Glycated hemoglobin (HbA(1c)) is a parameter broadly employed in the assessment of glycemic control in diabetes. The 2010 "Standards of medical care in diabetes", published by the American Diabetes Association (ADA), recommended performing the HbA(1c) test at least every 6 months in patients in whom disease is clinically stable, while subjects after modifications of therapy or in whom glycemic goals have not been met should be tested every 3 months. Moreover, the ADA suggested the HbA(1c) assay be implemented in the diagnosis of diabetes and in the detection of an increased risk of developing this disease. Among various approaches employed to measure the concentration of HbA(1c), high-pressure liquid chromatography is considered to be a reference method. HbA(1c) tests might not be clinically reliable in some circumstances. In cases when HbA(1c) levels do not correlate with glycemia and clinical symptoms, the results should be interpreted with caution, several conditions known to influence the measurement should be taken into account, and use of another diagnostic method, or even testing another marker of glycemic control, e.g., fructosamine or 1,5-anhydroglucitol, should be considered.