Long-term control of blood sugar concentration can be measured by the concentration of Hb A1c. Measuring it directly would require many samples because blood sugar levels vary widely through the day. Hb A1c is the product of the irreversible reaction of hemoglobin A with glucose. A higher glucose concentration results in more Hb A1c. Because the reaction is slow, the Hb A1c proportion represents glucose level in blood averaged over the half-life of red blood cells, is typically ~120 days. An Hb A1c proportion of 6.0% or less show good long-term glucose control, while values above 7.0% are elevated. This test is especially useful for diabetics.This Hb A1c level is only useful in individuals who have red blood cells (RBCs) with normal survivals (i.e., normal half-life). In individuals with abnormal RBCs, whether due to abnormal hemoglobin molecules (such as Hemoglobin S in Sickle Cell Anemia) or RBC membrane defects – or other problems, the RBC half-life is frequently shortened. In these individuals, an alternative test called "fructosamine level" can be used. It measures the degree of glycation (glucose binding) to albumin, the most common blood protein, and reflects average blood glucose levels over the previous 18–21 days, which is the half-life of albumin molecules in the circulation.
