HbA1c measures the percentage of haemoglobin molecules in your red blood cells that have glucose permanently attached (glycated). Because this process occurs continuously and red blood cells live for approximately 90–120 days, HbA1c reflects your average blood glucose over the past 2–3 months — providing a much more meaningful measure of long-term glycaemic control than a single fasting glucose test.

HbA1c is reported in NGSP/DCCT units (as a percentage) in the United States and some other countries. An HbA1c below 5.7% is normal; 5.7–6.4% indicates pre-diabetes; 6.5% or above on two occasions confirms a diagnosis of diabetes. It is also used to monitor ongoing diabetes management and the effectiveness of treatment.