What HbA1c is
Hemoglobin A1c is hemoglobin (the oxygen-carrying protein in red blood cells) with glucose chemically attached. The percentage of hemoglobin that's glycated reflects average glucose exposure over the lifespan of red blood cells.
The math
Red blood cells live ~120 days. Older RBCs have had more time to accumulate glycation; younger RBCs less. The HbA1c value averages across the entire RBC pool, weighted toward more recent glucose exposure.
Conversion to estimated average glucose (eAG):
- HbA1c 5.0% ≈ eAG 97 mg/dL
- HbA1c 6.0% ≈ eAG 126 mg/dL
- HbA1c 7.0% ≈ eAG 154 mg/dL
- HbA1c 8.0% ≈ eAG 183 mg/dL
Diagnostic ranges
- Normal: <5.7%
- Prediabetes: 5.7-6.4%
- Diabetes: 6.5%+ (confirmed on second test)
- Optimal target on therapy: typically 6.5-7.0% (varies)
Limitations
HbA1c can be misleading in:
- Anemia, short RBC lifespan lowers HbA1c
- Recent blood loss or transfusion
- Hemoglobinopathies (sickle cell, thalassemia)
- Pregnancy (RBC turnover changes)
- Chronic kidney disease
- Some medications
For these patients, fructosamine or CGM may be preferred.
vs CGM data
HbA1c gives a 3-month average but misses:
- Glucose variability (large excursions even with normal average)
- Time in range
- Postprandial spikes
- Nocturnal patterns
CGM provides this detail. For patients with prediabetes or wanting to optimize metabolic health, CGM data complements HbA1c.
Optimal target
- Healthy adults: under 5.4% considered metabolically optimal
- Diabetic patients: target 6.5-7.0% balances complications and hypoglycemia risk
- Older diabetics with frailty: 7.5-8.0% may be appropriate to avoid hypoglycemia
The clinical pearl: HbA1c is excellent for monitoring trends and diagnosing diabetes but misses meaningful day-to-day variability. CGM adds resolution; fasting insulin adds early-stage detection.
Bottom line
HbA1c reflects 3-month average glucose. Excellent workhorse but with limitations in anemia, hemoglobinopathies, and other contexts. Captures average but not variability. Optimal targets depend on patient context.