text: Hemoglobin in the red blood cells comes in several forms. F is 'fetal' and isn't produced after birth. S is the sickle cell type, and so on. A is one of these, and 1c is a subtype of A. There are several names / abbreviations for it; it is also called glycosolated hemoglobin. It reacts with glucose to form a permanent combination. The modification of this form of hemoglobin has no particular affect on oxygen transport and so is without much effect, save as a record of glucose levels. \n\nFurthermore the reaction goes forward at a nearly constant rate, depending on the concentration of blood glucose. Since red blood cells last about 90-120 days in the blood before being recycled, the amount of HbA1c is proportional to the average blood glucose over the last 3 months or so (with some increased weight to the last few weeks in that period). \n\nThe usual value in a non-diabetic is around 5, and in diabetics with good sustained glucose control will be 6 or perhaps 6.5 or less. Values above perhaps 7 are evidence of sustained poor blood glucose control. \n\n\n\nAnother similar test is the "fructosamine" test. Like HabA1c, it measures the results of a relatively constant reaction rate with glucose, but in this case with blood protein (mostly albumin). It is reprocessed much faster than red blood cells; the value reflects average blood glucose over about 2 weeks.