Problem: The Glycosylation Gap (GGAP) based on fructosamine (F) measurement and the Hemoglobin Glycation Index (HGI) based on mean blood glucose (MBG) are two indices of between-individual differences in glycated hemoglobin (HbA1c) adjusted for glycemia. We sought to simultaneously compare GGAP with HGI and other estimates of glycemia.
Methods: HbA1c, F, and MBG level were obtained at a clinic visit from 62 patients with Type 1 diabetes. GGAP and HGI were calculated from the data as previously described. The variables were compared by correlation analysis. The concordance of patient classification by GGAP and HGI was compared by weighted kappa test.
Results: The mean HbA1c=11.1+/-2.7%, F=372.0+/-136.6 mol/l, MBG=186.5+/-58.4 mg/dl, HGI=0.0+/-2.0, and GGAP=0.0+/-1.9. MBG, HbA1c, and F were all highly correlated with each other. The HGI and GGAP were highly correlated (r=.73, P<.0001) and similar in both magnitude and direction. There was good agreement between HGI and GGAP classifications of patients into high, moderate, and low glycation groups (P<.0075).
Conclusions: GGAP and MBG give similar information regarding between-patient differences in HbA1c among patients with diabetes. Thus, biological variation in HbA1c is not an artifact of variability in glucose measurements comprising the MBG. Individual patient factors influence the intracellular glycation of HbA1c in addition to the effect of extracellular glycemia, which is manifested as a between-individual biological variation in HbA1c.