By: Randie Little
In the simplest terms, hemoglobin A1C (known asHbA1c or A1C) is measured in people with diabetesto provide an index of average blood glucose for theprevious three to four months.
A1C is glucose attached to hemoglobin, a proteinfound in red blood cells that transports oxygen fromthe 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 thebinding of oxygen to hemoglobin.
A1C is hemoglobin that has been glycosolated,or modified by the addition of glucose. It issometimes referred to as glycosolated hemoglobinA1c or glycated hemoglobin.
The glucose in A1C is irreversibly bound to one orboth beta chains of hemoglobin. Glucose binds tohemoglobin continuously throughout the lifespan ofthe red blood cell. Since the lifespan of a red bloodcell in the circulation is approximately 120 days, A1C isan index of average blood glucose over approximately120 days.
However, blood glucose levels in the preceding 30days contribute substantially more to the level of A1Cthan do blood glucose levels 90 to 120 days earlier.This explains why the level of A1C can increase ordecrease relatively quickly with large changes in bloodglucose; it does not take 120 days to detect a clinicallymeaningful change in A1C following a clinicallysignificant change in average blood glucose.
The normal level of A1C in people without diabetesis approximately 4% to 6%. After the discovery, in1967, of A1C and the fact that people with diabeteshave higher levels of A1C, additional work was doneto clarify the clinical and biological significance of this modified hemoglobin.
A1C Testing at Home
Commercial methods for measuring A1C becameavailable in the late 1970s. By 1988 routinemeasurement of A1C for people with diabetes wasrecommended by the American Diabetes Association(ADA). Today, there are several testing methods, manyof which you can use in the comfort your own home.
However, it was not until 1993, when the landmarkDiabetes Control and Complications Trial (DCCT) wascompleted, that the importance of A1C as an indicatorof risks for the complications of diabetes (includingblindness, kidney disease and nerve damage) wasfirmly established.
In 1994, the ADA began recommending specific A1Ctreatment goals based on the results of the DCCT. Fromthat time on, the goal for most people with diabeteshas been less than 7%.
Each 1% change in A1C represents a change ofapproximately 35 mg/dl in average blood glucose.
At the time that the DCCT ended, A1C results in manylaboratories were not equivalent to those of the DCCT,which made it difficult for patients and healthcareproviders to utilize the ADA guidelines. Fortunately,this situation has improved dramatically due to effortsto standardize A1C testing, and almost all laboratoriesnow produce DCCT-equivalent results.
Proper interpretation of A1C results requires thatpatients and healthcare providers understand therelationship between test results and average bloodglucose. Also, as with any laboratory test, healthcareproviders need to be aware of specific measurementlimitations and interferences.