Coagulation laboratory tests

Factor Xa assay

Last author update: 1 June 2012
Last staff update: 28 September 2020

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PubMed Search: Factor Xa assay

Jeremy C. Parsons, M.D.
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Cite this page: Parsons JC. Factor Xa assay. website. Accessed February 26th, 2024.
Definition / general
  • Used to determine levels of heparin, low molecular weight heparin, danaparoid, etc.
  • Measures ability of heparin or other drugs in patients plasma to inhibit known amount of factor Xa
  • Usually reported out based on standard curve for the drug in question

  • Note: The term anti-Xa level should be avoided because this suggests measuring a factor X inhibitor, but the factor Xa assay is really a drug level using chromogenic Xa inhibition as the methodology; as the reference range and standard curve vary with the drug tested, the clinician should indicate the drug (heparin, danaparoid, etc.)

  • Used to monitor heparin, particularly if PTT has baseline prolongation due to lupus anticoagulant or factor XII deficiency

  • Note: Can cautiously use PTT to monitor heparin, even if lupus anticoagulant present, if factor Xa assay demonstrates that it is not affected by the lupus anticoagulant

  • Also used to monitor low molecular weight heparin and danaparoid, which dont prolong PTT; these drugs usually do not need to be monitored except if renal failure, pregnancy (increased dosage needed in third trimester), newborns (increased dosage needed), over- or underweight patients, prolonged use or high risk for bleeding/thrombosis

  • Chromogenic factor X assays:
    • Used to monitor warfarin in the presence of a lupus anticoagulant, hirudin or argatroban (which prolong the PT and increase the INR), because warfarin decreases factor X (also factors II, VII, IX), and the chromogenic assay has no interference from lupus anticoagulant, hirudin or argatroban
    • Patient plasma is added to a known amount of excess factor Xa with excess antithrombin
    • Anticoagulant binds to antithrombin and inhibits factor Xa
    • Residual factor Xa is inversely proportional to anticoagulant in plasma, cleaves a chromogenic substrate, and colored compound is detected by spectrophotometer
    • Results reported in antifactor Xa units/mL
  • Draw specimen 4 hours after subcutaneous injection of low molecular weight heparin or 6 hours after subcutaneous injection of danaparoid to avoid falsely low values
  • Must deliver to laboratory immediately (or separate plasma from cells within 1 hour), because platelets release platelet factor 4, which neutralizes heparin
  • Delays may cause falsely low values
  • Approximate therapeutic range for treatment of existing deep venous thrombosis:
  • Heparin 0.3 to 0.7 anti-Xa international units/mL
  • Low molecular weight heparin either 0.4 to 1.1 units/mL for twice a day dosing or 1 to 2 units/mL for once daily dosing
  • Danaparoid 0.5 to 0.8 units/mL
  • Low levels of factor Xa are due to:
    • Not collecting specimen at right time or delayed transportation to lab (see Laboratory above)
    • Higher therapeutic dose needed

  • High levels of factor Xa are due to:
    • Renal failure
    • Heparin contamination (specimen drawn from indwelling line containing heparin)
    • Lower therapeutic dose needed
Additional references
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