From Kim Kinney at Indiana University (formerly Clarian) Health Center:
Hi George, Even though I am not in coag much any more I still get a ton of questions. One of our TC’s was questioning our storage of PT/INR specimens. We follow the current recommendations of room temperature for 24 hours. We do not accept refrigerated whole blood or spun plasma refrigerated. We do accept frozen plasma. He wanted to know why you can freeze plasma without activation of VII but refrigeration may activate VII and therefore, shorten your PT. I was not sure of an answer other than frozen is a quick freeze. Can you help?
Hi, Kim, and thank you for your question. We miss you! As documented in CLSI H21-A5, specimen chilling activates factor VII and also causes large von Willebrand factor multimers to precipitate upon centrifugation. These effects are avoided if the specimen is frozen rapidly, for example, by placing platelet free plasma in a –70°C freezer, and thawed rapidly, typically in a 37°C water bath. Prolonged chilling over several minutes, such as transporting on ice or storing in the refrigerator, or repeated freeze-thaw cycles, create the adverse effects that may unpredictably shorten or prolong prothrombin time (PT) or partial thromboplastin time (PTT) results. I hope this helps. Geo.