Our November, 2019 Quick Question asked, “What may you substitute for the PT when LAC prolongs the PT?” We accumulated 53 votes.
A. None, use the PT anyway: 9% (5)
B. Chromogenic anti-Xa: 13% (7)
C. Chromogenic factor X: 30% (16)
D. Activated clotting time: 8% (4)
E. Thrombin time: 11% (6)
F. There is no substitute: 28% (15)
Thanks to all who voted. For those of us who use the PT anyway, it is important to determine from the package insert or local studies your thromboplastin reagent’s response to LAC, as the response varies among formulations. PT reagents that are prolonged by LAC are likely to generate a factitiously elevated INR, triggering an undesired Coumadin dosage reduction. The citations below provide evidence for the chromogenic factor X (CFX) assay as a valid substitute for the PT. The CFX is unaffected by LAC and by factor deficiencies other than factor X. CFX is also used when transitioning from argatroban to Coumadin. CFX is not to be confused with the chromogenic anti-Xa assay, used to monitor unfractionated and low molecular weight heparin and the anti-Xa DOACs. The ACT is employed to monitor unfractionated heparin, especially during cardiac surgery, whereas the thrombin time is seldom used to monitor Coumadin therapy. Here are two references recommending the CFX assay.
Mcglasson DL, Romick BG, Rubalb BJ. Comparison of a chromogenic factor X assay with international normalized ratio for monitoring oral anticoagulation therapy. Blood Coagulation and Fibrinolysis 2008,19:513–17.
Rosborough TK, Jacobsen JM, Shepherd MF. Factor X and factor II activity levels do not always agree in warfarin-treated lupus anticoagulant patients Blood Coagulation and Fibrinolysis 2010, 21:242–4.
Austin JH, Stearns CR, Winkler AM, Paciullo CA. Use of the chromogenic factor X assay in patients transitioning from argatroban to warfarin therapy. Pharmacotherapy 2012; 32:493–501.