From Heidi Hobbs: Adjusting anticoagulant for hematocrits greater than 55% has been a hot topic lately. It is a challenge to ensure this is being done in all cases and we have some great things in the works.
While doing some research, I ran across one lab that has a simplified adjustment table, where patients with HCT 56–68% had one adjustment and patients with HCTs of 69% or more had another, but I can't verify how they determined their cutoffs. Also, a recent CAP education activity addressed this topic and had a statement pulled from the textbook Kitchen S, Olson JD, Preston FE. Quality in Laboratory Hemostasis and Thrombosis. 2nd ed . that says, "In samples with a HCT between 55% and 65%, a simplified method to overcome citrate effect is to remove 0.1 mL of the citrate anticoagulant from a standard 5 mL evacuated tube with a 4.5 mL blood draw volume and not perform the calculations. This will account for most of the patients with a high HCT , since very few patients will have a HCT that is greater than 65%. For those patients with HCT of greater than 65%, the above formula should be utilized to determine the correct anticoagulant volume."
This leads to further questions. Why is only removing 0.1 mL sufficient, when based on the calculation that isn't even quite sufficient for a HCT of 55%, let alone a HCT of 65%. If you were going to cover up to 65% you would need to remove 0.2 mL of anticoagulant from a 4.5 mL tube. Given the same logic as not making adjustments for low hematocrits, would you expect an adjustment for a HCT of 65% or even up to 70% have an insignificant effect on a HCT of 55%?
I appreciate any insight you have on this matter. Using tubes with a standard anticoagulant adjustment would be very helpful!! If we did go that direction, what sort of validation would need to be done, or is referencing the literature sufficient? Thanks!
If you pardon a self-reference, here is the relevant material from Fritsma GA, Laboratory evaulation of hemostasis. In Keohane EM, Otto CN, Walenga JM. Rodak's Hematology; Clinical Principles and Applications, Elsevier, 2020.
The 9:1 blood-to-anticoagulant ratio is effective, provided the patient’s hematocrit is 55% or below. In polycythemia the decrease in plasma volume relative to whole blood unacceptably raises the anticoagulant-to-plasma ratio, causing falsely prolonged clot-based coagulation test results. The phlebotomist must prepare specially marked tubes with relatively reduced anticoagulant volumes for collection of blood from a patient whose hematocrit is known to be 55% or higher. The anticoagulant volume may be computed by using this formula: C (1.85X10–3 ) (100–HCT ) V, where C is the volume of sodium citrate in mL, V is volume of whole blood-sodium citrate solution in mL, and HCT is the hematocrit in %.
For example, to collect 3 mL of blood/anticoagulant mixture from a patient who has a hematocrit of 65%, calculate the sodium citrate volume as follows: C (1.85X10–3) (100–65) 3.0mL = 0.19 mL of 3.2% sodium citrate. Remove the stopper from a 3-mL blue closure collection tube, pipette and discard 0.11 mL from the 0.3 mL of anticoagulant, leaving 0.19 mL. Collect blood in a syringe and transfer 2.81 (2.8) mL of blood to the tube, replace the stopper, and immediately mix by gently inverting at least three times.
Alternatively the phlebotomist can prepare for collection of 10 mL of blood and anticoagulant solution in a 12-mL centrifuge tube as follows: C (1.85X10–3 ) (100–65) 10.0 mL = 0.64 mL of sodium citrate. In this instance 0.64 mL of sodium citrate is pipetted into the tube, and 9.36 (9.4) mL of whole blood is transferred from the collection syringe.
This material is referenced to CLSI Standard H21-A5, H21; Collection, Transport, and Processing of Blood Specimens for Testing Plasma-Based Coagulation Assays and Molecular Hemostasis Assays, 5th Edition 2009.
I [Geo] admit that even when we were writing the Rodak instructions I had misgivings about how cumbersome the process must be. At the minimum, the method is likely to require a re-draw unless the phlebotomist somehow knew the patient's HCT in advance. Also, since most of us use 3 mL plastic vacuum tubes, how accurately can someone withdraw 0.11 mL of citrate? Or if we chose to prepare a 10 mL centrifuge tube, the phlebotomist would have to prepare a fresh tube for each patient collection. It seems like the CAP recommendation could solve our problems, provided the anticoagulant volume were validated against the CLSI standard's procedure.
So, as you can see, I don't have an answer, but have reached out to authors of Quality in Laboratory Hemostasis and Thrombosis. 2nd ed . and to Dennis Ernst, president of the Center for Phlebotomy Education and chair of the CLSI committee that prepared CLSI GP41: Collection of Diagnostic Venous Blood Specimens, 7th Edition. 2017. Meanwhile, I invite comments from our participants to learn how they've dealt with these specimens. Click on Comments below or email me at firstname.lastname@example.org.