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POC INRs with an APA

From Elizabeth Kreuser:

Our pharmacy is using a point of care (POC) device to monitor prothrombin times with international normalized ratios (INRs).  However, they have a patient with an anti-phospholipid antibody (APA).  We did a quick study using Stago Neoplastin Cl Plus, Stago clotting factor II, and the POC device. Results were:

Assay  Test 1  Test 2
Factor II clotting assay  17% 14%
POC INR     3.5 4.5
Stago Neoplastin Cl Plus INR  2.6 2.8

They are satisfied with the correlation and are going to continue with POC keeping the patient between 2–3, because they feel this will correlate with a 24–45% factor II activity, which is what they want to achieve. I have been trying to find some references and not coming up with much. I think that the numbers are off.  Any advice?

Thank you for your question, Elizabeth. These data can’t really lead you to accept either the Neoplastin or the POC results.  I’m assuming that the APAs in the two test specimens are lupus anticoagulants (LAs), which are influencing the three assays differently. In the clot-based factor II assay, the LA prolongs the results despite the typical 1:10 dilution, thus rendering the assay results inaccurately reduced. Also, the LA appears to be influencing the POC and the Neoplastin results differently, as you may predict, since they use different technologies. Though the POC results seem to reflect the factor II clotting assay results, you can’t rely on the factor II results to serve as the reference method because of the LA effect on the assay.

I recommend comparing your POC and Neoplastin results to the chromogenic factor X (CFX) assay, available from DiaPharma, which is unaffected by LA and can serve as a reliable reference. Here are some supporting citations:

  • Rosborough TK, Jacobsen JM, Shepherd MF. Relationship between chromogenic factor X and INR differs during early warfarin initiation compared with chronic warfarin administration. Blood Coagul Fibrinolysis 2009 20:433-5.
  • McGlasson DL, Romick BG, Rubal BJ. Comparison of a chromogenic factor X assay with INR for monitoring oral anticoagulation therapy. Blood Coag Fibrinolys 2008;19:513–17.
  • Rosborough TK, Shepherd MF. Unreliability of international normalized ratio for monitoring warfarin therapy in patients with lupus anticoagulant. Pharmacotherapy. 2004;24:838–42.

By the way, please read our April 11 question that appears just below your, posted byCrystal Azevedo; New Oral A/Cs and POC. She asks about monitoring dabigatran, rivaroxaban, and apixaban with POC instruments. I suspect you may be facing the same question, perhaps you could each run studies that correlate LA and A/C POC results to the CFX.

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