Diabetes and the A1C Test: What Does It Tell You?

Blood sample on microscopic slide
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The A1C test (also known as HbA1C, glycated hemoglobin or glycosylated hemoglobin) is a good general measure of diabetes care. While conventional home glucose monitoring measures a person’s blood sugar at a given moment, A1C levels indicate a person’s average blood glucose level over the past few months.

Understanding A1C Numbers

For a person without diabetes, a typical A1C level is about 5 percent.

For someone with diabetes, experts disagree somewhat on what the A1C target should be. The American Diabetes Association (ADA) recommends an A1C target of less than or equal to 7 percent. The American Association of Clinical Endocrinologists recommends a level of 6.5 percent or below.

The ADA also emphasizes that A1C goals should be individualized. Those with diabetes should check with a healthcare professional to learn what their A1C targets should be. The National Institutes of Health (NIH) says that, in general, every percentage point drop in an A1C blood test results (e.g., from 8 percent to 7 percent) reduces the risk of eye, kidney and nerve disease by 40 percent.

The chart below shows what the A1C means in terms of average blood glucose levels. An average blood glucose of 150 mg/dL (milligrams per deciliter) translates into an A1C of about 7 percent. This is above normal, given that a diagnosis of diabetes is usually given when blood sugar levels reach about 126 mg/dL.

Note that the A1C is not the same as the estimated average glucose (eAG), which is the two to three-month average in mg/dL, but the A1C directly correlates to the eAG. When you are testing your blood sugars daily, the numbers you see on your monitor are also measured in mg/dL. They reflect a moment in time and are not the same as your eAG.

A1c to eAG Conversion Chart

HbA1c or A1ceAG
%mg/dlmmol/l
61267.0
6.51407.8
71548.6
7.51699.4
818310.1
8.519710.9
921211.8
9.522612.6
1024013.4

The formula used is: 28.7 X A1C - 46.7 = eAG.

How the Test Works

Hemoglobin A, a protein found inside red blood cells, carries oxygen throughout the body. When there is glucose in the bloodstream, it can actually stick (glycate) to the hemoglobin A protein. More glucose in the blood means that more glucose sticks to hemoglobin and a higher percent of hemoglobin proteins become glycated.

Once glucose sticks to a hemoglobin protein, it typically remains for the lifespan of the hemoglobin A protein—as long as 120 days. Therefore, at any moment, the glucose attached to the hemoglobin A protein reflects the level of the blood sugar over the last two to three months.

The A1C test measures how much glucose is actually stuck to hemoglobin A, or more specifically, what percent of hemoglobin proteins are glycated. Thus, having a 7 percent A1C means that 7 percent of the hemoglobin proteins are glycated.

A person’s A1C level will not change significantly over the course of a few days, but it will shift in response to a change in overall glucose control. It is estimated that the past month will account for about 50 percent of an A1C value, so that value can change within just a few weeks.

How Often the Test Is Necessary

People with diabetes should have their A1C tested every three months; if blood sugars are fairly stable at near-normal levels, twice a year may be enough. Health care providers should tell patients what is right for them and enable them to monitor their diabetes management more easily. More frequent A1C tests may be recommended if someone has recently changed his or her treatment plan.

How the A1C Test Is Done

The A1C test, typically performed by a healthcare professional, doesn’t hurt. Only a single drop of blood is needed, and the sample is either analyzed on-site or is sent to a laboratory for testing.

The Limitations of the Test

While the A1C is a good measure of overall glucose control, it cannot replace self-testing of blood glucose. Like other tests, results may vary from lab to lab. The A1C test is not calibrated the same everywhere, though an international effort is underway to standardize the A1C test to a new International Federation of Clinical Chemistry and Laboratory Medicine standard.

A1c results can be misleading when red blood cell survival is prolonged or reduced; some health conditions can result in falsely high A1c results (as in cases of anemia) or falsely low (as in cases of hemolysis).

One piece of good news: When someone has his or her A1C checked, that person does not need to worry about fasting; food eaten on the same day won’t affect the score.

Sources:

"A1C Test." Diabetes.org. American Diabetes Association. 

"A1C: The Test." Lab Tests Online. 29 Aug. 2005. American Association for Clinical Chemistry. 

"If You Have Diabetes, Know Your Blood Sugar Numbers!" National Diabetes Education Program. July 2005. National Institute of Diabetes and Digestive and Kidney Diseases.

"IFCC Standardization of HbA1c." National Glycohemoglobin Standardization Program. Aug. 2007. National Glycohemoglobin Standardization Program. 

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