Cold activation in PT-INR samples - Myth or reality in the modern laboratory?

Guidelines on storage for samples intended for Prothrombin Time - International Normalized Ratio (PT-INR) analysis have changed over time, sometimes advising against cold storage due to presumed cold activation of the coagulation cascade. Previous studies on PT-INR storage have mainly been underpowered, performed in glass tubes, and not in a modern laboratory setting. In this study, we re-analyzed 1149 PT-INR samples, divided into low-level samples (PT-INR <1.3), and high-level samples (PT-INR 1.8-3.5) after 3 h of cold storage. We found no statistical difference for high-level samples but statistically higher PT-INR values in low-level samples. The differences were minor and not considered clinically relevant. No cold activation could be detected, as cold activation would have diminished PT-INR. These findings open the possibility of transporting and storing centrifuged PT-INR samples refrigerated. The higher PT-INR values in low-level samples after cold storage impede a mechanistic principle that needs to be further investigated.