A1c is a test that shows the average level of blood sugar over the past 2 to 3 months. People who have diabetes need to have this test done regularly to see whether their blood sugar levels have been staying within a target range.This test is also used to diagnose diabetes.1c is a test that shows the average level of blood sugar over the past 2 to 3 months. People who have diabetes need to have this test done regularly to see whether their blood sugar levels have been staying within a target range. This test is also used to diagnose diabetes.