HbA1c is a weighted average of blood glucose levels during the preceding 120 days, which is the average life span of red blood cells. A large change in mean blood glucose can increase HbA1c levels within 1-2 weeks. Sudden changes in HbA1c may occur because recent changes in blood glucose levels contribute relatively more to the final HbA1c levels than earlier events. For instance, mean blood glucose levels in the 30 days immediately preceding blood sampling contribute 50% to the HbA1c level, whereas glucose levels in the preceding 90-120 day period contribute only 10%.