HbA1c
HbA1c captures the fraction of hemoglobin that glucose has permanently stuck to, so a single draw reports the average blood sugar a person has lived with over the past three months rather than the snapshot of the moment. Because red cells survive roughly 90 to 120 days, the number integrates glucose exposure across that window, weighted toward the most recent weeks. Each one-point rise tracks with about a fifth more cardiovascular and mortality risk, and the relationship continues well below the diabetic range, which is why an optimal value near 5.0 to 5.4 percent sits under the laboratory ceiling of 5.6 percent. The same long memory that makes HbA1c reliable also makes it slow, so it reveals trends rather than today. For most people it is the closest thing to a running average of metabolic health.
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