Coagulation laboratory tests
Reviewer: Jeremy Parsons, M.D. (see Reviewers page)
Revised: 10 February 2013, last major update November 2012
Copyright: (c) 2002-2013, PathologyOutlines.com, Inc.
● 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
End of Coagulation > Coagulation laboratory tests > Mixing studies
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