HbA1c measurement reflects the percentage of hemoglobin bound to glucose. In which blood component is HbA1c measured?

HbA1c is measured in red blood cells (erythrocytes). Hemoglobin A1c is formed when glucose in the bloodstream non-enzymatically binds (glycates) to hemoglobin A molecules within red blood cells. Since red blood cells survive for approximately 120 days (3-4 months), HbA1c provides a weighted average of blood glucose levels over that period, with more recent glucose levels contributing more to the measurement. It is expressed as the percentage of total hemoglobin that is glycated. The test is performed on a whole blood or capillary blood sample, but the measurement reflects glycated hemoglobin within the red blood cells, not dissolved in plasma or serum.