Figure 2. Clinical course during hospitalization. We diagnosed respiratory failure due to pneumonia, and disseminated intravascular coagulation (DIC) on admission. We began administering ceftriaxone, levofloxacin, steroid pulse therapy, peramivir, and recombinant human thrombomodulin. On day 3 of hospitalization, the platelet count decreased dramatically, and the lactate dehydrogenase (LDH) concentration increased markedly. Thrombocytopenia persisted despite platelet transfusion. On day 8, we diagnosed TMA because of the presence of fragmented erythrocytes in the blood smear, thrombocytopenia, and elevated LDH concentration, and we initiated plasma exchange therapy. We administered rituximab on day 8 because the patient had neuropsychiatric symptoms. After 4 plasma exchanges and rituximab once a week for 2 weeks, platelet numbers and LDH concentration normalized on day 16. The result of chromogenic ADAMTS13-act-ELISA was <0.5%. The patient was discharged home on day 41, and we subsequently submitted a nasopharyngeal specimen collected on admission to the National Institute of Public Health and the Environment, which identified influenza A virus infection. Therefore, we finally diagnosed secondary TTP associated with influenza A virus.0