What blood test measures average blood sugar over three months?

The blood test that measures average blood sugar over about three months is the hemoglobin A1c test.

Hemoglobin A1c, also written HbA1c or simply A1C, measures the percentage of hemoglobin molecules in red blood cells that have glucose attached to them. Because red blood cells circulate for roughly three to four months, their glycated hemoglobin provides a longer-term picture than a glucose meter reading, which is only a snapshot. Recent weeks generally influence the result more than the earliest part of the three-month period.

Clinicians use A1C to diagnose and monitor diabetes and prediabetes. In commonly used diagnostic ranges, an A1C below 5.7% is considered normal, 5.7% to 6.4% suggests prediabetes, and 6.5% or higher can indicate diabetes when confirmed appropriately. The result does not require fasting, unlike a fasting plasma glucose test.

A1C is not perfect for everyone. Conditions that alter red-cell lifespan, including some anemias, hemoglobin variants, recent blood loss, or transfusion, can make it misleading. A glucose tolerance test measures the body’s response to a glucose drink, while fructosamine reflects a much shorter period of roughly two to three weeks.