From Dennis Ernst, Center for Phlebotomy Education, Inc: I am chairing the CLSIDocument Development Committee on the venipuncture standard revision. At today’s web meeting the question came up about the necessity for discard tubes on heparinized patients. It’s been widely reported that discard tubes are not necessary, and haven’t been for some time. However, it came to our attention today that the passage in the CLSI coag standard (H21) that states discard tubes are not necessary for partial thromboplastin time assays (PTTs, APTTs) and prothrombin times (PTs), for that matter cites four studies, all of which have been conducted on patients who are not on heparin therapy. Here are the citations:
- Gottfried EL, Adachi MM. Prothrombin time and activated partial thromboplastin time can be performed on the first tube. Am J Clin Pathol. 1997;107:681–3.
- Adcock DM, Kressin DC, Marlar RA. Are discard tubes necessary in coagulation studies? Lab Med. 1997;28:530–3.
- Yawn B, Loge C, Dale J. Prothrombin time, one tube or two. Am J Clin Pathol. 1996;105:794–7.
- Bamberg R, Cottle J, Williams J. Effect of drawing a discard tube on PT and APTT results in healthy adults. Clin Lab Sci. 2003;16:16-19.
Are you aware of any study that had attempted to establish discard tubes may not be necessary for heparinized patients as well? In your opinion, is it safe to assume it shouldn’t matter if they are heparinized or not?
Hello, Dennis, and thanks for your question. While I know of no studies that clearly eliminate the need for discard tubes in heparinized patients, I can think of no reason why heparin, in contrast to Coumadin, other anticoagulants, or other drugs, should require a discard tube on the strength of its biochemical nature or viscosity. The original idea for a discard tube was based on expert opinion that theorized the needle could be contaminated by an excess of tissue thromboplastin while passing into the vein, a theory that failed on the basis of empirical evidence as presented in the references you cite. I know of no such theory involving heparin. I’ve found one study that uses Coumadin patients, among others: Raijmakers MT, Menting CH, Vader HL, van der Graaf F.Collection of blood specimens by venipuncture for plasma-based coagulation assays: necessity of a discard tube. Am J Clin Pathol. 2010;133:331-5. It confirms there is no difference in PT results from the first and second specimen of the specimens containing Coumadin. Perhaps these findings could be generalized to heparin and the PTT.
It may be that the committee is thinking of the common nursing practice of using heparin to flush vascular access devices, or the use of heparin-impregnated devices. These situations generate the need to collect and discard a volume of blood roughly equivalent to a multiple of the internal volume of the device when collecting through the device. I believe this is a legitimate reason to use a discard tube.
I invite comments from our contributors who may have seen published studies more specific to heparin. Thank you. Geo