Coagulation laboratory tests
Mixing studies

Topic Completed: 1 November 2012

Minor changes: 2 May 2020

Revised: 7 January 2019, last major update November 2012

Copyright: (c) 2002-2016,, Inc.

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Definition / general | Laboratory
Cite this page: Parsons J. Mixing studies. website. Accessed June 7th, 2020.
Definition / general
  • Used to determine if etiology of prolonged PT or PTT is due to a factor deficiency or an inhibitor
  • Laboratory should be notified of presence of therapeutic anticoagulant
  • Add heparinase to remove any heparin present (or perform thrombin time to check for even small amounts of heparin)
  • Experienced laboratories may omit mixing studies and move to more definitive testing, based on patient presentation
  • Although understanding the theory of mixing studies is educational, interpreting the actual data is not always straightforward
  • Add patient plasma to equal volume of normal plasma and repeat PTT
  • Various incubation times are usually assessed; the most rigorous testing uses 0, 30, 60 and 120 minutes; less rigorous testing omits the 30 and 120 minute incubations, although detecting some inhibitors requires the 120 minute incubation

  • Prolonged PTT becomes normal after mixing study and stays normal after 2 hours: indicates factor deficiency; perform assays for factors VIII, IX, XI and XII; if PT also prolonged, consider assays for common pathway factors

  • Prolonged PTT remains prolonged after mixing study: indicates inhibitor; most common is lupus anticoagulant (Thromb Res 2007;119:369); also therapeutic anticoagulant; rarely due to inhibitors to factors IX, XI or XII

  • Prolonged PTT becomes normal after mixing study, but prolonged after 1 - 2 hour incubation: indicates factor VIII inhibitor, rarely factor V inhibitor
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