A chart showing A1C and the typical corresponding average blood sugar levels | From the ADA Website
The A1C is a powerful tool for both patients and physicians. It can be used to diagnose both diabetes and pre-diabetes and as a measure of a person’s glycemic control over a 2 month period.
Yet it isn’t infallible. There are a variety of conditions that can cause the results to be either higher or lower than a person’s true average glucose percent.
The A1C represents a component of the hemoglobin molecule that has glucose attached. Hemoglobin is the portion of the red blood cell responsible for carrying nutrients and oxygen through the body. Hemoglobin A has an affinity for glucose and binds to it making the HbA1. The more glucose available the more hemoglobin becomes glycosylated. A1C is a sub-portion of HbA1 that is commonly measured in most labs today.
Red blood cells survive about 120 days in the blood stream, but at any given time some cells are dying and being replaced. Therefore, some cells will have more attached glucose than others. Overall the average age of the cells at any point is about 2 months and therefore A1C represents the average blood glucose level over a 2 month period.
An A1C over 6.5 is diagnostic for diabetes when used as a screening tool. And for most patients, practitioners aim for a goal A1C of less than 7.0 to limit the risk of developing diabetes complications.
Since practitioners rely on the A1C to help them make decisions about medication and lifestyle recommendations, it is important to know when it may not be accurate.
Three common reasons for A1C inaccuracies are
- Conditions that affect the lifespan of the red blood cell
- Abnormal hemoglobin,
- Medical abnormalities that produce testing errors.
Patients who have severe hemolytic anemias can have A1C’s that are significantly lower than would be expected by viewing their fingerstick results. This is because the blood sample contains a greater number of immature red blood cells.
Conversely those patients who have had their spleen removed, (the spleen filters out old and/or damaged red blood cells) would register a falsely elevated A1C Since the red blood cells are in the bloodstream for more than 120 days, some cells would have attracted more glucose over their longer lifespan.
Abnormal hemoglobin can affect the A1C results in a number of ways: changing how glucose binds to hemoglobin, making the red blood cells more likely to burst and die early and changing the way the chromatography testing machine reads the results
Most people are born with HemoglobinA. But people with inherited blood diseases have different hemoglobin types which do not attract glucose as strongly as the A type does. Examples of these are sickle cell anemia which carries HemoglobinS and thalassemia which carries HemoglobinS beta or HemoglobinC beta
Certain other factors can also cause measurement error, such as:
Severe lipemia
Salicylate containing medications (infrequently)
Fetal hemoglobin greater than 25 percent
If you have any of these conditions make sure your health care provider is aware when he or she orders an A1C.
