What HbA1c measures
HbA1c (hemoglobin A1c, glycated hemoglobin) reflects the percentage of hemoglobin molecules in your red blood cells that have glucose chemically attached. Because red blood cells live ~120 days, HbA1c reflects average blood glucose over the prior ~3 months, a rolling glycemic snapshot less prone to day-to-day variation than spot glucose.
It's the workhorse marker of long-term glucose control.
Diagnostic ranges
- Normal: under 5.7%
- Prediabetes: 5.7-6.4%
- Diabetes: 6.5%+
- Optimal target on therapy: typically 6.5-7.0% (varies by patient)
Typical reductions
GLP-1 therapy in Type 2 diabetes reduces HbA1c by:
- Semaglutide: 1.5-2.0 percentage points
- Tirzepatide: 2.0-2.5 percentage points
- Effect plateaus around 3-6 months
For patients with prediabetes (HbA1c 5.7-6.4%), normalization to under 5.7 is typical with weight loss and metabolic improvement.
Timeline
HbA1c is a lagging indicator (3-month average). Changes appear:
- Week 4: small change visible
- Week 8: meaningful change
- Week 12: near-maximum effect
- 6+ months: stable at new level
Tirzepatide vs semaglutide
SURPASS-2 head-to-head: tirzepatide produced 0.5 percentage point greater HbA1c reduction than semaglutide at comparable doses. The difference is meaningful, 0.5 percentage points correlates with measurable cardiovascular and microvascular outcome differences over years.
What HbA1c predicts
HbA1c trajectory predicts:
- Diabetic complications (retinopathy, nephropathy, neuropathy)
- Cardiovascular events
- Cognitive decline
- All-cause mortality
Each 1 percentage point reduction correlates with substantial reductions in microvascular complications. The HbA1c improvements on GLP-1 therapy translate to real long-term outcome improvements.
The clinical pearl: Watch HbA1c trajectory at 3 and 6 months on therapy. The trajectory tells you whether the medication is working; the absolute level tells you where you are. Patients who don't see HbA1c respond by 3 months may need dose adjustment or have other contributors to glycemia.
Bottom line
HbA1c reductions of 1.5-2.5 percentage points are typical on GLP-1 therapy in diabetes. Prediabetes often normalizes. The reduction is reliable, reproducible, and tracks long-term outcome improvements. For patients evaluating progress, HbA1c at 3 and 6 months is one of the most informative single markers.