[Acquired thrombotic thrombocytopenic purpura with rituximab-induced serum sickness].

The patient was a 38-year-old man who was admitted to our hospital with fever, vomiting and unclear speech. Blood tests showed hemolytic anemia, thrombocytopenia, and elevated creatinine, and a Coombs test was negative. ADAMTS13 activity was <1% and ADAMTS13 inhibitor was 0.7 BU/ml, so we diagnosed acquired thrombotic thrombocytopenic purpura (aTTP). We started plasma exchange and prednisolone 1 mg/kg on the day of admission (day 1). The platelet count recovered to normal on day 5, and hemolytic findings improved. However, symptoms worsened again, and thus we started rituximab for refractory aTTP on day 24. After rituximab administration, the patient showed no signs of infusion reaction (IR) or recurrence. On day 31, he received the second dose of rituximab and was discharged. On day 38, he had knee pain, fever, erythema, and difficulty moving. CRP was elevated and CH50 slightly decreased. Imaging findings were normal. Anti-rituximab antibody was positive (197 ng/ml), leading to the diagnosis of rituximab-induced serum sickness. Rituximab administration was discontinued, and the serum sickness has not recurred. The presence of anti-rituximab antibodies may aid diagnosis in patients with suspected symptoms of serum sickness or recurring IR-like symptoms.