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Pall Onundarson: Diluted Fiix Test Can Be Used to Measure DOACs

Pall T. Onundarson/LinkedIn

Jan 28, 2026, 12:59

Pall Onundarson: Diluted Fiix Test Can Be Used to Measure DOACs

Pall Onundarson, Professor Emeritus at Landspitali University Hospital, shared on LinkedIn:

”Diluted Fiix test (dFiix-PT) can be used to measure DOACs.

DOACs were developed to replace VKAs, which were considered difficult to manage because of fluctuating anticoagulant effect measured as PT-INR, commonly attributed to food and drug interactions.

Few considered that the measurement tool itself, the prothrombin time, might have an inherent flaw contributing to variability and amplifying need for dose adjustments.

About 20 years ago, coagulation experts at TandH meetings repeatedly expressed skepticism that oral anticoagulants should be used without monitoring or tailoring.

Nevertheless, DOAC manufacturers conducted RCTs comparing unmonitored DOACs with, in many cases, poorly monitored warfarin, aiming to replace cheap old warfarin entirely.

When results were favorable, DOACs – supported by many key opinion leaders -became first-line for non-valvular AF and most VTE patients and were reimbursed despite higher costs.

At the same time, potential major improvements in warfarin management, such as newer monitoring methods, were largely ignored.

PT-monitored warfarin nevertheless persisted, as it gradually proved superior to DOACs in certain high-risk conditions and remained cheaper, even when monitoring costs were included.

More recently, the need to measure DOAC levels has gained attention, with multiple semi-quantitative and quantitative wet-lab and POC assays described, most far more expensive than the old PT-INR, further increasing DOAC costs.

As many of my LinkedIn contacts know, the Fiix test stabilizes warfarin and appears to reduce thromboembolism substantially compared with PT-INR–monitored warfarin, with proportionally greater benefits than those reported for DOACs versus PT-warfarin.

Our studies have been limited by the small Icelandic population and funding, but Fiix has now shown measurement superiority at a major UK center (personal communication), and a large independent RCT comparing Fiix-warfarin to PT-warfarin in MHV patients is being prepared in North America.

Less well known is that a diluted Fiix test (dFiix-PT), and – less optimally – diluted PT (dPT), can assess DOAC concentrations.

Thromboplastin dilution enhances sensitivity to DOACs and heparins, enabling an inexpensive, easily implemented semi-quantitative DOAC assay, demonstrated and published by us 10 years ago (Loic Leterte et al, see headshot).

Would anyone be interested in a cheap DOAC assay applicable to coagulometers?”

Pall Onundarson: Diluted Fiix Test Can Be Used to Measure DOACs