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Quick Question: Discard Tube?

Our December, 2019 Quick Question asks, “When collecting hemostasis specimens, when do you use a discard tube?” Results from 46 respondents:

A. Just before every blue-closure tube: 6 (13%)
B. Never, the discard tube is unnecessary: 2 (4%)
C. Just after an additive tube to avoid carryover: 8 (18%)
D. When collecting through an infusion set to avoid a short draw: 10 (22%)
E: C and D above 20 (43%)

As documented in CLSI Standard H21, Collection, Transport, and Processing of Blood Specimens for Testing Plasma-Based Coagulation Assays and Molecular Hemostasis Assays, 5th Edition. Adcock D, 2008, it is inappropriate to routinely collect a discard tube before collecting a hemostasis specimen. Prior to a series of studies conducted in the 1990s, for example, McGlasson DL, More L, Best HA, et al. Drawing specimens for coagulation; is a second tube necessary?” Clin Lab Sci 1999;12:137–9, “expert opinion” held that the venipuncture needle accumulated “tissue thromboplastin” during its passage into the vein. The thromboplastin was imagined to contaminate the specimen and cause spurious clot-based assay results, but all the 1990–2000 study results consistently illustrated the falsity of this notion.

There are exceptions. We often collect specimens through the tubing of an infusion set (“butterfly” needle) from patients who have difficult veins. Infusion sets introduce approximately 0,5 mL of residual tube air into the first collection tube. The 0.5 mL of air reduces the whole blood volume collected into a 3 mL hemostasis tube by 19%, thus introducing a potential error as the anticoagulant to whole blood ratio is increased sufficient to influnece clot-based results. Consequently, if a hemostasis tube is the first or only tube to be collected, it must be preceded by a discard tube. The discard tube may be a tube with no additive or a blue-closure tube indentical to the hemostasis collection tube.

Further, the hemostasis tube may not immediately follow an “additive” tube such as a broth-infused blood culture tube or tubes containing heparin, EDTA, or clot-promoting silica. Since according to “order of draw” specifications, a blood culture tube, when ordered, must be collected first, the hemostasis collection tube must follow a non-additive tube or a discard tube. See Bennett A, Fritsma GA, Ernst DJ. Quick Guide to Blood Collection, 3rd Edition, 2016. AACC Press, Washington DC.

Often emergency department and ICU personnel collect blood while placing an IV cannula. Also, depending on local policies, phlebotomists may collect blood from a vascular access device such as a peripherally inserted central catheter (PICC line). Though CLSI Standard 41 discourages this practice, it may be the only way to obtain a specimen without undue patient trauma. The line must first be flushed with 5 mL of saline (not heparin), then 5 mL of blood is collected and discarded before collecing the specimen to eliminate contaminants.

As in all laboratory specimen collections, physicians, phlebotomists, and laboratory scientists limit blood volumes to avoid hospital-induced anemia. The average volume of blood collected from a patient in ICU exceeds 60 mL per day.

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