Immune Thrombotic Thrombocytopenic Purpura

Importance Immune thrombotic thrombocytopenic purpura (iTTP) is a life-threatening thrombotic microangiopathy that presents with microangiopathic hemolytic anemia (MAHA) and thrombocytopenia. Worldwide annual incidence of iTTP is 2 cases per million to 6 cases per million. Observations Immune TTP is caused by an autoantibody to a disintegrin and metallopeptidase with thrombospondin type 1 motif 13 (ADAMTS13), an enzyme that cleaves von Willebrand factor (vWF). With severely low ADAMTS13 activity (<10%), large multimers of vWF accumulate and bind platelets, forming microvasculature thromboses that cause ischemic organ injury (eg, myocardial infarction and stroke). The incidence of iTTP is higher in adults than children (incident rate ratio [IRR], 31.62 per million person-years [95% CI, 14.68-68.10]), females than males (IRR, 3.19 [95% CI, 2.65-3.85]), and Black compared with non-Black individuals (IRR, 7.09 [95% CI, 6.05-8.31]). Common presenting symptoms are neurologic (eg, headache, confusion, or seizures [39%-80%]) and abdominal pain (35%-39%). For patients presenting with MAHA and thrombocytopenia, clinical prediction scores for iTTP using laboratory data, such as platelet count less than 30 × 10 9 /L and creatinine level less than 2.0 mg/dL (176.8 μmol/L), can help guide empirical treatment initiation for iTTP before ADAMTS13 results are available. Prompt initiation of therapy with therapeutic plasma exchange, corticosteroids, and rituximab improves survival with iTTP from almost zero to approximately 93%. Caplacizumab, a synthetic small antibody (nanobody) that blocks platelet binding to vWF, administered concurrently with immunosuppression and therapeutic plasma exchange and continued until ADAMTS13 recovery, reduces the time to normalization of platelet count and decreases the risk of early recurrence (defined as within 30 days of completing therapeutic plasma exchange) compared with placebo (risk difference [RD], −29% [95% CI, −42 to −14%]) but increases bleeding risk (RD, 17% [95% CI, 4%-30%]). After obtaining clinical remission (defined as at least 30 days of sustained normalization of platelet count, decreased serum lactate dehydrogenase level, and absence of new or progressive ischemic organ injury without therapeutic plasma exchange or caplacizumab), 16% of patients have at least 1 relapse of iTTP. Regular monitoring of ADAMTS13 activity in remission and administration of rituximab when ADAMTS13 activity is less than 20% reduces risk of relapse (odds ratio, 0.09 [95% CI, 0.04-0.24]). Conclusions and Relevance Immune TTP is a rare immune-mediated disorder that presents with thrombocytopenia and MAHA and may cause life-threatening thrombosis. Treatment with therapeutic plasma exchange, corticosteroids, and rituximab is associated with 30-day survival rates of more than 90%. Addition of caplacizumab shortens time to normalization of platelet count and reduces recurrences while receiving the drug but increases bleeding risk. Monitoring ADAMTS13 activity in survivors and initiation of rituximab for those with low ADAMTS13 activity reduces the risk of clinical relapse.