Summary

We have established a new, enzyme-linked immunosorbentassay (ELISA) for the detection of ADAMTS13 antigen usingpurified polyclonal rabbit anti-human ADAMTS13 IgG. Normalplasma ADAMTS13 antigen levels span a concentration of 740–1420ng/ml (median 1080ng/ml) resulting in an ADAMTS13 activityto antigen ratio of 0.48 to 1.68 U/µg. In a cohort of HUSpatients,ADAMTS13 antigen was in the normal range, whereasin hereditary TTP patients antigen levels were low to undetectable,in concordance with severe deficient ADAMTS13 activity.Plasma of acquired TTP patients was found to contain free as well as autoantibody-bound ADAMTS13. We also present evidencefor circulating anti-ADAMTS13 antibody/ADAMTS13antigen immune complexes not only in acutely ill or activelytreated patients but also in patients who have already achievedclinical remission. This new developed ADAMTS13 antigenELISA assay allows rapid determination of ADAMTS13 antigenlevels in human plasma but is of limited predictive value for thediagnosis or treatment of acquired TTP due to the detection ofADAMTS13 in antibody complexes.