Methodologic considerations in measuring glycosylated hemoglobin in epidemiologic studies.

Multiple linear regression was used to predict an incubated glycosylated hemoglobin value from the unincubated value and blood glucose. Hemoglobin A1 (HbA1) was measured by a disposable microcolumn technique in a large, geographically defined population of diabetic persons in southern Wisconsin. During the study, incubation of blood samples to remove pre-A1c was implemented. Multiple linear regression using data from a group of 788 patients yielded the equation: incubated HbA1 = 0.897 (non-incubated HbA1)-0.00332 (blood glucose) + 0.388. This equation was "validated" by substituting the calculated value of incubated HbA1 for the actual value in a multinomial logistic regression with diabetic retinopathy as the dependent variable. Little change in the model resulted from the substitution. Further validation was obtained from an independent sample of diabetic persons. Calculated values of incubated HbA1 were an average of 0.4% lower than the actual values.