ADAMTS13 inhibitors are measured using the same ELISA method. The patient’s sample is heat-inactivated and mixed with an equal volume of pooled normal plasma (PNP) before testing. After testing, the residual activity is calculated, and the inhibitor concentration is expressed in Bethesda equivalent units (BEUs).
ADAMTS13 inhibitors are generally IgG antibodies to the ADAMTS13 molecule and are pathologic because they inactivate ADAMTS13 activity. Quantitative assays for ADAMTS13 antibodies exist, but they do not address the inhibitory activity of the antibody. Antibody tests are also not highly specific for the presence of ADAMTS13 inhibitors: up to 4% of healthy individuals and 13% of patients with systemic lupus erythematosus have detectable levels of ADAMTS13 antibodies.1,2