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What does an A1C (Hemoglobin A1C) Test Measure 

The A1C test measures your average blood glucose (sugar) levels over the last 3 months. The test looks at the amount of glucose that has attached to the red blood cells as they move through the blood stream. The more glucose in the blood, the higher the A1C percent and the higher the A1C, the more damage is occurring to your large and small blood vessels. 

Since each hemoglobin A1C cell has a lifespan of about 4 months, the A1C sample will include cells that are a few days, a few weeks and a few months old. As a result, the hemoglobin A1C test covers a span of about 3 months. 

How often do you need A1C? 

The American Diabetes Association recommends A1C testing every 3 - 6 months. For people who are under good control and are not changing their therapy, an A1C test every 6 months may be acceptable.3 Your doctor will help you decide what's right for you. 

Self-monitoring and A1C 

As important as the A1C is, however, it's not a substitute for frequent self-monitoring. Only regular blood glucose checks show how meals, activity, medications and stress affect your blood glucose at a single moment in time, as well as over the course of a day or week. 

How does an Average Daily Blood Glucose (Sugar) Level Compare to an A1C 

Your Estimated Average
Blood Glucose (eAG) is: 

Your A1C is:

298 mg/dL 12.0 %
283 mg/dL 11.5 %
269 mg/dL 11.0 %
255 mg/dL 10.5 %
240 mg/dL 10.0 %
226 mg/dL 9.5 %
212 mg/dL 9.0 %
197 mg/dL 8.5 %
183 mg/dL 8.0 %
169 mg/dL 7.5 %
154 mg/dL 7.0 %
140 mg/dL 6.5 %
126 mg/dL 6.0 %
  97 mg/dL 5.0 %

Formula for determining Estimated Average Glucose is 28.7 x A1C - 46.7 = eAG


1 American Diabetes Association. A1C test. Available at: Accessed March 2, 2009. 

2 Daily G. Assessing glycemic control with self-monitoring of blood glucose and hemoglobin A1C measurements. Mayo Clinic Proceedings. February 2007: 82(2);229-236. Available at: Accessed March 8, 2009. 

3 American Diabetes Association. Standards of medical care in diabetes—2009 [position statement]. Diabetes Care. 2009;32(1): S13-S61. Available at: Accessed March 2, 2009.