From Keith DeHaas, Southwestern Vermont Healthcare: Hi George I am currently enrolled in BLD 835 and am thoroughly enjoying the class. I have a question about sourcing specimens of plasma with unfractionated heparin for use in ex vivo Xa studies. We are a small hospital that does not use much heparin and getting enough heparinized plasma specimens from patients is difficult. Because of that, historically we have done heparin response curves using plasma spiked with heparin in vitro. This fulfills CAP and CLIA requirements but according to the studies I’ve been reading does not provide for an accurate UFH therapeutic range (HTR). I want to do an ex vivo study which seems to be the best method for determining the HTR, however I am having trouble locating a source for specimens. I am wondering if you know of a vendor that could provide 20 specimens of patient plasma with known concentrations of UFH between 0.1 and 0.7 U/mL?
Hi, Keith, thanks for you question. Keith mentions BLD 835, which is the graduate hemostasis and resource management course in the Michigan State University graduate Biomedical Laboratory Science curriculum.
Keith, I’m sorry to tell you no distributor has found a way to develop a heparinized plasma set, primarily because UFH patients, who are generally inpatients and are presumed to be quite ill, are inaccessible for large-volume plasmapheresis. Distributors would, of course, need large enough volumes of plasma to make a sustainable lot of calibrators. The alternative of administering UFH to healthy volunteers generates unacceptable risk and won’t be approved by most institutional review boards. Worse, most large-scale institutions that have an adequate supply of calibrators are constrained from sharing because their risk management folks don’t allow for distribution of plasmas that can’t be certified as free from HBV, HCV, and HIV. Sharing may occur only if your institution is in a business alliance with a larger institution.
This issue has remained a limitation for labs at community and acute care institutions, and the only advice available is to save, aliquot, and freeze the occasional specimens from patients on UFH whose PTs are normal. Hopefully, over the course of a few weeks or months enough samples come along to make for a reasonable collection. One piece of good news, Marlar RA, Gausman J. The optimum number and type of plasma samples necessary for an accurate activated partial thromboplastin time-based heparin therapeutic range. Arch Pathol Lab Med 2013;137:77–82 demonstrates that you can generate a relaible target range using only 20 samples provided you have no more than 10% repeat specimens from the same patient.
One additional piece of information, in our last BLD 835 unit we discuss anticoagulant monitoring. I will bring in a recent publication, Price EA, Jin J, Nguyen H, et al. Discordant aPTT and anti-Xa values and outcomes in hospitalized patients treated with intravenous unfractionad heparin. Annals Pharmacotherapy 2013;47:151–8. The authors contend the PTT and the anti-Xa are measuring different aspects of anticoagulation and their discordance calls the ex vivo calibration process into question. They reference Cuker A, Ptashkin B, Konkle BA, et al. Interlaboratory agreement in the monitoring of unfractionated heparin using the anti-factor Xa-correlated activated partial thromboplastin time. J Thromb Haemost 2009;7:80–6. Unfortunately, they offer no alternative.
The CAP may frown on in vitro “spiking” of normal plasmas as the curve tends to flatten out at the higher ranges. However, the unavoidable scatter associated with the ex vivo “Brill-Edwards” approach probably introduces at least as great an error component as spiking.
There may be more to this issue. I’ll contact several of our technical advisors to see if they have something to add, so please “watch this space.”