PB0308 - Method Discrepant ADAMTS-13 activity results in first presentation query TTP subjects

Background: Thrombotic thrombocytopenia purpura (TTP) is a rare life-threatening disease caused by severe deficiency of the von Willebrand factor cleaving protease ADAMTS13. A rapid automated chemiluminescent method (ACL Acustar) has recently been introduced. We have identified a small number of subjects who have Acustar results < 10 IU/dL that don’t match the classical presentation of either immune mediated acquired TTP (iTTP) or congenital TTP (cTTP) and often demonstrate method discrepancy when compared to ADAMTS13 activity by an ELISA method.

Aims: To identify and characterise potential method discrepancy subjects within first presentation query TTP subjects.

Methods: Data was examined on 401 first presentation patients tested for ADAMTS13 activity since 9/2019. Subjects with ADAMTS13 activity < 15 IU/dL (Acustar) and negative ADAMTS13 antibody (technozyme) were tested for ADAMTS13 activity with (technozyme) ELISA method.

Results: Method discrepancy was identified in 1.5 – 2.5% of first presentation ?TTP subjects tested and 17.5-20% of all 1st presentation ?TTP subjects with Acustar activity < 10 IU/dL.

Conclusion(s): ADAMTS 13 activity >10 IU/dl by Acustar could be safely used to exclude TTP. We identified two distinct groups amongst subjects with ADAMTS13 results of < 10 IU/dL by Acustar. New iTTP subjects are unlikely to present with results of >2.0 and subjects with results between 2-10 IU/dL by Acustar are unlikely to be ADAMTS13 antibody positive and were typically found to demonstrate method discrepancy. The reason for the method discrepancy is currently unclear and was not described during verification studies of the Acustar Adamts13 activity method.
Knowledge of the small but clinically significant group of method discrepant subjects alongside the typically pattern of new iTTP subjects seen on the Acustar is important for clinical and laboratory teams using the Acustar.