Defining the Relationship Between Plasma Glucose and HbA1c: Analysis of Glucose Profiles and HbA1c in the Diabetes Control and Complications Trial

Curt L. Rohlfing, BES, Hsiao-Mei Wiedmeyer, MS, Randie R. Little, PHD, Jack D. England, Alethea Tennill, MS and David E. Goldstein, MD


Diabetes Care. 2002;25(2) 

In This Article

Abstract and Introduction

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.

The results of the Diabetes Control and Complications Trial (DCCT), published in 1993, and the U.K. Prospective Diabetes Study, published in 1998, established the relationship between HbA1c levels and risks for diabetic complications in patients with type 1 and type 2 diabetes, respectively. Based on the results of the DCCT, the American Diabetes Association (ADA) has published recommendations for HbA1c and plasma glucose (PG) levels that are widely used [1,2]. However, it is important that the relationship between daily patient-monitored blood glucose determinations and HbA1c be clearly defined to enable patients and their health care providers to set appropriate daily PG testing goals to achieve HbA1c levels representing low risks for adverse outcomes.

Several previous studies have analyzed the relationship between blood glucose (BG) and HbA1c. Svendson et al. [3] assessed 15 subjects with type 1 diabetes who collected seven-point BG profiles over a 5-week period (three profiles per week) and used a curvilinear equation to correlate BG and HbA1c. Nathan et al. [4] obtained repeated preprandial and postprandial BG samples from 21 subjects with type 1 diabetes over an 8-week period and used a linear regression equation to describe the relationship between BG and HbA1c. In the DCCT, the correlation between HbA1c and mean BG was initially determined in a limited number of patients (n = 278) for the feasibility study [5]. However, a comprehensive analysis of the relationship of BG and HbA1c, examining BG at different time points and using the entire data set, was never performed. Here, we examine, in detail, the relationship between BG (converted to PG) and HbA1c, using data obtained from the entire DCCT data set to better define this relationship.