Reasoning and requirements.
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.
|Printer friendly Cite/link Email Feedback|
|Title Annotation:||Readers respond|
|Publication:||Medical Laboratory Observer|
|Article Type:||Letter to the Editor|
|Date:||Oct 1, 2005|
|Previous Article:||Blood, sweat, and fears.|
|Next Article:||Noah's Ark in LA.|