Glycated hemoglobin (hemoglobin A1c)—a blood test that reflects average blood glucose levels over the preceding 2–3 months. Glucose in the bloodstream attaches to hemoglobin in red blood cells; the higher the average glucose, the higher the percentage of glycated hemoglobin. Normal HbA1c is below 5.7%; prediabetes is 5.7–6.4%; diabetes is 6.5% or above.

HbA1c is the primary endpoint in most diabetes clinical trials, including the pivotal studies of semaglutide (SUSTAIN, STEP programs) and tirzepatide (SURPASS, SURMOUNT programs). When these trials report HbA1c reductions of 1.5–2.0 percentage points, that represents clinically meaningful improvement in glycemic control with demonstrated reductions in long-term complications. HbA1c changes are among the most robust clinical endpoints in the peptide literature because the GLP-1 agonist evidence base is exceptionally strong.