The main functional protein of erythrocytes is hemoglobin, which is not normally found in plasma, but during hemolysis, erythrocytes break down, it comes out of them and binds to the haptaglobin protein. Hemoglobin is a chromoprotein found in erythrocytes and carries out the transport of oxygen from the lungs to the tissues and the transport of carbon dioxide from the tissues to the lungs. These gases are loosely bound to the iron in the composition of heme as the non-protein component of hemoglobin, which also contains the protein component globin. In erythrocytes, there is also hemoglobin A1c in a certain amount, which is formed through non-enzymatic glycolysis of valine residues from the N-terminal end of the β-chain of hemoglobin A. It is created in pathological cases when the glucose in the blood has high concentrations, that is, the excess of glucose binds to the hemoglobin. Glycosylated HbA1c is of great importance for monitoring treatment and therapy in patients with diabetes mellitus, because its concentration is proportional to the concentration of glucose and in the long term, within three months, because HbA1c does not break down and remains present in the blood until erythrocytes are destroyed. The purpose of the research is to prove the significance of determining the concentration of HbA1c in monitoring diabetes mellitus and keeping it under control. This is the most reliable method for detecting poor metabolic regulation of diabetes, especially in cases where it remains undetected for a long time, and blood glucose is tested once every 2–3 months. The trials were conducted in young female patients aged 25 to 35 years with previously confirmed hyperglycemia. The patients were divided into three groups, a control group of patients with a normal level of glycemia in the circulation (n=30), a group of patients with diagnosed hyperglycemia (n=30), a group of patients treated with therapy to reduce the level of glycemia in the blood with Glucophage, which acts on the basis of dopamine (n=30). HbA1c is determined by an immunochemical inhibitory method that uses two reagents, one of which contains mouse antibodies to HbA1c applied to latex particles. The second reagent contains parts of HbA1c as antigens applied to agglutinators. The measurements were performed on an automated biochemical analyzer ARCHITECT 4000. The obtained results showed that patients diagnosed with diabetes mellitus have significantly increased results for the concentration of glycemia compared to patients from the control group whose results are within normal glucose values. The group of patients treated with therapy to reduce the level of glucose in the blood, that is, with Glucophage, have evidently reduced values for the level of glucose to a level close to the control group. There is a visible effect of Glucophage therapy in reducing the concentration of glucose in the blood. The drug reduces the concentration of glucose in the blood to normal values, that is, it regulates the level of sugar in the blood in such a way that it reduces the breakdown of glycogen from the liver to glucose, and increases the use of glucose in muscle cells. With the application of antidiabetic therapy with Glucophage, the negative effect of hyperglycemia is removed. In conditions of hyperglycemia, the concentration of glucose significantly increases compared to the control group, and in the treated group of patients, the level of glucose significantly decreases to values close to the control group of patients