glycated hemoglobin (HbA1c)

HbA1c is like a glucose ‘memory’ recorded inside your red blood cells. Glucose in the blood slowly and permanently sticks to hemoglobin, the oxygen-carrying protein inside red cells, and the higher the average sugar, the more hemoglobin gets coated. Because red cells live about three months, measuring how sugar-coated they are tells you the average blood glucose over roughly the past two to three months — no fasting required.

Glycated hemoglobin forms when glucose attaches non-enzymatically to the hemoglobin molecule (a process called glycation), and HbA1c specifically refers to glucose bound to the beta chain of adult hemoglobin. It is reported as a percentage (or in mmol/mol). A normal HbA1c is below 5.7%; 5.7–6.4% indicates prediabetes; and 6.5% or higher meets the diagnostic criterion for diabetes. Most people with diabetes aim for a target around 7% or individualized.

HbA1c is invaluable because it smooths out the moment-to-moment ups and downs of finger-stick glucose, and it predicts the long-term risk of complications. But it has limitations: it can be falsely low when red cells turn over quickly (as in hemolysis, pregnancy, or recent blood loss) and falsely high in conditions that lengthen red-cell life, and certain hemoglobin variants interfere with some assays. It also cannot detect the swings that day-to-day monitoring reveals.