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Jan, 2021 QQ: The Chromogenic Factor X Assay

Before 2010, several articles were published supporting the chromogenic factor X [CFX] assay [not to be confused with the chromogenic anti-factor Xa heparin assay] for monitoring warfarin therapy, especially in patients whose lupus anticoagulant [LAC} could falsely prolong the prothrombin time.

Two of these articles are attached below. While the data published made it clear the CFX could accurately follow warfarin efficacy despite interferences, interest appears to have waned, perhaps with the onset of the direct oral anticoagulants [DOACs] rapidly replacing warfarin for prophylaxis in non-valvular atrial fibrillation {NVAF, AFIB]. Nevertheless, Australian, US and European reference laboratories continue to offer the CFX, which is approved in all three continents.

Recently, George received a question from an AFIB patient with LAC whose warfarin therapy was being followed with the CFX in parallel to the PTINR. The questioner asked whether CFX results could be directly correlated to the INR at 3.0, 3.5, and 4.0. Dave McGlasson, in a 2009 platform presentation cited data from a half-dozen CFX-related publications. None provided a correlation for an INR of 4.0, and most stopped at 3.0. Interpreting from curves comparing CFX in % to INR, it appears available CFX assays reach the lower limit of detection at 11–13%, roughly equivalent to an INR of 3.0–3.5. If this is correct, is the CFX useful when documenting a hemorrhage-related warfarin overdose?

Our January, 2021 Quick Question polls our participants to learn if any laboratory policy advocates for the CFX in place of the PTINR, or if DOACs are being used for prophylaxis in AFIB patients with LAC. Please select your answer and add a comment to this post or by emailing to George at [email protected].

Our Quick Question now features a one-click “ReCaptcha” function to prevent spam. Here are our supporting articles:

McGlasson DL, Romick BG, Robals  BJ. Comparison of a chromogenic factor X assay with international normalized ratio for monitoring oral anticoagulation therapy. Blood Coagulation and Fibrinolysis 2008, 19:513–7.
Rosborough TK, Shepherd MF. Unreliability of international normalized ratio for monitoring warfarin therapy in patients with lupus anticoagulant. Pharmacotherapy 2004;24:838–42.

Comments (1)
Anticoagulant Therapy
Jan 5, 2021 5:52pm

From Dave McGlasson: One
From Dave McGlasson: One reason that the use of warfarin will continue is that if the renal function is not adequate the use of DOACs is not advisable. I have consulted with two patients on this subject and I asked why their MDs hadn’t suggested switching to DOACs. Both are home bound, so outpatient providers perform POC testing for PT/INR. The anticoagulation clinics told them that they would switch them over but they both had poor renal function. DOAC patients are supposed to be tested every 6 months for renal adequacy.

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