A1C and eAG
| Author: Judy Kohn, RN, BSN, CDE |
| Last Updated: Wednesday, July 30, 2008 |
Q: "Could you explain the A1C test? I was told it tells my overall level of control; if that is true, then why do I have to test my blood glucose every day?"
A: Defined: A1C (formerly called HbA1c, glycohemoglobin, glycated hemoglobin, or glycosylated hemoglobin) is a test that measures your overall diabetes control for the previous 2-3 months. I have heard it compared to "the semester grade" or "the batting average" because it indicates your average blood glucose over an extended period of time. I also have heard the A1C test compared to a candied apple: the apple represents the red blood cell, and the thickness of the candy coating on the apple represents the amount of glucose that has "stuck" to it.
How can the A1C test determine your average? About 30 years ago, it was discovered that a small amount of glucose normally combines with the hemoglobin molecule in your blood (that's the red-colored protein in your red blood cells that carries oxygen to the rest of your body). This binding occurs in direct proportion to the amount of glucose in your blood. Once it binds, it remains there for the lifespan of the red blood cells, usually between 2-3 months. So when your blood is tested for A1C, this test gives you an estimate of your control over the past 2-3 months.
Technical Details: Technically the various names for this test indicated different fractions of the hemoglobin (total versus just the A1c). But now, HbA1c (shortened to the new term A1C) is considered the standard test used in research and guidelines for care.
A new term introduced in 2008 is the estimated Average Glucose (eAG). The American Diabetes Association joined forces with international organizations to provide a standardized way to compare your daily glucose checks with your A1C. You can read more details about how this term was derived by clicking on the Diabetes Professional link in the related links below.
How does the A1C equate to the estimated average glucose? Please see the chart below, but be sure to read the details that follow:
A1C Levels and Estimated Average Glucose (eAG)(source: Diabetes Care, August 2008) Comparison of A1C and Estimated Average Glucose (eAG) levels
NOTE:mg/dL is used as the unit of glucose measure in the U.S. while mmol/l us used in some countries |
||
| A1C (%) | eAG (mg/dl) | eAG (mmol/l) |
| 6% | 126 | 7.0 |
| 6.5% | 140 | 7.8 |
| 7% | 154 | 8.6 |
| 7.5% | 169 | 9.4 |
| 8% | 183 | 10.1 |
| 8.5% | 197 | 10.9 |
| 9% | 212 | 11.8 |
| 9.5% | 226 | 12.6 |
| 10% | 240 | 13.4 |
| The relationship between A1C and eAG is described by the formula 28.7 X A1C - 46.7 = eAG. | ||
Those pesky details: An article in Diabetes Care 25 (2): 275-278, 2002 provided insight as to why the A1C is not the sole method for evaluating glucose control:
- Although A1C reflects the average glucose over the past few months, the more recent glucose levels (i.e. the past 3-4 weeks) contribute approximately 50% to the final result, while the glucose levels from 90-120 days earlier contribute much less to the final result.
- It would be misleading to equate the fasting glucose (FPG) with the A1C, because this article showed that the FPG underestimated the A1C, while the post meal glucose contributed significantly to the A1C results, although this relationship depends on the level of the A1C.
- The post breakfast glucose results overestimated the A1C while the post lunch results were more closely related to the A1C; finally, the post lunch glucose and the bedtime glucose both were more closely related to the A1C compared to the fasting glucose.
Many have asked: "Can't I go by how I feel, along with regular A1C tests?"
- NO. Research has shown that very few people can estimate their glucose by how they feel.
- You still need the daily tests to fine tune your daily control, and to verify if you are running too low or too high; also, regular testing helps you see the immediate effect of your meal, activity, and medication, as well as any variation in your usual routine, such as illness or stress.
- Often, there is a discrepancy between your A1C and your home blood glucose monitoring results; this could be due to many factors: you might not be testing enough, so you might be missing the spikes in your blood glucose; your meter technique might be incorrect (such as the meter is coded improperly); you might tend to test only when you know you've been following your prescribed regimen (i.e. it is common that people don't want to test after they have overeaten). So whenever there is a discrepancy, it will alert both you and your healthcare team to look further to find the cause.
- In fact, you may notice that if you compare the new A1C /estimated average glucose (eAG) chart to the former A1C chart published in the former ADA Standards of Care, you will see that the new chart shows a slightly higher eAG compared to the previous chart. This is because the former chart was based on quarterly A1C tests and 7-point glucose measurements over a 3-month period with no overnight glucose readings; the new eAG chart incorporates results from continuous glucose monitoring over an average of 52 days, which generated a larger pool of data. So the eAG chart represents a more refined and accurate formula describing the relationship between A1C and eAG. The formula is: 28.7 X A1C - 46.7 = eAG.
What is the goal for the A1C?
The American Diabetes Association (ADA) recommends an A1C under 7%, while the American Association of Clinical Endocrinologists (AACE) has taken a stricter stance and advocates a goal of under 6.5%. Note however that the ADA goals are for nonpregnant adults, and the ADA also recommends that the A1C goal for selected individuals should be as close to normal (<6%) as possible without significant hypoglycemia.
How often should the A1C be measured?
The ADA recommends at least every 6 months for those in stable control; however, if your therapy has changed or if you are not meeting target goals, then it should be done every 3 months. Naturally, the frequency of this test depends on your treatment regimen as well as the judgment of your healthcare team.
Related Questions
Why test if not on diabetes medication?
Q:
"I have Type 2 and am getting high numbers when I test. A type 1 can take
insulin if high. What can I do to lower my number? Almost seems senseless to
test if I cannot take action to remedy the results..."
Glucose Goals
Q:
"What is a good blood glucose reading?"
Related Links
Professional Diabetes
http://diabetes.org
American Diabetes Association (ADA)
http://www.diabetes.org
National Diabetes Education Program (NIDDK)
http://www.ndep.nih.gov
Important Notice: The responses provided by the team of Diabetes Educators are based on their personal experiences and expertise as practicing diabetes healthcare professionals, and are not to be considered diabetes management advice from Abbott Laboratories. Remember that information provided by the team of Diabetes Educators is for general background purposes and is not intended as a substitute for medical diagnosis or treatment by a trained professional. You should always consult your physician about any healthcare questions you may have, especially before trying a new medication, diet, fitness program, or approach to healthcare issues.
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