Reassessment of dextran sulfate in anti-Xa assay for unfractionated heparin laboratory monitoring

BACKGROUND: Anti-Xa assays are used for unfractionated heparin (UFH) monitoring. Dextran sulfate (DS) is used in some assays to overcome the artifactual preanalytical release of platelet factor 4. However, the practical implications of this test modification have not been studied extensively.

OBJECTIVES: To investigate the impact of the presence of DS in the anti-Xa assay for UFH laboratory monitoring.

METHODS: We studied factor Xa inhibition, using an assay without DS (Stago Liquid Anti-Xa), in normal pool plasma spiked with various concentrations of UFH (up to 1 IU/mL) in the presence of increasing concentrations of DS (up to 2560 μg/mL). We also investigated the effect of DS on FXa inhibition measured after the addition of UFH and heparin antagonists (protamine and Polybrene; Sigma Aldrich). Eventually, we compared the anti-Xa levels measured using the assay without DS to those measured with an assay containing DS (BIOPHEN Heparin LRT, Hyphen BioMed).

RESULTS: DS per se had a detectable anti-Xa effect. FXa inhibition in UFH-spiked plasma linearly increased with increasing concentrations of added DS, with a plateau at approximately 160 μg/mL DS, at which the apparent anti-Xa level had almost doubled. In the presence of heparin antagonists, the addition of DS increased anti-Xa levels, corresponding to the dissociation of the UFH-antagonists complexes in vitro. With the anti-Xa assay containing DS, UFH inhibition was not detected.

CONCLUSION: In the presence of high concentrations of DS, FXa inhibition was much higher than that predicted from added UFH amounts, presumably related to the greater availability of UFH for interaction with antithrombin. While the relevance of measuring this "masked" heparin has not been demonstrated, the presence of DS renders the result inaccurate in the presence of protamine or Polybrene.