Translating the A1C assay into estimated average glucose values.
Level 3 - non-randomized controlled study
Prospective multicenter observational calibration cohort study
PubMed 18540046 · doi:10.2337/dc08-0545
What was done
A multicenter study across 10 international centers evaluated 507 subjects (268 with type 1 diabetes, 159 with type 2 diabetes, and 80 nondiabetic controls) over 3 months. Average glucose (AG) was derived from approximately 2,700 glucose values per subject by combining continuous glucose monitoring (at least 2 days, repeated 4 times) with 7-point fingerstick self-monitoring (at least 3 days per week). A1C was measured at a central laboratory at the end of the 3-month period and compared with AG using linear regression.
What was found
Linear regression revealed a tight relationship between A1C and AG: AG (mg/dL) = 28.7 × A1C − 46.7 (R² = 0.84, P < 0.0001). The regression equations did not differ significantly across subgroups defined by age, sex, diabetes type, race/ethnicity, or smoking status.
Why it matters
This study established the standardized mathematical formula used internationally to convert laboratory HbA1c values into estimated average glucose (eAG), presenting chronic glycemia in the same units used for daily self-monitoring.
Limits
The linear model leaves 16% of variance unexplained (R² = 0.84), reflecting biological variation in glycation rates and red cell lifespan. The abstract does not provide prediction intervals for individuals, nor does it address populations with hemoglobinopathies, anemia, or kidney disease.
Cited by
- supports A hemoglobin A1c of 6.5% translates to an estimated average blood glucose level of 140 mg/dL.
- supports A hemoglobin A1c level of 5.0% corresponds to an average blood glucose level of approximately 100 mg/dL, while 5.5% corresponds to approximately 115 mg/dL.