Glycosylated Hemoglobin

It measures the percentage of hemoglobin A molecules that have a glucose moiety attached. These molecules can be separated from normal HbA by electrophoresis into 3 fractions called HbA1a, HbA1b, and HbA1c; normally only HbA1c is quantitated. The percent of glycosylation is proportional both to time and to concentration of glucose in the blood.

Although minor variations occur from lab to lab, the HbA1c correlates to the average blood sugar over the past three months (which is the mean lifespan of a red blood cell.)

          HbA1c                   Avg. Glucose
           14         awful             380
           13                           340
           12                           310
           11         poor              280
           10                           240
            9                           210
            8                           180
            7         fair              140
            6        excellent          110
            5                            80
    

These values are used to drive the therapy regimen; if the HbA1c is above 6.5, adjustments should be made. It is checked every three to four months.
Incidentally, a diagnosis of DM is made if a fasting blood glucose is 126 gm/dl or greater; the HbA1c is not used for diagnosis.