An A1C under 5.7 percent is normal, according to Healthline. A1C levels from 5.7 to 6.4 percent indicate prediabetes, and A1C levels at 6.5 percent or higher indicate diabetes.
A1C tests measure a person's average blood glucose level over a period of two to three months, explains Healthline. Specifically, A1C measures how much hemoglobin in the blood is attached to glucose. An excessive amount of glucose attached to hemoglobin cells results in an elevated A1C.
A1C tests are generally accurate, but they can be as much as half a percentage lower or higher than the actual percentage, notes Healthline. For example, an A1C of 6 can indicate a range of 5.5 to 6.5. People of African, Southeast Asian and Mediterranean ancestry have a type of hemoglobin that can interfere with A1C testing.