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Licensed Unlicensed Requires Authentication Published by De Gruyter May 22, 2020

Quality performance for indirect Xa inhibitor monitoring in patients using international external quality data

Martine J. Hollestelle EMAIL logo , Felix J.M. van der Meer and Piet Meijer



Chromogenic anti-activated factor X (FXa) assays are currently the “gold standard” for monitoring indirect anticoagulants. However, anti-FXa has been shown to vary according to the choice of reagents. In the present study, the performance of anti-FXa measurement was evaluated in order to gain more insight into the clinical applications. Furthermore, the longitudinal coefficient of variation (CV) was studied to investigate whether there is improvement over time.


Laboratory tests results were evaluated for samples spiked with unfractionated heparin (UFH), low-molecular-weight-heparin (LMWH), fondaparinux and danaparoid sodium. External quality assessment (EQA) data from multiple years were used from more than 100 laboratories.


Comparison of the results for all methods showed significant differences in measured values between the frequently used methods (ANOVA: p < 0.001). The largest differences were observed for LMWH and UFH measurements. These differences may be caused by differences in method composition, such as the addition of dextran sulphate. Substantial interlaboratory variation in anti-FXa monitoring was observed for all parameters, particularly at low concentrations. Our results showed that below 0.35 IU/mL, the CVs for UFH and LMWH increase dramatically and results below this limit should be used with caution.


Our study demonstrates that the choice of the anti-FXa method is particularly important for UFH and LMWH measurement. The variation in measurements may have an effect on clinical implications, such as therapeutic ranges. Furthermore, the longitudinal EQA data demonstrated a constant performance and, in at least 50% of the cases, improvement in the CV% of the anti-Xa results over time.

Corresponding author: Martine J. Hollestelle, PhD, External Quality Control for Assays and Tests (ECAT) Foundation, P.O. Box 107, 2250AC Voorschoten, The Netherlands, Fax +31.71.3030919


The authors would like to acknowledge Dr. P. Tsiamyrtzis for his statistical support.

  1. Research funding: None declared.

  2. Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.

  3. Competing interests: Authors state no conflict of interest.


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Received: 2020-02-07
Accepted: 2020-04-30
Published Online: 2020-05-22
Published in Print: 2020-10-25

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