ABSTRACT
WHAT IS KNOWN AND OBJECTIVE: Unexplained thrombocytopenia is a common clinical problem, and the possibility of drug-induced thrombocytopenia must be considered. Moreover, chemotherapy agents are known to typically cause thrombocytopenia by suppressing haematopoiesis, but they can also cause immune thrombocytopenia in which the mechanism involves antibody-mediated platelet destruction. CASE DESCRIPTION: We report a case of severe thrombocytopenia related to trastuzumab administration in which the patient exhibits a slow drift downward in platelet counts with repeated cycles of trastuzumab. A 70-year-old woman received a diagnosis of breast cancer and was treated with trastuzumab. She was confirmed to have a severe trastuzumab-induced thrombocytopenia after the 4th cycle. Retreatment with trastuzumab was not attempted. WHAT IS NEW AND CONCLUSION: Considering that trastuzumab is being increasingly used and that severe thrombocytopenia is a potentially life-threatening complication, its association must be seriously considered.
