This is a message first posted on November 8, 2012 from Dave Easler, who originally sent his question to Dave McGlasson, Wilford Hall USAF Hospital, San Antonio. Both Daves gave me permission to post it here.
It was a pleasure to listen to your presentation at the 2012 Northeast Laboratory Conference in Portland, Maine last month. I have been asked if it is recommended to not use a point of care (POC) international normalized ratio (INR) method for patients bridging from low molecular weight heparin (LMWH, Lovenox) to Coumadin. I am in total agreement with you when it comes to the limitations of testing INRs with POC. I have started a literature search to try to find an answer to this question. There isn’t much specific information about this, but I am getting the impression that the recommended method for testing INRs in this class of patients is with a laboratory analyzer rather than with POC. Your thoughts would be much appreciated.
McGlasson answers, When bridging from heparin or a direct thrombin inhibitor (DTI) like argatroban to Coumadin the recommended test is the chromogenic factor X (ten, CFX) assay. It is impervious to the effects of either of the other anticoagulants. The POC PT/INR is a clottable assay that can be affected by the anticoagulants, thus making it unreliable. Measure the CFX until it gets to be <30.0% activity. The POC is just like a reference prothrombin time (PT) INR and is affected by anticoagulants. Lovenox will affect the PT but you can’t monitor it with a clottable assay. The CFX is just as easy to perform as an anti-Xa assay. See McGlasson DL, Romick BG, Rubal BL. Comparison of a chromogenic factor X assay with international normalized ratio for monitoring oral anticoagulation therapy. Blood Coagul Fibrinolys 2008;19:513–7.