What can cause a falsely elevated or low hemoglobin A1c (HbA1c)?

I was prompted to write this article when one of my patients. who happens to be a physician, told me that the hemoglobin A1c test that I wanted to order for her would not be appropriate because she was a regular blood donor, which affects HbA1c levels. Hemoglobin A1c (HbA1c or HgbA1c) is a type…