Valproic acid (VPA)是一個常用於治療癲癎的藥物。成人服用VPA引起的血小板減少症機率約為1-30%；其中老年人發生的機率約為年輕人的兩倍。在女性VPA血中濃度如果高於110 μg/mL，男性如果高於135 μg/mL時，引起血小板減少症的可能性升高。另外VPA造成血小板減少的數量約在15,000/dL至85,000/dL。VPA引起血小板減少症的可能性與劑量呈正相關(dose-related)，但是無法預測。本文將報告一位77歲的女士服用VPA 1200 mg，q8h，測得血中VPA濃度為134.2 μg/mL，血小板數量由133,000/uL降至39,000/uL。當停止給予VPA後血小板數量即回升至正常。VPA引起的血小板減少症之機轉至今仍不明。臨床上若使用VPA應監測病人的血小板數量，特別是使用於老年人時。本文將報告服用VPA引起血小板減少症的治療經過，並回顧相關文獻。

Valproic acid, an antiepileptic drug, is frequently used to treat seizure disorders. The incidence of valproic acid induced thrombocytopenia has been reported to vary from 1% to 30%. The rate of occurrence of thrombocytopenia among elderly patients was nearly double as compared to younger patients. The risk is also increased associated with the increase of total plasma trough level of valproic acid, which is 110 µg/mL in females and 135 µg/mL in males. Nadir platelet counts after valproic acid administration ranged from 15,000/µL to 85,000/µL and the time course was variable. Valproic acid induced thrombocytopenia may be transient and dose-related. We described a 77-year-old patient with valproic acid-induced thrombocytopenia. She was taking valproic acid 1200 mg q8h, with valproic acid concentration of 134.2 µg/mL. Her platelet count decreased from 133,000/uL to 39,000/uL. Five days after discontinuation of valproic acid, platelet count returned to baseline. The etiology of valproic acid induced thrombocytopenia has not been elucidated at present. We suggested that regular monitoring of platelet counts on valproic acid, especially for elderly patients is indispensable.