How are HbA1c and blood sugar related?

HbA1c (glycated hemoglobin) forms when blood glucose non-enzymatically binds to hemoglobin. It reflects average blood glucose over the preceding 8–12 weeks, because red blood cells live about 120 days. Higher blood sugar leads to more glycation and thus a higher HbA1c. The relationship is not linear but exponential: even mildly elevated glucose can raise HbA1c significantly. Clinically, HbA1c is used to diagnose and monitor type 2 diabetes (e.g., ≥6.5% = diabetes). However, limitations exist: hemoglobin variants (e.g., sickle cell), anemia, kidney failure, or pregnancy can distort HbA1c. Iron deficiency or blood transfusions also affect results. Therefore, HbA1c should not be interpreted alone but alongside fasting glucose and oral glucose tolerance tests. For non-diabetics, HbA1c is a good marker for long-term glucose control but not a warning for short-term spikes. Evidence is human-strong, based on large clinical trials and guidelines.