Dr. Tony Tang asks, In a mixing study, how many seconds difference between the APTT of the test plasma+normal plasma (NP) incubated after mixing together, and the APTT of the test plasma and NP incubated separately before mixing, can suggest a positive result for factor VIII inhibitor? Are there any references? Thank you!
Hello, Dr. Tang, and thank you. We posted a Quick Question on this topic in 2015, please click here to see the summary. The survey results reveal that there is no consensus. Most of us use either the upper limit of the reference interval (RI) or 10% over the normal plasma value. I advocate for the latter approach, as this enables the operator to normalize on the NP value, be it an immediate mix or incubated mix. If you use the RI upper limit you must develop a separate limit for the incubated mix. A handful of us use the more precise Chang index or Rosner index. I’m attaching a PowerPoint PDF of a presentation I gave last week in Portland, Oregon on the subject of mixing studies that provides the Cang and Rosner formulae with examples and limits.
Also, for a general discussion of the approach to mixing studies, although it doesn’t address the issue of limits, see Ajzner E, Rogic D, Meijer P, et al, on behalf of the joint Working Group on Postanalytical Phase (WG-POST) of the European Federation of Clinical Chemistry and Laboratory Medicine (EFLM) and European Organisation for External Quality Assurance Providers in Laboratory Medicine (EQALM). An international study of how laboratories handle and evaluate patient samples after detecting an unexpected APTT prolongation. Clin Chem Lab Med 2015; 53: 1593–1603.
Here is my PowerPoint: /sites/default/files/mix_studs_6pp_5-7-16.pdf