A question from Joe Lamb:
I’m sure this never happens at other institutions, but occasionally here a patient is started on heparin therapy before a baseline PTT specimen is collected. Could you collect a specimen, absorb with heparinase and use it for the baseline PTT time?
This is a great question, as I am sure this happens everywhere. In fact, I’d be willing to bet a significant percentage of people who are started on standard, unfractionated heparin never have a baseline PTT collected.
The purpose for the baseline PTT is to ensure there is nothing in the plasma that factitiously prolongs the results, such as a lupus anticoagulant, coagulation factor inhibitor or coagulopathy. In theory at least, you could absorb any specimen with heparinase and test. If the PTT on the treated specimen is prolonged, switch to the anti-Xa heparin assay to monitor using 0.3 to 0.7 as the target therapeutic range.
Since I am theorizing, I’d like to hear from others experienced in heparin neutralization.
Meanwhile, I heard a terrific presentation by Karen Wrona of Stago-US at the Northeast Laboratory Conference in Portland, Maine in September. Karen’s the heparin initiative product manager for Stago. She lists some of the PTT weaknesses: the therapeutic range is reagent and lot-dependent, it is not specific for heparin and is prolonged by warfarin, lupus anticoagulants, factor deficiencies and liver disease. The PTT is shortened, and thus less responsive to heparin, when the factor VIII or fibrinogen activities are elevated in acute inflammation.
Stago is in the vanguard of an initiative to convert to the anti-Xa heparin assay for routine heparin therapy monitoring. Besides its inherent precision, the anti-Xa excels because it may be used to monitor low molecular weight heparin and synthetic pentasaccharides in addition to standard unfractionated heparin. Though per-test expense exceeds the PTT, the downstream benefits of accurate, reproducible monitoring may outweigh the costs. Several large institutions have completed the switch with positive outcomes results.