Effects of Acute Changes in Blood Glucose on HbA1c

Abstract

SUMMARY Although hemoglobin A1c (HbA1c) is generally considered to be an accurate index of long-term blood glucose regulation, several recent studies suggest that HbA1c may be acutely responsive to changes in blood glucose. We have examined the effects of acute changes in glucose concentration in vivo and in vitro on HbA1c. HbA1c was measured by a high-performance liquid chromatography (HPLC) method. HbA1c and plasma glucose were measured in seven diabetic patients and five control subjects before and 2 h after a standard breakfast. Only diabetic patients showed increases in HbA1c and plasma glucose values 2 h after the test meal (Δ HbA1c = 0.87 ± 0.24% and Δ glucose = 210 ± 0.33 mg/dl). The increment in HbA1c correlated significantly with the increment in plasma glucose (r = 0.73, P < 0.05). to examine the lability of these postmeal increments in HbA1c, erythrocytes from pre- and postmeal blood samples were incubated in 0.9% NaCl for 5 h at 37°C and HbA1c was re-measured. After saline incubations %HbA1c in pre- and postmeal blood samples decreased in both diabetics and controls, and from each subject time 0 and 2-h HbA1c values were nearly identical. HbA1c was then measured before and after incubations of erythrocytes from a larger group of diabetic patients (N = 55) and control subjects (N = 7). In both diabetic and control cells HbAlc decreased after saline incubations. Pre- and post-saline HbA1c values in diabetics were (means ± SEM) 10.07 ± 0.30% and 9.15 ± 0.27%, respectively; values in controls were 5.87 ± 0.11% and 5.46 ± 0.09%, respectively. The mean decrement in HbA1c was significantly greater in cells from diabetics than from controls (P < 0.001). In diabetics the HbA1c decrement correlated with both plasma glucose (r = 0.58, P < 0.001) and the preincubation HbA1c (r = 0.55, P < 0.001). After dialysis of hemolysates for 5 h at 37°C or 48–72 h at 4°C, HbA1c values showed decreases comparable to those after saline incubations of intact erythrocytes. However, decreases in HbA1c after dialysis were accompanied by increases in HbA1a+b, findings that were not observed after saline incubation. The results suggest that HbA1c exists in two chromatographically indistinguishable forms: one that represents the major portion of HbA1c in normals and diabetics, is not altered by short-term changes in plasma glucose, and can be estimated by measuring HbA1c after saline incubation of erythrocytes; and a second form that is responsive to short-term fluctuations of the blood glucose level. These labile and stable fractions may be identical to the labile Schiff-base and stable ketoamine forms of HbA1c, which have been previously described. For HbA1c to be an accurate indicator of long-term glucose control, saline incubation of erythrocytes or perhaps dialysis of hemolysates before HbA1c assay may be necessary. Otherwise the assay results will reflect recent changes in blood glucose levels.