From Madan Verma:
Why are we allowed to add on a prothrombin time (PT) assay even 24 hours after collection? I was trying to find the factors’ half lives in vivo and wondering that factor VII has the shortest half life. Please explain. Can you please cite some literature regarding factor half lives? At http://uabcoag.net I found a summary, but it does not indicate these are in vivo or in vitro. Many-many thanks.
Hello, Madan Verma, and thank you for your question. The table on http://uabcoag.netis an adaptation of Table 45, page 632 of Fritsma MG, Fritsma GA. Chapter 40: Normal hemostasis and coagulation. In Rodak BF, Fritsma GA, Keohane EM. Hematology: Clinical Principles and Applications, 4th Edition, Elsevier Saunders, St. Louis, 2011. Factor VII’s in vivo plasma half-life is 6 hours, perhaps owing to its tendency to become activated and consumed as it binds tissue factor. In vitro, however, factor VII is stable, accounting for the stability of the PT, as documented in Adcock D, Kressin D, Marlar RA. The effect of time and temperature variables on routine coagulation tests. Blood Coagul Fibrinolysis 1998;9: 463-470. The Adcock publication is referenced in Clinical and Laboratory Standards Institute: Collection, transport, and processing of blood specimens for testing plasma-based coagulation assays: approved guideline, ed 5, CLSI Document H21-A5, Wayne, PA, 2008.
Contrast the PT to the partial thromboplastin time (PTT), which Adcock illustrates is stable for only four hours in vitro. The PTT depends upon an intact intrinsic coagulation pathway, which includes factor VIII, whose in vivo half-life is 12 hours but whose in vitroactivity deteriorates by approximately 10% per hour. Factor VIII’s deterioration accounts for the progressive lengthening of the PTT during storage.
By the way, specimens for PT and PTT should be stored at room temperature, 18°C–24°C. Refrigeration activates factor VII and platelets and causes large VWF multimers to precipitate, whereas storage above 24°C hastens factor VIII deterioration. Geo