From Lauren Ball: Do you suggest that most labs use a 4 hour rule for specimen acceptability? We offer PT, PTT, fibrinogen, D-dimer, and anti-Xa tests. PTTs and Anti-Xa tests may need to be performed within 2 hours but the manufacturer’s guidelines suggest that a specimen is good for up to 8 hours for tests such as fibrinogen and D-dimer. The CLSI standards are not available at our small lab, but I would love to make sure that our practices are up-to-standard. I would love to keep samples for the whole 8 hours, but I am unsure about the fibrinogens and D-dimers because I suspect that plasma needs to be removed from the cells. Is it safer to simply discard after 4 hours?
Hello, Lauren, this is a great question that affects many of our subscribers. The best article I have is Adcock-Funk DM, Lippi G, Favaloro EJ. Quality standards for sample processing, transportation, and storage in hemostasis testing. Semin Thrombos Hemostasis 2012; 38:575–85. Dr. Adcock is also one of the authors of CLSI H21-A5. The authors record the CLSI requirements, which document 4 hours’ ambient temperature whole blood stability for PT, PTT, and all factor assays; also D-dimer, antithrombin, and proteins C and S. The CLSI document limits PTT or anti-Xa assay specimens to 1 hour for unfractionated heparin or 4 hours for low molecular weight heparin. The 1-hour limitation reflects platelet factor 4 secretion, which rapidly neutralizes unfractionated heparin. Specimens should not be chilled or allowed to exceed customary room temperatures.
The paper also reports that publications other than the CLSI document extend most specimen stability to 24 hours except for specimens that are collected to measure unfractionated heparin levels by PTT or anti-Xa, still 1 hour; 18 hours for other PTT applications, and 4 hours for protein S activity, which is labile. These publications also insist the specimens be held at room temperature and should be stored upright. I regret I can’t post the Adcock article, as this would violate copyright laws. I hope your local librarian can obtain the article for you, it is packed with good information. Thank you, Geo.