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Presentation Details
| Validation of the Fiix Assay on Werfen ACL TOP 750 Analyzer A Eshaghpour1, K Moffat1, 2 , S Carlino2, L Letertre3, E Belley-Cote1, D.M Witt4, B Gudmundsdottir3, P Onundarson3, 5, M Crowther1. 1Department of Medicine, McMaster University, Hamilton, ON, Canada.2Hamilton Regional Laboratory Medicine Program, Hamilton, ON, Canada.3Departments of Medicine and Laboratory Medicine, Landspitali National University of Iceland, Reykjavik, Iceland.4University of Utah College of Pharmacy, Salt Lake City, UT, USA.5Faculty of Medicine, University of Iceland, Reyjkavik, Iceland |
Abstract
Background: Warfarin is a vitamin K antagonist widely used to prevent and treat thromboembolism. Its effect is traditionally monitored using the prothrombin time (PT), reported as the International Normalized Ratio (INR). Because the PT/INR is highly sensitive to factor VII (FVII) activity, fluctuations in FVII levels during warfarin therapy can cause marked variability in the INR, even when the overall anticoagulant effect, primarily determined by reductions in factors II and X, remains relatively stable. In 2012, Gudmundsdottir et al. described a new assay to monitor warfarin based on its effect on factors II and X only, thereby removing the influence of FVII on clotting time; this test was named the Fiix assay.(1) Since 2015, most patients receiving warfarin in Iceland have been monitored using the Fiix assay. Prospective studies in Iceland have demonstrated reduction in thromboembolism with Fiix monitoring as compared to PT/INR monitoring. Testing is performed with a reagent from Hart Biologicals (Hartlepool, UK) on a Diagnostica Stago analyser (Asnières-sur-Seine, France) using a modified protocol, and results are reported as a Fiix normalized ratio with a therapeutic range of 2.0 to 3.0. Objective: To establish the Fiix assay protocol on the Werfen ACL TOP750 (Bedford, MA, USA) in Hamilton, Ontario, Canada and validate the performance in comparison with the Icelandic Fiix assay. Methods: 60 anonymized patient samples, encompassing a broad distribution of Fiix-NR values, were sent from Iceland to Hamilton for an interlaboratory challenge. The Icelandic test protocol was used and adapted to the TOP750. The same lot numbers for Hart FIIX reagents (kit containing Thromboplastin, buffer, FII&FX deficient plasma), DEKS Calibrators (Glostrup, Denmark), and George King (Overland Park, Kansas, USA) A-FACT quality control were used as identical to testing completed in Iceland. Seven replicates on 3 levels of DEKS calibrators were run on the TOP750. Linear regression analysis was used to assign a local ISI to the Hart kit thromboplastin using the DEKS Calibrators and Excel Template provided by the Icelandic group. An instrument specific geometric mean was determined using 20 individual normal plasmas (13 females; 7 males). An additional comparison of 64 anonymized warfarin plasma samples from Hamilton evaluated agreement between Fiix-NR and PT/INR. Results from both assays were classified as below, within, or above a therapeutic range of 2.0-3.0 and deemed concordant if classifications agreed. Results: The Icelandic Fiix-NR values had range of 0.9 to 4.6. Linear regression analysis was excellent for results from 60 anonymized samples from patients receiving warfarin in Iceland (R2 = 0.984; y=0.974x+0.11) (figure 1). Fiix-NR and PT/INR results were concordant in 73.4% of samples, and no consistent pattern was observed among discordant results (figure 2). Conclusions: Our study is the first to evaluate an adapted FIIX assay outside of Iceland. Overall, the performance was comparable between the results on the Stago analyzer (Iceland) and the TOP750 (Hamilton, Canada). Prospective studies are needed to confirm the clinical utility of the adapted Fiix assay and to better characterize areas of discordance for warfarin monitoring in comparison to the PT-INR.
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