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Relationship between ADAMTS13 antigen (Ag) and free IgG anti-ADAMTS13 antibody (free IgG ab), functional ADAMTS13 inhibitor (FI) and circulating immune complexes (CIC) using two ELISA techniques, in patients (P) with acquired thrombotic thrombocytopenic purpura (aTTP). Ana C. Kempfer1, Juvenal Paiva 2, Analía Sanchez-Luceros12, Adriana I. Woods1, María M. Casinelli2, María A. Lazzari1.1 Laboratorio de Hemostasia y Trombosis, IMEX-CONICET, 2Hemostasia y Trombosis, Academia Nacional de Medicina de Buenos Aires, Argentina. NdXHrl12 A-859-0081-00433B TTP is caused by the absence of functional ADAMTS13. The deficiency may be congenital or acquired. Thomas (2015) suggests that enhanced ab-mediated clearance provides a major pathogenic mechanism in aTTP, corroborated by the reduced levels of circulating Ag in P with non-inhibitory ab.The aim of this study was to evaluate the relationship of ADAMTS13-measurements in 64 P, [(55 in the acute phase of their first episode (ap) and 9 in non-acute phase (nap)] at first consultation.MActivity (Ac), Ag, FI and free IgG ab were evaluated by ELISA. CIC by ELISA using ab: clone 20A5 (A) or monoclonal from Ag kit (B).RThe P were 46 female/ 18 male. The median age was 37±19 (SD).Coefficients of variance were acceptable for CIC in A and B. The cut off for normal levels of ADAMTS13-specific CIC was 0.501, n=28 in A and 0.503, n =25 in B. The medians of A and B were not significantly different p=0.1069 (p < 0.05). A is the elected ELISA. CIC median of all P was 0.42, n=60 and CIC median of P with higher levels was 0.55, n=16.All ADAMTS13-related measurements were in Table I.In ap P, there was a very weak correlation found between Ag and CIC (Spearman?s coefficient r= -0.017, n=51), and between Ag and FI (r=-0.061, n=40). There was a moderate correlation (r=-0.477, n=55) between Ag and free IgG ab (including two P with high titre ≥850 U/mL). The 95th percentile said that the free IgG ab was below 318 U/mL, then there was a strong correlation (r =-0.643, n=53).C The prevalence (pr) of ap P with aTTP who had CIC pr=27% was lower than that found by Lotta (2014) and Mancini (2016), pr=47% and pr=39% respectively, although we substituted the ab and doubling the number of ap P of our previous report (pr=33%).Thomas (2015) said that not all anti-ADAMTS13 ab promote clearance, as free IgG ab did not correlate with Ag, exemplified by two P in remission with high-titre ab. In our study, only 95% of free IgG ab (without high titres) might be a variable with prognostic relevance.