Factor Xa (Activated Factor X) for Anti-Factor Xa Test
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This article is compiled based on the United States Pharmacopeia (USP) – 2025 Edition
Issued and maintained by the United States Pharmacopeial Convention (USP)
—Factor Xa is the proteolytic enzyme derived from bovine plasma, and it cleaves prothrombin to form thrombin. It is a glycoprotein having a molecular weight of 40,000. One Factor Xa Unit is the amount of activated endogenous Factor Xa (55,000 Da when nonactivated) that is contained in 1 mL of normal plasma. Russell's viper venom is used to activate the enzyme, and is then removed. The preparation is stabilized
and lyophilized.
Factor Xa is free of thrombin at the concentration used in the test. When tested against pure brinogen, no clotting takes place within 24 hours.
It has not less than 40 Factor Xa Units per mg of protein, and exhibits 90% homogeneity when tested by disk electrophoresis.
One Factor Xa Unit per mL gives a 15-second clotting time when tested in the following manner. Mix 0.1 mL of a saturated solution of cephalin (derived from rabbit brain-acetone powder or from an equivalent amount of rabbit brain thromboplastin) in 0.1 mL of citrated bovine plasma and 0.1 mL of 0.025 M calcium chloride. Immediately add 0.1 mL of a solution of factor Xa (1 in 10), and incubate at 37°.
