A1c is a blood test that measures the average level of glucose in a person's blood over the past two to three months. It is often used to diagnose and monitor diabetes. The results are expressed as a percentage, with higher percentages indicating poorer blood sugar control. For example, an A1c level of 5.7% or lower is considered normal, while levels of 6.5% or higher indicate diabetes. This test helps healthcare providers assess how well a person's diabetes management plan is working and make necessary adjustments to improve overall health.