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A1C is glucose attached to hemoglobin, a protein found in red blood cells that transports oxygen from the lungs to other parts of the body.
Hemoglobin is composed of four globin (protein) chains; two beta (b) and two alpha (a) chains. Each chain has a heme attached, the site for the binding of oxygen to hemoglobin.
A1C is hemoglobin that has been glycosolated, or modified by the addition of glucose. It is sometimes referred to as glycosolated hemoglobin A1c or glycated hemoglobin.
The glucose in A1C is irreversibly bound to one or both beta chains of hemoglobin. Glucose binds to hemoglobin continuously throughout the lifespan of the red blood cell. Since the lifespan of a red blood cell in the circulation is approximately 120 days, A1C is an index of average blood glucose over approximately 120 days.
However, blood glucose levels in the preceding 30 days contribute substantially more to the level of A1C than do blood glucose levels 90 to 120 days earlier. This explains why the level of A1C can increase or decrease relatively quickly with large changes in blood glucose; it does not take 120 days to detect a clinically meaningful change in A1C following a clinically significant change in average blood glucose.
The normal level of A1C in people without diabetes is approximately 4% to 6%. After the discovery, in 1967, of A1C and the fact that people with diabetes have higher levels of A1C, additional work was done to clarify the clinical and biological significance of this modified hemoglobin.
A1C Testing at Home
Commercial methods for measuring A1C became available in the late 1970s. By 1988 routine measurement of A1C for people with diabetes was recommended by the American Diabetes Association (ADA). Today, there are several testing methods, many of which you can use in the comfort your own home.
However, it was not until 1993, when the landmark Diabetes Control and Complications Trial (DCCT) was completed, that the importance of A1C as an indicator of risks for the complications of diabetes (including blindness, kidney disease and nerve damage) was firmly established.
In 1994, the ADA began recommending specific A1C treatment goals based on the results of the DCCT. From that time on, the goal for most people with diabetes has been less than 7%.
Each 1% change in A1C represents a change of approximately 35 mg/dl in average blood glucose.
At the time that the DCCT ended, A1C results in many laboratories were not equivalent to those of the DCCT, which made it difficult for patients and healthcare providers to utilize the ADA guidelines. Fortunately, this situation has improved dramatically due to efforts to standardize A1C testing, and almost all laboratories now produce DCCT-equivalent results.
Proper interpretation of A1C results requires that patients and healthcare providers understand the relationship between test results and average blood glucose. Also, as with any laboratory test, healthcare providers need to be aware of specific measurement limitations and interferences.
Mar 1, 2005
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