Updated February 29, 2016

The hemoglobin A1C is a blood test conducted in your doctor's office. It measures the amount of sugar stuck to red blood cells, which helps determine how well blood sugar has been controlled over the past two to three months.

"Think of a cornflake and then think of a frosted flake," says Diana Berger, MD, a diabetes medical specialist at the Diabetes Prevention and Control Program at the New York City Department of Health and Mental Hygiene. "The cornflake is the red blood cell and the frosted flake is the red blood cell coated with sugar. The A1C test measures the coating of sugar around a red blood cell."More about blood sugar monitoring

People with type 2 diabetes should have this test at least twice a year. An ideal test result is below 7%, according to the American Diabetes Association. (People without diabetes typically have an A1C of 4% to 5.9%.)

A 10-year trial known as the United Kingdom Prospective Diabetes Study found that a 1% drop in A1C (from 9% to 8%, for example) translated into a 35% drop in the risk of diabetes-related complications, such as eye, kidney and nerve problems.

However, it's not clear if more aggressive treatment to get hemoglobin A1C even lower is beneficial. In 2008, the National Institutes of Health halted part of a trial when an aggressive strategy to lower hemoglobin A1C (to below 6%) in patients at high risk of heart attack and stroke was linked to higher risk of sudden death from a variety of causes, including heart attack, than a less intensive strategy.

The jury is still out on whether it is safe to aim for the lowest possible A1C levels. What is clear is that monitoring blood sugar daily—and making the appropriate changes in lifestyle and medication—can help reduce hemoglobin A1C to below 7%. This will lower your risk of damaging blood vessels and nerves in the eyes, kidneys, heart, and limbs.