· Reviewed by Dr. Lauren Foster, MD, FACE
Fasting glucose is a snapshot. HbA1c is a 3-month average. Most people get the snapshot annually and never see the average. Understanding what each test measures helps you interpret your results and ask better questions of your doctor.
HbA1c (glycated hemoglobin) measures how much glucose has stuck to your red blood cells over the past 2-3 months. Red blood cells live about 120 days. Glucose binding is irreversible. The percentage of hemoglobin that's glycated reflects your average blood sugar over the cell's lifespan.
| Status | HbA1c | Fasting Glucose |
|---|---|---|
| Healthy | Below 5.7% | Below 100 mg/dL |
| Prediabetes | 5.7-6.4% | 100-125 mg/dL |
| Type 2 Diabetes | 6.5% or above | 126 mg/dL or above |
For diagnosis and long-term monitoring, HbA1c is more reliable. It's not affected by what you ate yesterday or whether you fasted properly. It captures the average glucose burden your body has been processing for months. For tracking the effect of an intervention (diet change, supplement, medication), the 3-month follow-up A1C is the most useful single number.
For day-to-day tracking and detecting acute changes (medication adjustment, illness, dietary changes within the last few weeks), fasting glucose responds faster than HbA1c. For someone using a home glucose meter, daily fasting readings give early signals weeks before HbA1c reflects them.
HbA1c measures average blood sugar over 2-3 months (glucose stuck to red blood cells). Fasting glucose is a single-moment snapshot. Reference ranges: Healthy A1C below 5.7% / fasting under 100. Prediabetes 5.7-6.4% / 100-125. Diabetes 6.5%+ / 126+. HbA1c is better for diagnosis and tracking long-term intervention effects. Fasting glucose is better for detecting acute changes (illness, medication, recent dietary change).
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