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Establishing PTT Rx Range

From “Kkehoe“: Recognizing that the preferred method for establishing a partial thromboplastin time (PTT) therapeutic range for unfractionated heparin (UFH) is plotting the PTT against the anti-Xa, is there an alternative for labs that don’t perform anti-Xa testing? Are you aware of any commercial kits that can be purchased and used? We currently purchase a normal donor kit from Precision BioLogic Inc for validating our normal reference range when changing reagent lot numbers.

Hello, and thank you for your question. Regrettably, there is no alternative to plotting PTT against anti-Xa, often called the ex vivo “Brill-Edwards curve,” for establishing the PTT therapeutic range. This creates difficulty for labs that don’t perform the chromogenic anti-Xa heparin assay routinely and whose volume of UFH patient testing is low. Spiking normal plasma doesn’t work, as spiked curves don’t match up with ex vivo curves. I’ve had a couple of conversations with Stephen Duff, Co-CEO of Precision BioLogic about kits and he points out the practical limitations to providing a standardized commercial set of UFH plasmas. A distributor would need several large-volume sources for UFH plasma, implying the need for plasmapheresis of patients on heparin, an unlikely scenario. That’s why you won’t find commercial plasma sets. High-volume labs that collect and aliquot patient plasmas for internal validations are reluctant to share aliquots outside their health systems as the products must meet good laboratory practice standards to avoid liability.

Given these concerns, experts advise low-volume labs to identify, aliquot, and freeze candidate UFH specimens daily until at least 50 (plus 20 normal plasma aliquots) are available to meet the demands of the Brill-Edwards procedure and to continue the process to maintain an adequate inventory. Aliquots may be shipped to reference labs where the chromogenic anti-Xa heparin assay is available. The assay need be performed only once per patient specimen.

But really, why do you want to continue monitoring UFH with the PTT? The anti-Xa assay is readily available on counter-top coagulometers and in manual chromogenic microtiter formats, and acute care labs now need the assay to test for low molecular weight heparin (LMWH, enoxaparin), the synthetic pentasaccharide fondaparinux, and now the oral direct anti-Xa antithrombotics rivaroxaban and apixaban. Further, the PTT is notorious for producing spuriously prolonged results when a lupus anticoagulant or specific inhibitor is present or when there is a coagulopathy secondary to liver disease or vitamin K deficiency. Even more troublesome, the PTT is insensitive to UFH if the factor VIII activity is elevated or antithrombin level is reduced, both are consequences of acute inflammation. A glimpse of the degree of scatter seen in a routine Brill-Edwards curve may be enough to convince you of the PTT’s unreliability, as would a comparison of assay results on random UFH samples.

To conclude, rather than going to the effort of setting up the Brill-Edwards procedure, why not investigate a means for providing the more precise and accurate chromogenic anti-Xa heparin assay?

Here is a link to a January, 2011 discussion of the anti-Xa heparin assay, another to an August 2, 2012 discussion with several references, and a link to our audio module that describes the ex vivo Brill-Edwards curve protocol in detail. I hope these are helpful.

Comments (1)
Anticoagulant Therapy
Herb Crown
Aug 17, 2012 6:16am

Hi Kkehoe. George is correct, the alternative to performing
Hi Kkehoe. George is correct, the alternative to performing the anti Xa assay in your lab is to store samples in a -70 freezer and send to a reference laboratory for testing. This leads me to a much broader answer to your question and also prompted by some work we performed for a client a while back.

We were perplexed when we plotted the anti Xa values against the aPTT seconds. What we saw were elevated aPTTs and minimal anti Xa values significantly beyond what we were expecting. When we assigned a therapeutic range of 0.3-0.7 to the clients aPTTs, the therapeutic range of the aPTT was much higher than what we thought we should be seeing.

We investigated the situation carefully and we found the samples were left sitting in a rack on the bench until the end of the shift and then they were poured off and frozen. This allowed the platelets to release PF4 which neutralized the heparin. One would think that the proper handling of heparin monitoring tubes would be common knowledge.

The take home message here is to carefully handle all heparin monitoring specimens and test them promptly. This is important whether a lab is using the anti Xa or aPTT to monitor heparin therapy.

Herb Crown
St. Louis Hospital Coagulation Reference Lab

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