The red blood cell count of 3.5 x 10¹²/L is low. There are many reasons for the low index, and the common ones are physiological reasons such as large amount of infusion, and pathological reasons such as iron-deficiency anemia, acute blood loss, malignant blood system diseases, etc. 1. Physiological reasons: for example, large amount of infusion will cause increase of plasma volume, blood dilution, and relative decrease of red blood cell count, and the red blood cell count will return to normal after excessive water is discharged from the body. 2. Iron deficiency anemia: Iron is an important component of hemoglobin in red blood cells. When iron deficiency occurs, the amount of hemoglobin synthesized by red blood cells decreases, leading to a decrease in the production of red blood cells and a decrease in their count. 3. Acute blood loss: excessive loss of red blood cells due to trauma, surgical hemorrhage, etc. can also lead to lower red blood cell count. 4. Malignant hematologic diseases, such as leukemia, lymphoma, myelodysplastic syndrome, etc., can cause low red blood cell count. There may be other causes of low red blood cell count, such as megaloblastic anemia due to folic acid deficiency, hemolytic anemia due to various reasons (burns, malaria, etc.). If you find a low red blood cell count, you should have a complete examination in time to find out the cause, and standardize the treatment under the guidance of your doctor.