Acquired Thrombotic Thrombocytopenic Purpura Without Anti-ADAMTS13 Antibody Caused by Influenza A (H1N1) Virus Successfully Treated by Plasma Exchange: A Case Report.

BACKGROUND Thrombotic microangiopathy, characterized by microangiopathic hemolytic anemia, thrombocytopenia, and organ damage by microvascular thrombosis, has a high mortality rate; therefore, early diagnosis and treatment are important. Thrombotic thrombocytopenic purpura is caused by a deficiency of a disintegrin-like and metalloproteinase with thrombospondin type 1 motifs 13 (ADAMTS13), and results in thrombotic microangiopathy. Influenza virus causes thrombotic thrombocytopenic purpura by inducing immunoglobulin G autoantibodies against ADAMTS13. We report a rare case of thrombotic thrombocytopenic purpura caused by influenza A without anti-ADAMTS13 antibody that was treated by plasma exchange. CASE REPORT A 57-year-old woman was admitted to our hospital because of hypoxemia. We diagnosed pneumonia and disseminated intravascular coagulation. Despite treatment, she developed thrombocytopenia, and we diagnosed thrombotic microangiopathy and started plasma exchange. With a PLASMIC score of 6 points and neuropsychiatric symptoms, we strongly suspected thrombotic thrombocytopenic purpura and started rituximab. However, ADAMTS13 activity by FRETS-VWF73 assay was 65%, and anti-ADAMTS13 antibody was negative. After 4 plasma exchanges and 2 rounds of rituximab, platelet numbers and lactate dehydrogenase and creatinine concentrations normalized on the 16th day of hospitalization. Subsequently, influenza A (H1N1) was identified in a nasopharyngeal swab collected on admission. Plasma enzyme-linked immunosorbent assay testing for chromogenic ADAMTS13 activity showed a significant decrease (<0.5%). Therefore, we diagnosed thrombotic thrombocytopenic purpura caused by influenza A without anti-ADAMTS13 antibody. CONCLUSIONS We present a rare case of thrombotic thrombocytopenic purpura without anti-ADAMTS13 antibody caused by influenza A virus successfully treated by plasma exchange. Influenza A may reduce ADAMTS13 activity without inducing autoantibodies.