Anti-factor Xa (Anti-Xa) Assay

The anti-factor Xa (anti-Xa) assay is a functional assay that facilitates the measurement of antithrombin (AT)-catalyzed inhibition of factor Xa by unfractionated heparin (UFH) and direct inhibition of factor Xa by low-molecular-weight heparin (LMWH) (Kitchen, Br J Haematol 111:397–406, 2000; Walenga et al., Semin Thromb Hemost 11:17–25, 1985; Levine et al., Arch Intern Med 154:49–56, 1994; Barrowcliffe et al., J Pharm Biomed Anal 7:217–226, 1989; Triplett, Ther Drug Monit 1:173–197, 1979; Nelson, Clin Lab Sci 12:359–364, 1999; Laffan and Manning, Dacie and Lewis: practical haematology, Churchill Livingstone, London, pp 465–479, 2001; Olson et al., Arch Pathol Lab Med 122:782–798, 1998). Whilst automated methods for the determination of the abilities of UFH and LMWH to inhibit factor Xa have been available since the 1970s, their cost was viewed to prohibit their broad use in the clinical management of UFH and LMWH until relatively recently. The anti-Xa assay can also be used to guide the determination of therapeutic APTT ranges in the clinical management of UFH (Hirsh and Raschke, Chest 126:188S–203S, 2004). As a result, the anti-Xa assay is commonly viewed as a heparin assay , despite the fact that it actually provides a measure of UFH effect as opposed to a measure of UFH concentration.