What Is A1C?

Our bodies are constantly working to keep our blood-sugar levels in a healthy range. This helps ensure our cells receive a steady supply of energy. Yet sometimes the complex interplay of hormones and other physiological processes — as well as the quantity, quality, and timing of the food we eat — destabilizes these levels.

If blood glucose falls too low, we may feel dizzy, fatigued, and shaky, or we may even pass out. High blood ­sugar — a hallmark of diabetes — can lead to extreme thirst, excessive urination, blurred vision, nausea, and headaches. If high blood sugar becomes chronic, it can result in nerve and organ damage, heart disease, and stroke.

Routine lab work during a doctor’s visit often includes a fasting glucose test; this measures how much glucose remains in the blood after an eight-hour fast. A level less than 100 milligrams per deciliter is considered normal. A higher reading might prompt your doctor to order an A1C test, which reveals the body’s blood-glucose levels over time.

Here’s how the test works: Glucose binds to hemoglobin, a protein in red blood cells that carries oxygen throughout the body, in a process called glycation. The A1C test measures the percentage of hemoglobin that is glycated.

Because red blood cells live for about three months, the test reflects the average blood-glucose levels over that period. “The higher your blood sugar is, and the longer it is high, the higher your A1C will be,” explains Jill Weisenberger, MS, RDN, author of Prediabetes: A Complete Guide.

An A1C level below 5.7 percent is considered normal. A level of 5.7 to 6.4 percent is classified as prediabetes, and a level of 6.5 percent or higher indicates diabetes.