HbA1c reflects average blood glucose over the past 2-3 months by measuring the percentage of hemoglobin proteins that are glycated (sugar-coated). It provides a more stable picture than a single fasting glucose measurement. Normal is below 5.7%, prediabetes is 5.7-6.4%, and diabetes is 6.5%+.
One caveat for TRT patients: elevated hematocrit and hemoglobin can falsely lower A1C readings, so interpret in context with fasting glucose and insulin.