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Day-to-day fluctuations in blood concentration of the glycosylated hemoglobin, HbA1c or HbA1, are mainly due to variations in content of the intermediate aldimide form, preHbA1c. Using electrofocusing we have studied this component adjacent to the HbA1c band, which represents the stable ketoamine form. The fraction of preHbA1c present rapidly increases upon incubation of erythrocytes in 20 mM glucose at +37 degrees C, and the reaction is fully reversible if glucose is removed. In children with insulin-dependent diabetes mellitus with different degrees of metabolic control, levels of preHbA1c varied between 0.2 and 3.7% of total hemoglobin (median value 1.3%, N = 25). A clear correlation was found between preHbA1c and urinary glucose concentrations during the 12 hours preceding blood sampling (r = 0.75, p less than 0.001, df = 18). As expected, pre-HbA1c did not correlate to HbA1c or glycosylated albumin, which reflect long- and medium-term diabetic control.