Overview
Clinical Meaning
Hemoglobin A1C (glycated hemoglobin) measures the percentage of hemoglobin that has undergone non enzymatic glycation by glucose, reflecting average blood glucose over the prece...
Hemoglobin A1C (glycated hemoglobin) measures the percentage of hemoglobin that has undergone non-enzymatic glycation by glucose, reflecting average blood glucose over the preceding 2-3 months (corresponding to the 120-day lifespan of red blood cells). The glycation reaction is irreversible and proportional to ambient glucose concentration: each 1% increase in A1C corresponds to approximately 1.6 mmol/L (29 mg/dL) increase in mean plasma glucose. The clinician interprets A1C in context: conditions causing falsely elevated A1C include iron deficiency anemia, splenectomy, alcoholism, and chronic kidney disease (carbamylated hemoglobin); conditions causing falsely low A1C include hemolytic anemias, blood loss, transfusion, pregnancy (hemodilution and increased RBC turnover), and hemoglobin variants (HbS, HbC, HbF). The ADA/CDA diagnostic threshold for diabetes is A1C 6.5% or greater; prediabetes is 6.0-6.4% (CDA) or 5.7-6.4% (ADA). Target A1C for most adults with diabetes is less than 7.0%, with individualization based on age, comorbidities, hypoglycemia risk, and life expectancy (tighter targets of less than 6.5% for younger patients with short disease duration and no cardiovascular disease; relaxed targets of less than 8.0-8.5% for elderly patients with multiple comorbidities, limited life expectancy,...