Time course of measured HbA1c fraction versus mean age of biotinylated RBCs. HbA1c fraction was measured in autologous biotinylated RBCs, sampled at various times (triangles) after reinfusion in 6 NDM (left) and 6 DM (right) human subjects. Mean RBC age at sampling was calculated based on the time of sampling and the measured survival characteristics of each patient as described in “Analysis of RBC life span.” The initial mean cell age differs among both NDM and DM subjects resulting from differences in RBC survival. The slope of the biotinylated RBC HbA1c fraction vs time plot represents the Hb glycation rate. Slopes differ among subjects and are, in general, greater in the DM subjects. □ represent HbA1c fraction values in isolated TfR(+) cells; (—), regression line; (), 95% confidence intervals.