The hemoglobin A1C (HbA1C) test is a blood test that measures the average blood sugar (glucose) level over the past two to three months. Glucose in the blood sticks to hemoglobin, a protein in red blood cells, and as blood glucose levels increase, more of the hemoglobin will be coated with glucose. An A1C test measures the percentage of red blood cells that have glucose-coated hemoglobin. The test is used to diagnose diabetes, monitor blood glucose control in people with diabetes, and screen for or diagnose prediabetes. The Centers for Disease Control (CDC) recommends A1C testing for diabetes and prediabetes if you are over age 45. The normal range for the hemoglobin A1c level is between 4% and 5.6% for people without diabetes. A high A1C level is a sign of high blood glucose from diabetes, which can cause serious health problems, including heart disease, kidney disease, and nerve damage.