PB1075 - Discrepant results for ADAMTS13 assays with different activity assays. Artefact or cause for concern?

Background: ADAMTS13 is a plasma metalloprotease which regulates the multimeric size of VWF by cleaving the protein at Tyr1605-Met1606. ADAMTS13 deficiency (acquired or congenital) occurs in thrombotic thrombocytopenic purpura (TTP). This disease primarily requires a clinical diagnosis, but ADAMTS13 assays can provide confirmation, help characterise the phenotype of the deficiency, may be useful in monitoring treatment, and can be used to identify family members with ADAMTS13 deficiency.

Aims: Different assay methodologies are available for measurement of ADAMTS13 activity, which may give rise to differences in levels of ADAMTS13 depending on method used. Proficiency testing exercises may help identify differences in accuracy and precision of these methods.

Methods: We describe here data from a series of proficiency testing exercises for ADAMTS13 assays in 2020 and 2021, in which an average 25 UK and International centres performed assays on lyophilised plasma samples. All samples were obtained through plasma exchange from patients with suspected or confirmed TTP. Samples 2103 and 2104 were prepared from plasma from a patient with acquired TTP, to which normal plasma had been added.

Results: Results from 8 samples are shown in table 1. In samples with normal levels of ADAMTS13 activity, there was good agreement between results obtained by FRET, ELISA and chemiluminescence immunoassays (CLIA). For samples from donors with acquired TTP, and low levels of ADAMTS13 activity, results obtained by FRET-based assay methods were significantly higher than those obtained by ELISA and CLIA methods.

Conclusion(s): It is unclear whether these findings are a consequence of plasma collection following plasma exchange, sample processing, related to presence of ADAMTS13 antibodies in the samples, or reflect a genuine difference in results obtained with these different methods. Further studies are required to elucidate the clinical relevance of these findings.