Two sensitive and rapid chromogenic assays of fondaparinux sodium (Arixtra) in human plasma and other biological matrices

Fondaparinux is a synthetic selective inhibitor of factor Xa recently approved for thromboprophylaxis after major orthopedic surgery. Determination of its concentration gives valuable insight into specific pharmacokinetics or safety studies. The aim of the study was to develop direct, sensitive, precise and accurate assays of fondaparinux sodium in different biological matrices. Consistency with the recommended chromogenic assay for low molecular weight heparin required a similar method. However, recent data indicated some variability in the determination of anti-Xa level between commercial chromogenic assays. Consequently, we developed and validated two chromogenic methods (A and B) for assaying fondaparinux in plasma and other biological matrices. The assays are calibrated with fondaparinux, a pure chemical entity, and the result is expressed as amount (microg) of the fondaparinux calibrator. Results showed that precision was lower than 5.2% in plasma or plasma water and 13% in placental medium. The accuracy was lower than 7.6% in plasma or plasma water and 10.2% in placental medium. The lower limit of quantification in plasma was 0.042 microg/mL with automated Method A and 0.019 microg/mL with Method B. The assay was not affected by the source of the samples, the presence of blood cells, EDTA, citrate or repeated cycles of freezing and thawing. The two chromogenic assays calibrated with fondaparinux sodium reach the equivalence criteria for plasma samples and provide reliable and reproducible results.