Testing of hemoglobin A(1C) (HbA(1C)) levels has become widespread in the management of patients with diabetes mellitus. Since the 1980s, it has proven to be an invaluable tool correlating with a patient's average blood glucose levels as well as with their disease morbidity. Clinicians often base treatment decisions and make adjustments depending on a patient's HbA(1C) level. As useful as the HbA(1C) is, it does have notable limitations. A number of conditions can lead to a falsely elevated or a falsely low HbA(1C) level. When one of these conditions is present, it is important to recognize the inaccuracy of the HbA(1C) test to prevent a delay or error in the diagnosis or care of patients with diabetes mellitus. It is also important to be aware of alternative methods of monitoring a patient's diabetes such as a fructosamine assay or home and office blood glucose measurements. Presented is the case of a patient with diabetes mellitus and hereditary spherocytosis, a condition that interfered with her HbA(1C) value and resulted in a delay in her care.