High blood concentration refers to increased blood viscosity, which slows down blood flow and increases resistance to blood flow in the blood vessels, making it easier to induce thrombotic disorders. There are two common causes of high blood concentration: the first is the increase of cellular components in the blood, which is commonly caused by primary thrombocytosis, true erythrocytosis and when the deformability of red blood cells is reduced. The second is the alteration of various solute components in the plasma. The most common causes are increased blood glucose, increased lipids, increased fibrinogen concentration, and increased antibody concentration. Due to the increase of these solute components, the blood concentration increases, which also causes slow blood flow and tends to induce thrombotic disorders. Therefore, the increased blood concentration is caused by changes in blood cells or plasma solute components, and it is necessary to identify which factor is causing the increased blood concentration and then provide corresponding treatment to avoid thrombotic diseases.