Glycated hemoglobin HbA1c is considered the gold standard for assessing the compensation and treatment of diabetes. After the success of the DCCT and UKPDS studies, which showed that HbA1c is an independent indicator of risk for the development of chronic vascular complications of diabetes, this parameter was used for routine monitoring of metabolic control. In 2010, the American Diabetes Association introduced HbA1c determined by HPLC or HPLC-standardized methods for the diagnosis of diabetes as a parameter more accurate and more stable than the determination of fasting plasma glucose or oral glucose tolerance test. HbA1c measurements can be performed by different analytical methods including HPLC, affinity chromatography, electrophoresis and immunoenzymatic and immunoturbidometric methods. Due to differences in the results of HbA1c determined by laboratories using various methods, it is extremely important to standardize methods for the results given by the HPLC method compared to the methods used in the DCCT and UKPDS studies. The problem is even greater as the large discrepancies are observed between the frequency of carbohydrate disorders diagnosed on the basis of determination of HbA1c and glucose, which raise important questions concerning the widespread introduction of HbA1c for the diagnosis of diabetes.