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Patterns of changes of anti‐ADAMTS13 after plasma exchange

An enzyme-linked immunosorbent assay (ELISA) has recently been developed to detect antibodies against the von Willebrand factor-cleaving protease ADAMTS13 in patients with thrombotic thrombocytopenic purpura (TTP). The ELISA is based on incubation of plasma with immobilized recombinant ADAMTS13 followed by visualization of IgG and IgM antibodies by means of secondary enzyme-labeled antibodies [1]. In a recent study, anti-ADAMTS13 IgG antibodies were detected in most patients with TTP (97%) characterized by severe ADAMTS13 deficiency (< 10% of normal) [1]. The ELISA was more sensitive than the standard inhibitor assay based upon ADAMTS13 neutralizing activity, which gave positive results in 80% of patients [1]. The study, carried out in patients with acute untreated TTP, gave no information on the value of the ELISA in monitoring patients with immunomediated TTP during treatment with plasma exchange. We subsequently investigated three patients (two with chronic recurrent TTP and one with a first acute episode that recurred) before, during and after plasma exchange. These clinical cases illustrate examples of the parallel changes of platelet count, ADAMTS13 activity, anti-ADAMTS13 inhibitory activity and IgG antibody titers. A 53-year-old male with chronic recurrent TTP (Fig. 1) was first investigated during clinical remission (platelet count 92 · 10 9 L )1 ), when his ADAMTS13 activity was below 6%, and anti-ADAMTS13 antibodies undetectable by inhibitor assay and ELISA (panel A, time I). Six days later he had a clinical relapse and his platelets spontaneously decreased to 42 · 10 9 L )1 , with inhibitory activity 0.5 BU mL )1 and the ELISA detected a weak anti-ADAMTS13 IgG titer (1:20) (panel A, time II). The patient was then treated with two sessions of plasma exchange, which rapidly normalized his platelet count. Twelve days later, when the patient was off therapy, his platelet count decreased again to 10 · 10 9 L )1 , with an increase in inhibitory activity to 16 BU mL )1 (data not shown). He was treated again with nine sessions of plasma exchange within 28 days (five plasma exchange sessions daily and subsequently with four additional sessions within the next 23 days). After the last exchange session he had a stable platelet count (148 · 10 9 L )1 ), but persistently severe ADAMTS13 deficiency (< 6%), and his inhibitory activity rose to 5B U mL )1 and IgG antibody titer was markedly increased