Hereditary Thrombotic Thrombocytopenic Purpura in a 9-Month Old: Diagnosing and Managing an Ultra-rare Disorder.

Hereditary thrombotic thrombocytopenic purpura is an ultra-rare disorder caused by biallelic mutations in the ADAMTS13 gene. Because it can be difficult to diagnose, plasma ADAMTS13 activity assessment should be considered in patients with thrombocytopenia, anemia, and schistocytes on peripheral blood smear. We present the diagnostic evaluation of a patient with hereditary thrombotic thrombocytopenic purpura. Genetic testing revealed one known pathogenic mutation and one novel mutation of ADAMTS13 classified as likely pathogenic on the basis of parental genetic testing and in silico analyses. We further discuss off-label use of prophylactic plasma-derived Factor VIII (Koate-DVI) and the benefit of rare disease registries.