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Heparin Therapeutc Range

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.”


Comments (4)
Anticoagulant Therapy
Oct 14, 2015 10:47am

Thanks for the replies and
Thanks for the replies and the references, everyone. Very good info. For my upcoming lot change, I guess I’ll do a response curve using spiked plasma as usual and keep looking for a better alternative.

Oct 10, 2015 1:47am

Hi all,

Hi all,
I’m sure we had a similar question before, and I’m sure I added my two cents worth before, but a search on the website failed to find my previous (potentially imaginary) response. Anyway, I subscribe to the view that the science on this topic is weak, and unlikely to get any better, as current practice is so ingrained, and unlikely to encourage a proper clinical trial (especially in the age of the DOACs). A good reference is: Cuker A. Unfractionated heparin for the treatment of venous thromboembolism: best practices and areas of uncertainty. Semin Thromb Hemost. 2012;38:593–9. Essentially, there is limited evidence even in respect to the utility of monitoring UFH therapy, whether this be by anti-Xa or APTT, let alone which method is clinically the best. In regards to establishing a therapeutic range by correlating the two, Cuker showed in a cross-lab comparison in 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.) a great deal of variation between what labs would have established as their APTT range. As George indicates, the scatter is so high that different ranges will be identified using different sets of samples, even when using the same APTT reagent and instrument. However, spiking is also not great. The correlation is much sharper, but spiking tends to over call the heparin concentration compared to ex-vivo samples, so the resultant APTT range may also be less than ideal. But we have no alternative options available. So, monitor by anti-Xa if you like–but the choice of 0.3-0.7 U/mL is based on limited data. Or monitor by APTT if you like – but the therapeutic range will not be the most accurate range you will establish in your laboratory.

Oct 8, 2015 10:27am

Posted on behalf of Dr. Tim
Posted on behalf of Dr. Tim Exner: Hi George, thanks again for the interesting discussion. I enjoy speculating about some of these issues.
The APTT prolonging and anti-FXa effects of unfractionated heparin are known to disappear from circulation at different rates. This may account for some of the variability between these two assay methods in samples from patients with different clinical problems.
It suggests that the larger heparin molecules are inactivated or lost faster than the smaller, lower MW forms that act against FXa through antithrombin III. This loss of activity might also be through partial neutralization by platelet factor 4 (or similar) or in vivo enzyme activity. Thus it might be possible to mimic the effect of “in vivo” heparin by using a mix of unfractionated and low MW heparins. However if UFH becomes heterogeneous in vivo it may never be possible to get exact correlations between APTT and anti Xa testing. Cheers from Tom Exner, Haematex, Sydney, Australia.

Steve Duff
Oct 8, 2015 5:56am

To Geo and Fritsma Factor
To Geo and Fritsma Factor followers… greetings from the world of early stage venture capital . Keith, I had a ‘back-to-the-future’ flashback reading your post. The holy grail of ex vivo UFH calibrators, apparently, still eludes the industry. A big problem in standardization of UHF management. What an opportunity for an innovative IVD start-up 🙂
(Keith, Steve could be serious, he is CEO for a Canadian “incubator” agency.)

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