Diagnosis of Thrombotic Thrombocytopenic Purpura by ADAMTS13 Activity Quantification

Relationship Between Intraluminal Thrombus Volume and Circulating ADAMTS-13 Activity in Abdominal Aortic Aneurysms

Introduction

Abdominal aortic aneurysms (AAAs) with intraluminal thrombus (ILT) are suggested to be more prone to rupture than AAAs without. Prior studies indicate that the von Willebrand factor (vWf) plays a role in the formation of ILT since a positive correlation between ILT volume and vWf has been shown. vWf mediates the tethering of platelets at sites of endothelial injury, and the protease ADAMTS-13 cleaves larger forms of vWf, thus counteracting the thrombosis cascade and maintaining haemostatic balance. When investigating the largest quantifiable thrombus in the human body, it was hypothesised that circulating ADAMTS-13 activity may be associated with ILT size in patients with AAA and the aim was to explore this potential relationship using 3D contrast enhanced ultrasound (3D-CEUS) for ILT volume determination.

Report

In this retrospective, exploratory study, 60 patients with AAA were evaluated, and the association between ILT volume and thickness and ADAMTS-13 was estimated using 3D-CEUS. ADAMTS-13 activity was measured in plasma samples obtained the same day. No association between ILT volume (r = -0.03, p = 0.84) or ILT thickness (r = 0.02, p = 0.87) and ADAMTS-13 activity was found. Likewise, when subdividing the group into lowest and highest 50% of ADAMTS-13 activity, the half with the lowest ADAMTS-13 activity (mean ILT volume ±standard deviation [SD]: 32 ± 34 mL) did not differ from the half with the highest ADAMTS-13 activity (43 ± 24 mL) when comparing ILT volume (p = 0.172, F = 2.95) and thickness (p = 0.070).

Discussion

After evaluating the largest quantifiable intraluminal thrombus in the vasculature, it was concluded that, surprisingly, circulating ADAMTS-13 activity seems unrelated to ILT formation in AAA.

Assay for ADAMTS-13 Activity with Flow Cytometric Readout

A disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13 (ADAMTS-13) is a metalloprotease that regulates the size of circulating von Willebrand factor (vWF) multimers. Severe lack of ADAMTS-13 activity [<10% of normal (0.1 IU/mL)] leads to thrombotic thrombocytopenic purpura (TTP), a specific type of thrombotic microangiopathy (TMA). Timely determination of plasma ADAMTS-13 activity is essential to discriminate TTP from other types of TMA with respect to adequate treatment. Identification of the minimal substrate motif for ADAMTS-13 within the A2 domain of vWF (vWF73) as well as the generation of monoclonal antibodies (mAbs) that specifically recognize the ADAMTS-13 cleavage site enabled the development of a variety of methods for determination of plasma ADAMTS-13 activity. In order to further extend the range of analytical platforms applicable for quantitative determination of plasma ADAMTS-13 activity, a specific, vWF/mAb-based assay with flow cytometric readout was developed and validated. Basic assay characteristics include a total assay time of 80 to 90 min, a near linear dynamic range from 0.005 (lower limit of quantification) to 0.2 IU/mL, and intra- and interassay coefficients of variation below 5 and 30% at input plasma ADAMTS-13 activities of 0.015 and ≤0.050 IU/mL, respectively. When compared to the results obtained with a commercially available quantitative ADAMTS-13 activity ELISA, analysis of 18 plasma samples obtained from patients with suspected TTP revealed full agreement of results with respect to the clinical 0.1 IU/mL TTP threshold. Based on these data, it is assumed that the described assay principle can be successfully transferred to virtually all laboratories that have a flow cytometer available.