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Failing to Mix Coagulation Specimens

A participant in Medlab-L has witnessed phlebotomists who do not mix the blue top tubes for coagulation testing as they are drawn. They draw and label the tubes, laying them down as they are collected without mixing. Though not for coagulation testing, I have had the same experience when having blood collected for a CBC for my annual physical exam. I’ve attached a 2004 Chest abstract describing a study that indicates coagulation specimen mixing is not essential.


As one who observes the number of clotted specimens that arrive in the coagulation laboratory, I was surprised by the conclusions of this study, and passed them on to Dr. Larry Brace, lab director at Edwards Hospital in Naperville, Illinois and to Dave McGlasson, hemostasis researcher at Wilford Hall USAF Medical Center, Lackland AFB, Texas.

Here is Dave’s comment: I question the number of specimens. I would like to have seen a power analysis to see if they had a high enough number of subjects to make it statistically meaningful.

Here is a paraphrase of Dr. Brace’s comments:

1. This study was performed on normal, healthy volunteers and patients on oral anticoagulant therapy. I assume these are all adults and not pediatric patients or infants.

2. Although I am a little surprised by their results, I don’t have any real problems with the “fill volume” part of this study. Personally, I have not seen problems with fill volume until the volume is less than about 70%.

 3. I also assume that the samples for this study were obtained by venipuncture (since the study subjects appear to be adults). Thus sample acquisition technique can have an impact. If the sample was drawn by a Vacutainer system, some level of mixing occurs during sample acquisition. However, samples from pediatric patients and infants are often obtained from lines. If the sample is collected in a syringe and then gently distributed into the tubes, then recapped without mixing, this is a very different situation. If we assume that the sample then sits on a phlebotomy tray for some period of time before it is sent to the lab, the results could be very different.

4. Note that all samples in the study were mixed (1-5 times) with a max wait time of 4 minutes. The samples were drawn in siliconized glass collection tubes which should provide a non-thrombogenic surface. I would not expect a cleanly drawn sample to become activated in that time period. However, pediatric collection tubes are often used in patients expected to be difficult draws, so if the draw is less than “free flow”, the sample may be activated during the phlebotomy procedure. This is not the population used in this study.

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