The severity of your child’s platelets 400 x 10^9/L needs to be determined by the cause. The normal value of platelets is (100~300) x 10^9/L, and 400 x 10^9/L belongs to a slightly high condition. 1. Thrombocytosis in children may be caused by iron deficiency, hemolytic anemia, acute blood loss, infection, trauma, stress or surgery, etc. Usually, platelet count of 400×10^9/L caused by the above conditions is not a serious condition, and can be gradually recovered after treatment. For example, levofloxacin can be used to treat the infection. 2. However, thrombocytosis in children may also be caused by myeloproliferative diseases such as primary myelofibrosis, true erythrocytosis, chronic myeloid leukemia, and so on, which are generally more serious. Because of the slow course of primary thrombocytosis, the symptoms are easy to recur, and the prognosis is poor. It is recommended to go to the hospital in time to improve the relevant examination, to clarify the cause of the disease, according to the cause of the disease to determine whether it is a serious situation, diagnosis and treatment under the guidance of the physician.