Hba1c and Glycemic Monitoring

Glucose binds nonenzymatically and irreversibly to hemoglobin within circulating erythrocytes. Because erythrocytes circulate for about 120 days, A1C reflects average glycemia over roughly the preceding 8–12 weeks. It is weighted toward the most recent month because newer erythrocytes contribute more heavily to the result. A1C is useful for judging the longer-term direction of glycemic management, but it cannot show the route taken to get there. A person with frequent lows and marked post-meal highs may have the same A1C as someone whose glucose remains relatively stable. A1C also cannot identify an overnight hypoglycemic event, a dangerous current glucose value, or rapid deterioration during illness. An A1C of 7.0% corresponds approximately to an estimated average glucose of 8.6 mmol/L. That estimate is only an average. Treat it like a semester grade, not a record of every clinical event that occurred along the way.