Thrombotic thrombocytopenic purpura (TTP) is often associated with acquired or congenital deficiency of the von Willebrand factor-cleaving metalloprotease, ADMATS13 (Lammle B et al., J Thromb Haemost 2005;3:1663-1675; Schneppenheim et al., Blood 2003;101:1845-1850). Although undetectable levels of enzyme activity (<10%) are diagnostic of inherited or acquired TTP in the correct clinical setting (absence is specific), not all patients diagnosed with TTP have severe protease deficiency, and it is therefore not recommended as an initial test for diagnosis (Copelovitch and Kaplan, Pediatr Nephrol, in press). Many prospective and retrospective studies have demonstrated that patients with severe protease deficiency have a higher likelihood of relapse, making it helpful as an indicator of recurrence. The short-term prognostic usefulness of ADAMTS13 testing during acute TTP warrants further investigation because of limited prospective studies (Ferrari S et al., Blood 2007;109:2815-2822; Peyvandi et al., Haematologica 2008;93:232-239).
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