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
These suggested references
These suggested references will answer most questions related to mixing studies/inhibitors: Kershaw G, Orellana D. Mixing tests: diagnostic aides in the investigation of prolonged prothrombin times and activated partial thromboplastin times. Semin Thromb Hemost. 2013;39:283–90. Also Kershaw G, Jayakodi D, Dunkley S. Laboratory identification of factor inhibitors: the perspective of a large tertiary hemophilia center. Semin Thromb Hemost. 2009;35:760–8.
These suggested references
These suggested references will answer most questions related to mixing studies/inhibitors: Kershaw G, Orellana D. Mixing tests: diagnostic aides in the investigation of prolonged prothrombin times and activated partial thromboplastin times. Semin Thromb Hemost. 2013;39:283–90; and Kershaw G, Jayakodi D, Dunkley S. Laboratory identification of factor inhibitors: the perspective of a large tertiary hemophilia center. Semin Thromb Hemost. 2009;35:760-8.
Thank you George. However to
Thank you George. However to my knowledge, to screen for factor VIII inhibitor, the APTT of incubated 1:1 mixed plasma should be compared to APTT of patient plasma incubated separately and then mixed with normal plasma, not to APTT of incubated normal plasma, because it’s not easy to distinguish factor VIII inhibitor and immediate acting anticoagulant (such as LAC) through the latter in some cases, is that so?
Response: please refer to Dr. Favaloro’s post and in particular to Kershaw G, Orellana D. Mixing tests: diagnostic aides in the investigation of prolonged prothrombin times and activated partial thromboplastin times. Semin Thromb Hemost. 2013;39:283–90. When using the Rosner index or Chang percentage, the mix result is compared to the difference between the patient plasma and the normal pooled plasma. In these instances the patient plasma is incubated for two hours along with the normal pooled plasma and the mix, and the same formula is applied. From the publication, it appears that the key to generating a favorable predictive index is to employ a series of positive plasmas and normal plasmas to carefully establish the most discriminatory decision limit.