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Coagulation laboratory tests

Factor Xa assay

Reviewer: Jeremy Parsons, M.D. (see Reviewers page)
Revised: 21 June 2012, last major update June 2012
Copyright: (c) 2002-2012, PathologyOutlines.com, Inc.


● Used to determine levels of heparin, low molecular weight heparin, danaparoid, etc.
● Measures ability of heparin or other drugs in patient’s 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 don’t 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


● 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

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


● Low levels of factor Xa are due to:
      (a) not collecting specimen at right time or delayed transportation to lab (see Sampling above)
      (b) higher therapeutic dose needed
● High levels of factor Xa are due to:
      (a) renal failure
      (b) heparin contamination (specimen drawn from indwelling line containing heparin)
      (c) lower therapeutic dose needed

Additional references

Thromb Haemost 2012;107:379

End of Coagulation > Coagulation laboratory tests > Factor Xa assay

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