The A1C blood test indicates average blood glucose levels over several months. Results from self-administered tests are volatile, shooting up after a meal, for example. A1C results are considered a far better indicator of health factors involving blood glucose levels.
A large study found that A1C numbers were higher in older, nondiabetic test subjects. (Reference 1)
Race and Ethnicity
Numerous studies have attempted to determine whether race and ethnicity affect A1C levels, although any answer is complex at best because of the many factors that affect glucose levels. There are indications that race and/or ethnicity may play a role independent of subject behavior. (Reference 2)
According to a study reported in “Occupational and Environmental Medicine,” workplace stress and social support at the workplace had significant effects on A1C results, with stress raising the percentage and social support lowering it. (Reference 3)
Diet and Exercise
As the A1C test measures the average blood glucose level over several months, diet and exercise will factor in. If your glucose levels are high due to inappropriate diet and lack of exercise, that will be reflected in your A1C percentage.
Sickle cell hemoglobin (hemoglobin S) causes A1C results to be less reliable. Anemia causes false low results, while iron deficiency can increase A1C numbers. Blood transfusions raise levels because blood preservative solutions are full of sugar. (See link in Resources)