According to the Mayo Clinic, the hemoglobin A1C is a blood test to indicate the level of sugar in a patient's blood for the two to three months previous to the test. Doctors often use the A1C to diagnose type 1 or type 2 diabetes and to determine the effectiveness of the treatment plan for a diabetic patient. While some diabetic tests require fasting, the hemoglobin A1C requires no special preparation, allowing the patient to eat and drink according to the normal diet laid out by their doctor.
All food turns into sugar in the bloodstream, however, diabetics lack the ability to turn that sugar into usable energy. Blood tests are used to calculate the amount of sugar in the blood at any given time. The hemoglobin A1C averages the level of sugar in the blood for the previous two to three months, thereby providing an overall trend of the levels of sugar. As noted by the National Diabetes Information Clearinghouse, the average for normal blood sugar levels from a hemoglobin A1C test is below 5.7 percent. Prediabetes is sometimes indicated in a blood sugar level from 5.7 to 6.4 percent, while full diabetes is indicated at 6.5 percent and above.
The hemoglobin A1C sometimes provides inaccurate results if the patient has a hemoglobin variant such as sickle cell anemia or thalassemia that interferes with the test. Patients of African, Mediterranean or Southeast Asian ethnicities are more likely to have one of these variants. A variant is generally indicated if the A1C results differ drastically from a regular blood test.