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August 15, 2003

A1c (Glycohemoglobin, HgbA1c)

Question from Indian Harbor Beach, Florida, USA:

I have fairly well controlled type 2 diabetes along with thalassemia minor and microcytosis. My A1c levels have been as high as 7.0%, but my last one was 6.1% which seemed to be contradictory to my history of daily finger stick glucose checks which run from 100-250 mg/dl [5.6-13.9 mmol/L]. Will the thalassemia and microcytosis affect the accuracy of the results of the automated A1c testing? (i.e., Is the automated reading of the glucose bonded to the cell misread due to the small size of the cell?). My doctor and I are on very good terms, and I have discussed this situation but at this time he is not sure of the relationship. I thought I would try to research some information myself.

Answer:

The thalassemia minor may result in increased amounts of hemoglobin F that persist in the blood which is measured with the glycosylated hemoglobin and can give falsely elevated values. In addition, anything that decreases the number of red blood cells, such as hemolysis of the blood cells, causes a falsely lower glycosylated hemoglobin. In order to understand why you have had a change, and to make sure it is not from a change in control, you would have to know about the amounts of different hemoglobins in your red blood cells and any change in your red cell survival. This is generally too complicated to be useful.

There are other glycated proteins that can be used to monitor glucose control. For instance, glycated albumin or fructosamine may be used. They are much more expensive and only allow for evaluation of glucose control over the previous two weeks, compared to hemoglobin A1c which allows for determination of your average blood sugar over the previous three months.

JTL

[Editor’s comment: There are some newer A1c assays that eliminate interference from hemoglobin variants. You and your doctor should check with the laboratory about availability.

SS]