such as haemoglobin. This may account for the fact that the HbA1c in some elderly patients seems too high when compared with their blood glucose test results.
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