Here’s a note from T. Spratley:
We have protime specimens delivered from offsites that are older than 4 hours. Are they acceptable?
I know some places accept them up to 24 hours old.
Hi, T. Spratley. Thank you for your question. Yes, in accordance with CLSI Guideline H21-A5, “Specimens for PT assays uncentrifuged or centrifuged with plasma remaining on top of the cells in an unopened tube kept at 18-24° C should be tested within 24 hours from time of specimen collection. Storage at 2-4° C may result in cold activation of factor VII and therefore alter PT results. If the patient is on both heparin and coumarin-based oral anticoagulant therapy, the PT may vary with time of storage of the specimen unless the PT reagent contains a heparin neutralizer.”
I hope this is helpful. Geo