Anti-factor Xa assay

Kristensen HI, Ostergaard R Nordfang O, Abilolgaard U, Effect of tissue factor pathway inhibitor (TFPI) in the HEPTEST assay and in an amidolytic anti factor Xa assay for LMW heparin, Thromb Haemost I 992 68(3)3 10-3 14. 

Patients with end-stage renal failure may require monitoring with an anti-factor Xa assay because this condition may prolong the half-life of low-molecnlar-weight heparin. Specific dosage recommendations for varions low-molecnlar-weight heparins may be obtained from the mannfactnrer s literature. 

Harris, L.F., Rainey, R, Castro-Lopez, V., O Donnell, J.S., KiUard, A.J., 2013. A microfluidic anti-factor Xa assay device for point of care monitoring of anticoagulation therapy. Analyst 138 (17), 4769 776. 

Harris LF, Castro-Lopez V, Hammadi N, O Donnell JS, Killard AJ. Development of a fluorescent anti-factor Xa assay to monitor unfractionated and low molecular weight hep-arins. Talanta 2010 81 1725-30. 

The anti-Factor Xa activity of fondaparinux can be measured by anti-Xa assay using the appropriate calibrator. 

When used in full therapeutic doses, UFH must be monitored to determine the appropriate dose to administer. The choice of assay is based on chnician preference and institutional availabihty. The aPTT remains the most commonly used test to monitor UFH therapy in North America. In some European countries, anti-factor Xa heparin activity is used commonly. The aPTT should be measured prior to the initiation of therapy to determine the patient s baseline. When administered by intravenous infusion, the response to therapy... 

When anti-factor Xa activity is used to monitor LMWH therapy, the sample should be drawn approximately 4 hours after the subcutaneous injection, during the peak period of anti-factor Xa activity. A calibrated LMWH heparin should be used to establish the standard curve for the assay. The therapeutic range for anti-factor Xa activity is not well defined and to date has not been correlated clearly with efficacy or the risk of bleeding. Eor the treatment of VTE, an acceptable target range is 0.5 to 1.0 unit/mL. Specific algorithms for dosing adjustments based on anti-factor Xa activity are not available at the present time. 

Heparin resistance is defined as the requirement of more than 35,000 units in a 24-h period to maintain a therapeutic activated partial thromboplastin time. In some cases, elevated factor VIII levels can factitiously lower the aPPT without influencing heparin activity measured by anti-Xa assays, leading to a misdiagnosis of heparin resistance. A case of apparent heparin resistance due to increased factor VIII levels in an elderly male with infective endocarditis was reported [18 ]. 

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