Coagulation
Coagulation laboratory tests
Factor Xa assay
Author: Jeremy C. Parsons, M.D.
Last author update: 1 June 2012
Last staff update: 28 September 2020
Copyright: 2002-2025, PathologyOutlines.com, Inc.
PubMed Search: Factor Xa assay
Cite this page: Parsons JC. Factor Xa assay. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/coagulationfactorxaassay.html. Accessed January 10th, 2025.
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
Laboratory
- 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
Interpretation
- 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
