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.
Dr. King and George please
Dr. King and George please see the following reference for answers to your questions.
McGlasson DL, More LE, Best HA, Norris WL, Doe RH, Ray H. Drawing specimens for coagulation testing: is a second tube necessary? Clin Lab Sci 1999:12. We took on this problem 20 years ago. We used three different reagent/instrument combinations on over 100 specimens for PT/INR, PTT and fibrinogen assays. We found no clinical and statistical differences for either the first or second draw tubes on any of the systems or between the 3 testing sites. Also we were never able to find a predicate document that had done a definitive study to prove there was a difference. We searched all the way back to 1948 to find this mystical “tissue thromboplastin” contamination that would cause a difference in using a discard tube or not.
Another study also came up with similar conclusions. Smock KJ, Crist RA, Hansen SJ, Rodgers GM, Lehman CM. Discard tubes are not necessary when drawing samples for specialized coagulation testing. Blood Coagul Fibrinolysis 2010;21:279–82.
Comparison of clot-based and chromogenic assay for the determination of protein C activity. Blood Coagul Fibrinolysis. 2019.
Review: preanalytical investigations of phlebotomy: methodological aspects, pitfalls and recommendations. Biochem Med (Zagreb). 2017.
Review: Pre-analytical issues in the haemostasis laboratory: guidance for the clinical laboratories. Thromb J. 2016.
I hope this is helpful. Dave McGlasson