Generally, routine CT examination will not directly cause white blood cell decrease, if found that the white blood cell decrease may be caused by other reasons, it is recommended to go to the hospital to clarify the cause of the disease. The radiation dose of CT examination is generally in the range of 2mSV~20mSV, within this range, it will not cause damage to the human body, and the radiation dose will not be accumulated, so CT examination will not directly cause leukocyte decrease. There are mainly the following reasons for the decrease of white blood cells. 1. Infection: influenza, chickenpox and other viral infections, etc., usually combined with fever and other symptoms, blood tests mostly show a decline in white blood cells, lymphocytes, lymphocytes, lymphocyte ratio, etc., which need to be examined by a doctor and then targeted medication. 2. Blood system diseases: such as aplastic anemia, hypersplenism, etc., the blood test mostly shows the decrease of whole blood cells, including the decrease of red blood cells, platelets and white blood cells. It is necessary to further improve the bone puncture, imaging examination to clarify the diagnosis, and early treatment after diagnosis. 3. Drug factors: In the clinic, there are many drugs that may cause leukocyte decline, such as chemotherapy drugs, sulfonamides, anti-hyperthyroidism, malaria, tuberculosis drugs and so on. Therefore, we need to pay attention to the current medication, timely feedback to the doctor, in order to adjust the medication. Warm reminder: everyone’s white blood cell level is in a fluctuating state, white blood cell reduction may be transient, so it is recommended that patients with lowered white blood cells regularly follow up the blood routine, and consult the doctor in case of discomfort.