Hemoglobin A1c (HbA1c) is a blood test that measures the amount of glucose (sugar) attached to hemoglobin, a protein in red blood cells. The test shows what your average blood sugar level was over the past two to three months. Glucose in your blood sticks to hemoglobin, and as your blood glucose levels increase, more of your hemoglobin will be coated with glucose. The A1C test measures the percentage of your red blood cells that have glucose-coated hemoglobin. The test is used to diagnose prediabetes and diabetes, and it is also the main test to help manage diabetes. Higher A1C levels are linked to diabetes complications, so reaching and maintaining your individual A1C goal is important if you have diabetes. The target A1C level for people with diabetes is usually less than 7% .

Factors that can falsely increase or decrease your A1C result include kidney failure, liver disease, or severe anemia, a less common type of hemoglobin that people of African, Mediterranean, or Southeast Asian descent and people with certain blood disorders (such as sickle cell anemia or thalassemia) may have, certain medicines, including opioids and some HIV medications, blood loss or blood transfusions, and early or late pregnancy. Hemoglobin variants are more common among people of African, Mediterranean, or Southeast Asian descent, and they can affect A1C results.

The A1C test is an important tool for managing diabetes, but it does not replace regular blood sugar testing at home.