Hello George, I just completed the Quick Question concerning PTT mixing studies. Our PTT mixing study consists of two parts, a control of NPP and factor VIII deficient plasma tested immediately upon preparation and after 60 minutes incubation at 37 degrees. The patient is also mixed 50/50 with NPP and tested immediately upon preparation and after 60 minutes incubation.
We established a normal reference range for these studies by testing a group of normal donors from our lab and surrounding offices. All donors were lupus anticoagulant negative by Sta-Clot LA, DRVVT and Dilute PT index. All donors had normal PTs, PTTs and thrombin times.
We always perform these mixing studies at “zero time” and after 60 minutes. Without the additional incubation step, a physician would be unable to determine the presence of a factor inhibitor.
St. Louis University Hospital Coagulation Laboratory
(Herb’s comment got misplaced to an unrelated post, so I am re-posting it here)
Herb, it looks like your approach provides better standardization than most of us use. Could you go into more detail about your use of factor VIII deficient plasma/NPP mix? Thank you.
More from Herb:
Hi George, I am happy to further explain our mixing study thought processes.
The rationale for using factor VIII deficient plasma is to simulate a worse case scenario involving a hemophiliac. Of course this strategy is not without its problems, first and foremost, it doesn’t account for a lupus anticoagulant or factor inhibitor, but it pushes us to think about low factor VIII patients.
Here is where it gets kind of interesting. The process of setting our “normal range” for patient studies consists of mixing our previously tested normal donor plasmas with factor VIII deficient plasma thereby simulating the same worse case scenario mentioned above. We then tested those “normal range” test plasmas at zero time and after 60 minutes 37 degree incubation. The reference range was then established at 2 SD. Establishing the normal range in this manner involves us thinking that the patient (the factor VIII deficient plasma) is being mixed with normal pool plasma (the normal donor plasma) which is somewhat counter intuitive, but remember, this part of the process is only for establishing the normal reference range. As I always tell my students, you have to wrestle with it to get it.
Let me add that I am not the original lab tech that performed this work. David Chance, under the direction of Dr Heinrich Joist, performed this work a number of years ago in our laboratory. I should be considered a secondary source concerning this process.
Regards, Herb Crown
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