ADAMTS13 Testing Overview

Introduction

ADAMTS13 (A Disintegrin And Metalloproteinase with Thrombospondin Motifs 13) is a protease that cleaves ultra-large von Willebrand factor (vWF) multimers. An ADAMTS13 deficiency plays a key role in the pathogenesis of thrombotic thrombocytopenic purpura (TTP).¹

Clinical Indications for Testing

• Evaluation of suspected TTP  
• Distinguishing TTP from other thrombotic microangiopathies (e.g., HUS, aHUS, DIC) and HIT
• Monitoring disease activity or treatment response in known TTP
• Evaluation of congenital TTP  

Testing Pearls

• Imperative to order and draw samples for ADAMTS13 testing prior to initiation of plasma exchange.
• In congenital TTP(cTTP) expect severely low ADAMTS13 activity.
• In acquired TTP (aTTP) expect autoantibodies to ADAMTS13.

ADAMTS13 Test Algorithm

• ADAMTS13 Activity if <10% –> Inhibitor if < 0.4 IU –> Autoantibody²
  

ADAMTS13 Tests

• ADAMTS13 Activity:
  • Overview:
    • Sent to reference labs, first test performed in cascade.
    • Quantitative measure of ADAMTS13 enzymatic function/activity.
    • Deficiency defined as ADAMTS13 activity <10%; considered diagnostic of TTP
    • Activity assay measures degradation of full-length VWF or synthetic peptides of VWF by ADAMTS13 in plasma.
  • ADAMTS13 activity assays detecting VWF cleavage products:
    • Fluorescence resonance energy transfer (FRET)-based assays:
      • Most common methodology employed.
      • Calibrated against WHO International Standard ADAMTS13 plasma’ ‘normal’ 100%.
      • ADAMTS-13 activity determined by using a truncated, synthetic VWF peptide as substrate.
      • VWF substrate is labeled with a donor and acceptor fluorophore on the A2 domain of VWF.
      • A2 domain of vWF contains the cleavage site of ADAMTS13.
      • When VWF substrate is cleaved a change in fluorescence is observed.
      • Patient plasma is incubated with VWF substrate, ADAMTS13 activity is detected/measured based on changes in fluorescence measured.³
    • Enzyme-linked immunosorbent assay (ELISA)
    • Surface-enhanced laser desorption/ionization time-of-flight mass spectrometry (SELDI-TOF)
    • Electrophoresis
    • Reduced collagen binding
    • Reduced ristocetin induced platelet agglutination⁴ (1,4)
• ADAMTS13 Inhibitor:
  • Detects neutralizing autoantibodies in acquired TTP.
  • Bethesda-type assay; mixing study, inhibitor units (IU/mL).
  • Only detects functional inhibitory antibodies.⁵
• Testing for anti-ADAMTS13 autoantibodies:
  • When ADAMTS13 activity is deficient, next determine if an autoantibody to ADAMTS13 is present.
  • This distinguishes acquired TTP from congenital TTP.
  • Important for obstetric and pediatric population where cTTP rate is higher.
  • Auto antibody to ADAMTS13 can be inhibiting/neutralizing (most common) or non-neutralizing; both can be present.
  • Anti-ADAMS13 IgG antibodies are measured using one of the following methods:
    • Enzyme-linked immunosorbent assay (ELISA) – can detect both inhibitory and non-inhibitory IgG antibodies
    • Western blot (qualitative)⁶
• ADAMTS13 antigen 
  • ADAMTS13 antigen can be measured by ELISA but this is not yet part of routine clinical practice.⁷
• ADAMTS13 gene analysis
  • Sequence analysis of ADAMTS13 gene can be considered to confirm cTTP.
    
    

ADAMTS13 Assay Limits

• To maximize sensitivity, assays may be performed at pH 6 and use low ionic strength saline; these conditions are where ADAMTS13 activity is at its highest.
• These conditions may falsely increase ADAMTS13 activity, making it appear the patient has normal ADAMTS13 activity by lab testing.
• Dilutions; patient plasma may be diluted to minimize interference from other substances which may also dilute autoantibodies, limiting detection of autoantibodies.
• The combination of enhanced ADAMTS13 activity and possible antibody dilution could lead to a false negative interpretation.
• Despite the theoretical risk there has only been 1 case reported of a false negative.⁸

Turnaround Time & Practical Points

• Testing is often a send-out to a reference lab; due to time consuming and complex nature, thus turnaround time is days.
• Empiric treatment with plasma exchange (PLEX) should not be delayed for test results in suspected TTP.⁹

Stephanie Conrad, MD is a clinical pathologist specializing in Transfusion Medicine and Clinical Pathology at Beth Israel’s Lahey Hospital and Medical Center in Burlington, Massachusetts. She serves as the Medical Director of Clinical Chemistry and Molecular Diagnostics and as the Associate Medical Director of Transfusion Medicine. She is deeply passionate about medical education and is committed to teaching across diverse formats and settings. Her multifaceted role reflects her dedication to advancing laboratory medicine, fostering interdepartmental collaboration, and mentoring the next generation of healthcare professionals.