In TTP how can ADAMTS13 activity deficiency be detected? When should the assay be performed? How can the cause of ADAMTS13 deficiency be identified?

• ADAMTS13 activity is tested using specialist functional assays which are conducted at reference centres:
  • Principle involves degradation of vWF substrate (either full length or small peptides)
  • The value is expressed as a % compared against activity from normal pooled plasma which is considered as 100%

• A blood sample for an ADAMTS13 assay should be obtained as soon as the diagnosis of TTP is suspected:
  • Helps to avoid false-negative results
  • However ADAMTS13 activity was usually still severely decreased within the first 3 days after PLEX initiation
• The treatment of TTP should be started before the ADAMTS13 activity assay results become available:
  • May take several days to get results

If ADAMTS-13 activity deficiency confirmed further testing can include:

• Identification of autoantibodies against ADAMTS-13
• Searching for an ADAMTS-13 inhibitor
• ADAMTS-13 gene sequencing