This question arrived as a comment from “skierktc;” I have reproduced it as a post here:
I am preparing to update our laboratory’s procedure for prothrombin time (protime, PT) and partial thromboplastin time (PTT) mixing studies and wonder if anyone screens additionally with the thrombin clotting time (TCT, TT) or the TT plus protamine sulfate to rule out heparin in the specimen as the source of
prolongation. If yes, what is their policy for continuation of the study? We have encountered physician insistence even when heparin effect is evident.
Hello, and thank you for your question. Yes, when you encounter a prolonged PTT that is not corrected by the normal platelet free plasma in the first step of your mixing study, perform a thrombin time to determine if the patient is receiving therapeutic unfractionated (standard) heparin. Heparin will prolong the thrombin time from its normal mean, which is often around 18 or 19 seconds, to 40 seconds or longer. You can confirm using protamine sulfate, which neutralizes heparin, however you are reasonably safe to conclude that heparin is the culprit without taking this step. You may also confirm using the anti-Xa heparin assay.
Once you have concluded heparin is present, treat the specimen with Hepsorb or Hepzyme, then repeat the mixing study. The same rules may apply to a PT mixing study, though they are rarely performed and heparin has relatively little effect upon the PT.
You also asked, “What commercial controls do you use for the pathogenic control for both PT and PTT mixing studies?
Generally, no normal nor any “positive” or pathological controls are employed in mixing studies, as the results are compared directly to the original PTT prolongation.
I hope this is helpful. You may also review our Audio Modules 7 and 8 for details on mixing studies and lupus anticoagulant testing. Geo.