Our April 2025 Quick Question asked, “How do you measure warfarin in a patient with LAC?” The poll drew 47 respondents and was a sequel to our March 2025 question, “How do you test for lupus anticoagulant?” that referenced the ISTH‘s most recent LAC guideline, Devreese KMJ, Bertolaccini ML, Branch DW, et al. An update on laboratory detection and interpretation of antiphospholipid antibodies for diagnosis of antiphospholipid syndrome: guidance from the ISTH-SSC Subcommittee on Lupus Anticoagulant/Antiphospholipid Antibodies. J Thromb Haemost. 2025;23(2):731-744. doi: 10.1016/j.jtha.2024.10.022. In this question, we addressed the possibility that LAC could factitiously prolong some PT results as it binds reagent phospholipid. Here are your answers:
- Thrombin time: 5 (11%)
- PT/INR range 2–3: 9 (19%)
- PT/INR range 2.5–3.5: 16 (34%)
- Chromogenic anti-Xa: 5 (11%)
- Chromogenic factor X: 12 (25%)
Given that the VKA suppresses thrombin production, some have attempted to use the TCT to measure warfarin therapy. However, PT with INR is the preferred method worldwide. Many chose the PT/INR using the standard INR Rx range of 2–3, and more chose 2.5–3.5, which roughly compensates for the potential partial LAC neutralization of the PT reagent phospholipid. In either choice, the laboratory QC manager establishes the range for the local population.
Answer 4 was a “red herring,” the chromogenic anti-Xa is the standard assay for UFH, and the RUO assays for anti-Xa DOACs, rivaroxaban, and apixaban.
The CFX assay has enjoyed little market penetration; however, several chose answer #5, perhaps having navigated to our April 1, 2025 entry, Chromogen FXA versus PT/INR, featuring coagulation experts Malissa Norfolk, Dave Cabral, and David McGlasson. The ISTH Guideline referenced above briefly references the CFX, as it offers acceptable precision, a low limit of detection, and is insensitive to the LAC phospholipid binding properties. As always, we invite your comments.
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