Defining the Relationship Between Plasma Glucose and HbA1c

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Abstract
OBJECTIVE— To define the relationship between HbA1c and plasma glucose (PG) levels in patients with type 1 diabetes using data from the Diabetes Control and Complications Trial (DCCT). RESEARCH DESIGN AND METHODS— The DCCT was a multicenter, randomized clinical trial designed to compare intensive and conventional therapies and their relative effects on the development and progression of diabetic complications in patients with type 1 diabetes. Quarterly HbA1c and corresponding seven-point capillary blood glucose profiles (premeal, postmeal, and bedtime) obtained in the DCCT were analyzed to define the relationship between HbA1c and PG. Only data from complete profiles with corresponding HbA1c were used (n = 26,056). Of the 1,441 subjects who participated in the study, 2 were excluded due to missing data. Mean plasma glucose (MPG) was estimated by multiplying capillary blood glucose by 1.11. Linear regression analysis weighted by the number of observations per subject was used to correlate MPG and HbA1c. RESULTS— Linear regression analysis, using MPG and HbA1c summarized by patient (n = 1,439), produced a relationship of MPG (mmol/l) = (1.98 ○1 HbA1c) - 4.29 or MPG (mg/dl) = (35.6 ○1 HbA1c) - 77.3, r = 0.82). Among individual time points, afternoon and evening PG (postlunch, predinner, postdinner, and bedtime) showed higher correlations with HbA1c than the morning time points (prebreakfast, postbreakfast, and prelunch). CONCLUSIONS— We have defined the relationship between HbA1c and PG as assessed in the DCCT. Knowing this relationship can help patients with diabetes and their healthcare providers set day-to-day targets for PG to achieve specific HbA1c goals.