Platelets (400-500) x 10⁹/L are usually not serious in themselves; the normal range for platelets is (100-300) x 10⁹/L, and platelets of (400-500) x 10⁹/L indicate the presence of mild thrombocytosis. It is commonly caused by secondary factors, such as chronic inflammatory and neoplastic diseases. It can also be seen in primary diseases, such as primary thrombocythemia. 1. Chronic inflammatory diseases such as chronic hepatitis, chronic cholecystitis, chronic pyelonephritis, etc., can lead to mild thrombocytosis, which can be in the range of (400~500)×10⁹/L when tested. Neoplastic diseases such as lung cancer, gastric cancer, liver cancer, etc., can cause an increase in the body’s platelet reactivity, which can be in the (400~500)×10⁹/L range when tested. Whether it is serious or not is related to the primary disease, such as lung cancer, liver cancer, etc. It is more serious and can be life-threatening due to disease progression. 2. Primary thrombocythemia, when the platelet index is (400~500)×10⁹/L, it is not serious yet, and it needs to monitor the index changes continuously, and it can be treated with oral drugs such as hydroxyurea and aspirin. Patients should be under the guidance of the doctor, do further examination, in order to clarify the cause of the disease, and actively carry out treatment.