George, I would like to get your thoughts and references if possible about the "proper" amount of blood to discard before drawing samples for coagulation testing. In the current environment of minimizing blood waste and limiting volume of blood draws, we are reviewing our current policy. Our current SOP states to draw and discard 20 cc of blood before coagulation tube draws. We cannot find documentation to support that amount and would like to reduce that amount with some justification. If you have any insights, we would greatly appreciate it. Thanks much.
Bruce King, M.D.
Director of Hematology laboratory
Hello, Dr. King, and thank you for your question, which raises an important issue. Please pardon my self-reference, but if convenient, go to Fritsma GA, Laboratory Evaluation of Hemostasis in Keohane EM, Otto CM, Walenga JM. Rodak's Hematology, 6th Edition, Elsevier, 2020. pages 765-70. This provides referenced guidelines for blood discard, order of draw, and specimen management. In short, discarding blood prior to a coagulation specimen draw is contraindicated, and in fact, using the "order of draw" standards, the blue-enclosure tube comes first. Exceptions including collecting through a "butterfly" infusion set, when a discard tube corrects for approximately 0.5 mL air volume in the tubing; from a vascular access device that must be first flushed with 5 mL of saline followed by a discard volume of 5 mL; or if collecting after an evacuated tube with an additive, such as a blood culture tube. In the latter circumstance, collect one non-additive or blue-closure discard tube just after the blood culture tube and just ahead of the coag tube. I hope this is helpful. Watch here for additional comments.
Added September 9, 2019: Thanks to Dave McGlasson for the various citations provided in his comment. The article by Dr. Smock (ARUP) is particularly useful as it documents the discard tube contraindication for special coagulation tests as well as the PT and PTT. As Dave writes, the concept of tissue thromboplastin contamination is an expert opinion from the "data-free" zone.