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Reasoning and requirements.

I would like to comment on Mr. [Roy] Midyett's article titled "Under the blue top: coags, corrections, and crits" in the February [2005] issue. (1) I agree with his rationale for not adjusting the citrate concentrations to the patient's hematocrit when >55%. I would also submit that it would not be necessary for low hematocrits either. The notion for this practice was an assumption that altered plasma volume would affect the ability of a fixed concentration of citrate in the coagulation tubes to chelate calcium in the patient's plasma. The effectiveness of citrate to prevent coagulation is based on the number of moles of citrate relative to the moles of divalent cations in the patient's blood and is independent of the amount of water in the system (either plasma or additive water). Based on the citrate concentration in evacuated tubes, and even at extreme levels of divalent cations, there is approximately double the amount of citrate in the tube to bind all of the divalent cations in the blood. Further, the water in the system has no impact on PT/INR or aPTT results that are determined in seconds, but may slightly influence factors measured in concentration (e.g., fibrinogen). However, the citrate solution mixes faster than a dry additive enhancing anticoagulation.


Adjusting the blood level in the tube as described by the author for patients with high or low hematocrits is questionable for several reasons. First, just as the amount of water is independent of the anticoagulation mechanism, changes in the amount of blood or plasma are negligible to coagulation in the reaction. Standard aPTT reagents replace a large excess of calcium (at least 6 times the amount required) to clot the plasma in the reaction. Secondly, adjusting the headspace in the tube by altering blood volume has been shown to influence aPTT values, particularly in heparinized patients using 0.129M citrate tubes. (2) There was less of an effect with the 0.109M citrate tubes which are currently recommended by CLSI (formerly NCCLS). Lastly, there is a biohazard exposure risk in opening the tube and transferring blood from a syringe.

These points can be demonstrated by simple experiments and have been shown by Pai, et al. (3) Perhaps coagulation tubes will someday follow other anticoagulated tubes and become dry additive tubes.

--Valerie Bush, PhD, Director

Clinical Laboratory & Point-of-Care Testing

Bassett Healthcare, Cooperstown, NY


1. Midyett R. Under the blue top: coags, corrections, and crits. MLO. February 2005;20-22.

2. Adcock DM, Kressin DC, Marlar RA. Minimum specimen volume requirements for routine coagulation testing, Dependence on citrate concentration. Am J Clin Pathol. 1998; 109:595-599.

3. Pai SH, Michalaros K. Effect of sample volume on coagulation tests. Lab Med. 1990;6:371-373.

Roy Midyett's reply: I agree with Dr. Bush's well-written comments. My description of a means to adjust the tube was only included because some procedure is required by CAP. Of course, I do not agree with the reasoning behind the requirement.

MLO welcomes letters to the editor. We ask that you include a phone number for verification. While we prefer to publish the writer's name, we will publish a letter with "name withheld by request," but our editorial staff must have the writer's name confirmed for our files. MLO reserves the right to edit any letter for style and length.
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Title Annotation:Readers respond
Publication:Medical Laboratory Observer
Article Type:Letter to the Editor
Date:Oct 1, 2005
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