Unstable glycosylated hemoglobin in patients with diabetes mellitus.

Incubation of washed erythrocytes for 4 h at 37 degrees C in saline resulted in the disappearance of an unstable glycosylated fraction. In a reference group, its amount was very small, but its presence had a significant influence on the upper level of the reference range. In non-diabetic pregnant women, the unstable fraction was significantly higher than in the non-pregnant individuals, which resulted in a lower reference range for the stable HbA1. In diabetic patients, the average unstable HbA1 was about 10% of the "unincubated" level, with marked differences between individuals (0--30%), even in patients with a slightly elevated HbA1. Therefore it is important to determine the concentration of stable HbA1.