Adjusting coag tubes for high hematocrits

[Editor's note: the following question was originally posted on The Fritsma Factor, an online coagulation blog. We responded to the question and are grateful to George Fritsma for permission to reproduce the discussion here.]

Question: Do you have any recommendations on how the sample should be collected for coag testing if the patient’s hematocrit is above 55 percent? We have the formula in our procedure for adjusting the amount of anticoagulant in the tube to obtain the 9:1 ratio of blood to anticoagulant, but once we break the vacuum seal on the vacutainer tube to adjust the anticoagulant volume it cannot be filled without using a syringe. Since the risk of the sample clotting or hemolyzing is greater when using a syringe to draw the blood, we don’t recommend that either. Any suggestions?

Fritsma Factor Response: This is indeed a tricky maneuver. The CLSI coag standard (H21) has a chart that tells you how much sodium citrate is required based on the patient's hematocrit, and states "Place the volume (of anticoagulant) in a collection tube and add blood up to the required total volume." This seems to require knowledge of the patient's hematocrit in advance of the coag collection, and the use of a syringe to transfer the blood to the "doctored" citrate tube. That's not the ideal solution, but I'm afraid there is no ideal solution.

Our follow-up response: On page 20 in the February 2005 issue of MLO, an article titled "Under the Blue Top: Coags, Corrections, and Crits" explains that the cap should be removed and the tube filled 8 mm from the top edge of the tube using a syringe. The tube should then be recapped with a new cap because the overfill will prevent re-fitting the original stopper.  The author suggests hematocrit-based tinkering is not worth the effort and may cause more problems than it solves.
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