A question from “Bmolpus:”
Hello, I am an emergency physician who has started an urgent care practice and find myself reviewing more patient records and getting more exposure to laboratory tests which were out of the scope of my previous ER practice. I have been reviewing the site and am curious to see if you could answer the following question:
Knowing that heparin and low molecular weight heparin (but not coumadin) can cause a false positive lupus anticoagulant (LA) assay, my question is as follows: Does treatment with heparin/heparin substitutes cause a false positive LA screening assay only, or can it also cause a false positive hexagonal phase confirmatory test? Thanks much.
Hello, and thank you for your question. Yes, heparin, particularly standard unfractionated heparin, does interfere with both the partial thromboplastin time (PTT) assay, which is the preliminary screening assay, and the confirmatory hexagonal phase phospholipid assay. Low molecular weight heparin may also interfere, though to a lesser degree. When the initial screening PTT is prolonged, perform a thrombin time test, which is significantly prolonged by unfractionated heparin. If heparin is confirmed by the thrombin time, neutralize the heparin with Heparinase, then proceed with the remainder of the lupus anticoagulant profile.
By the way, it is possible, though less frequent, for Coumadin to also interfere, particularly with the dilute Russell viper venom time (DRVVT). This should be considered, and there is no method available for neutralizing the effect of Coumadin in vitro.