The presence of more than 400 platelets in a child usually refers to a platelet count of around 400×10^9/L in a routine blood test. In general, a child’s platelet count is mostly normal at (100-300)×10^9/L, and more than 400×10^9/L is a mildly high platelet count. If a child has a high platelet count due to physiological factors, it is usually not serious. If the platelet count is high due to a disease, the cause of the abnormal platelet count needs to be analyzed and the severity judged based on the child’s symptoms and other relevant tests. A single routine blood test result does not fully diagnose that the child has a related disease. It may be caused by external influences during the blood test and other reasons, such as too little water, excessive emotion, strenuous exercise and other physiological causes, which are not serious at this time and can mostly be recovered after a period of rest. If a child has repeated platelet tests and there is a persistent elevation in a short period of time, it may be caused by some diseases such as acute infection, cold, Kawasaki disease, bronchitis, pneumonia, anemia, etc. However, the child can generally recover after the disease is cured, and the prognosis for these diseases is generally good, so there is no need to worry too much. If a child has persistent abnormal platelet count, it may be due to myeloproliferative neoplasms, such as true erythropoiesis, primary thrombocytosis, early primary myelofibrosis and chronic myeloid leukemia, which are more difficult to cure and more serious.