HbA1c to Fasting Glucose Conversion
Yes, there is an established conversion relationship between HbA1c and average glucose levels, but HbA1c correlates better with overall average glucose than specifically with fasting glucose alone. 1
Standardized HbA1c to Average Glucose Conversion
The most authoritative conversion comes from the A1C-Derived Average Glucose (ADAG) study, which established a strong correlation (r = 0.92) between HbA1c and mean glucose levels: 1
The conversion formula is: Average Glucose (mg/dL) = 28.7 × HbA1c (%) - 46.7 1, 2
Standard Conversion Table
| HbA1c (%) | Estimated Average Glucose (mg/dL) | 95% Confidence Interval |
|---|---|---|
| 5 | 97 | 76-120 |
| 6 | 126 | 100-152 |
| 7 | 154 | 123-185 |
| 8 | 183 | 147-217 |
| 9 | 212 | 170-249 |
| 10 | 240 | 193-282 |
| 11 | 269 | 217-314 |
| 12 | 298 | 240-347 |
HbA1c to Fasting Glucose Relationship
While HbA1c reflects average glucose over 2-3 months, fasting glucose represents only a single time point and shows moderate correlation (r = 0.65-0.82) with HbA1c-derived average glucose. 3, 4, 5
Empirical Fasting Glucose Targets by HbA1c Level
Based on the ADAG study's self-monitored blood glucose data, the average fasting glucose levels needed to achieve specific HbA1c targets are: 6
| Target HbA1c Range (%) | Average Fasting Glucose (mg/dL) | 95% CI |
|---|---|---|
| 5.5-6.49 | 122 | 117-127 |
| 6.5-6.99 | 142 | 135-150 |
| 7.0-7.49 | 152 | 143-162 |
| 7.5-7.99 | 167 | 157-177 |
| 8.0-8.5 | 178 | 164-192 |
Each 1% increase in HbA1c corresponds to approximately a 35 mg/dL (2 mmol/L) increase in mean plasma glucose. 2, 7
Critical Limitations and Pitfalls
Why Fasting Glucose Alone Is Insufficient
- Fasting glucose captures only one time point and misses postprandial hyperglycemia, which significantly contributes to HbA1c. 2
- Day-to-day variability in fasting glucose is substantial due to stress, illness, recent meals, and medications. 2
- Fasting glucose consistently underestimates average glucose compared to HbA1c-derived estimates (mean difference approximately 1.6 mmol/L). 4
Conditions That Invalidate HbA1c-Glucose Correlations
You must use glucose criteria exclusively (not HbA1c) in these situations: 1
- Hemolytic anemia or conditions with shortened red blood cell lifespan (falsely lowers HbA1c) 1, 7, 8
- Iron deficiency anemia (falsely elevates HbA1c) 7, 8
- Hemoglobinopathies (sickle cell trait, thalassemia) - use interference-free assays or glucose criteria only 1
- Recent blood transfusion 1
- Pregnancy 1
- End-stage kidney disease 1, 8
- Use of erythropoiesis-stimulating agents 1
Ethnic and Age Variations
- African Americans have HbA1c values approximately 0.4% higher than whites for the same glucose levels. 7
- HbA1c increases approximately 0.1% per decade after age 30 in non-diabetic individuals. 7
- The ADAG study was 83% non-Hispanic White, limiting generalizability to other populations. 1
Temporal Weighting of HbA1c
HbA1c is not an equal average of the past 3 months: 2, 7
- 50% reflects glucose from the most recent month 2, 7
- 25% reflects glucose from 30-60 days prior 2, 7
- 25% reflects glucose from 60-120 days prior 2, 7
This means recent glucose changes disproportionately affect HbA1c compared to older values.
Diagnostic Thresholds
For diabetes diagnosis, the established criteria are: 1
- HbA1c ≥ 6.5% (must use NGSP-certified laboratory method) 1
- Fasting plasma glucose ≥ 126 mg/dL (7.0 mmol/L) (no caloric intake for ≥8 hours) 1
- 2-hour OGTT glucose ≥ 200 mg/dL (11.1 mmol/L) 1
These criteria show imperfect concordance - HbA1c ≥6.5% identifies approximately one-third fewer cases than fasting glucose ≥126 mg/dL in screening studies. 1
Practical Clinical Application
For monitoring glycemic control, use HbA1c as the primary metric supplemented by glucose monitoring, not fasting glucose as a surrogate for HbA1c. 2, 8
When HbA1c and glucose measurements are discordant, consider: 1, 7, 8
- Conditions affecting red blood cell turnover 1, 8
- Hemoglobin variants 1, 8
- Measurement timing and variability 1
- Postprandial hyperglycemia that fasting glucose misses 2
A calculator for converting HbA1c to estimated average glucose is available at professional.diabetes.org/eAG. 1