It should be noted that the HbA1c is an approximation to an average of the glucose levels over the period of the life of red cells for an individual. There is considerable variability in the result
RESULTS—The slope (95% CI) for mean sensor glucose concentration (area under the curve)
versus a centrally measured HbA1c was 24.4 mg/dL (22.0–26.7) for each 1% change in HbA1c,
with an intercept of 216.2 mg/dL (232.9 to 0.6). Although the slope did not vary with age or
sex, there was substantial individual variability, with mean sensor glucose concentrations ranging from 128 to 187 mg/dL for an HbA1c of 6.9–7.1%. The root mean square of the errors
between the actual mean sensor glucose concentration versus the value calculated using the
regression equation was 14.3 mg/dL, whereas the median absolute difference was 10.1 mg/dL
The test measures how much hemoglobin becomes glycosalated which means that how many glucose molecules attach to the hemoglobin molecule. The amount of hemoglobin around depends on the number of red cells, and the number of red cells depends on the red cell life, their production and destruction. Some conditions shorten the life of a red cell from the average of 120 days but in sickle cell disease it can drop to 28 days
As red cells age they accumulate more glycosalated hemoglobin so conditions which shorten red cell life span would tend to lower the HbA1c
Blood HbA1c is a mean for RBCs with values that range from very low for reticulocytes to approximately twice the mean for the oldest RBCs.
We also see that diabetic patients can have RBCs with shorter lives
The mean age of circulating RBCs ranged from 39 to 56 days in diabetic subjects and 38 to 60 days in nondiabetic controls
but anemia, and even exercise can shorten the life of a red cell
Exercise and professional sport increase RBC turnover and maximize oxygen delivery to the tissues (Mairbäurl, 2013). Maturation and aging of RBCs is accompanied by multiple processes occurring at various rates driving the circulating RBCs from adolescence to senescence within approximately 120 days (Lew and Tiffert, 2013; Lutz and Bogdanova, 2013). The resulting “markers of senescence” are recognized by the macrophages and clearance of RBCs is promptly initiated (de Back et al., 2014). Premature clearance is a hallmark of various disorders associated with anemia. In each case one or multiple markers of senescence appear prematurely. Those include excessive oxidative stress (Mohanty et al., 2014), excessive cation leak with the following dehydration (Wang et al., 2014), decrease in RBC size and loss of RBC membrane through vesiculation (Alaarg et al., 2013), metabolic abnormalities (Vives-Corrons et al., 2013), or following auto-immune diseases (Lutz and Bogdanova, 2013). Blood storage damages RBCs facilitating aging. As a result clearance of transfused cells is dramatically facilitated (Bosman, 2013; Flatt et al., 2014).
With normal activity, and no other health conditions that affect red cell life, one can expect the HbA1c to apply over a period of 115 days +/- 15% which is the average life span of red cells.