Factor X, Chromogenic
Stuart Prower Factor
The chromogenic factor X activity test can be useful in monitoring patients on vitamin K antagonist therapy where baseline PT is prolonged.
Factor X is a 54.8 kilodalton vitamin K-dependent glycoprotein coagulation factor that is produced by the liver. Normal factor X’s plasma concentration is approximately 10 mg/mL with a half-life of about 40 hours. Factor X activation occurs by both the extrinsic and intrinsic pathways. Factor X deficiency should be considered when a patient with bleeding history has both extended protime (PT) and activated partial thromboplastin time (aPTT).
Oral anticoagulation with warfarin inhibits vitamin K-dependent carboxylation of several procoagulant factors, including factor X. Overdosing with warfarin can increase the risk of hemorrhage and inadequate dosing decreases the efficacy of anticoagulation. Unfortunately, a large number of factors can affect the pharmacological potency of these oral anticoagulants. These factors are reviewed in considerable detail in the American Heart Association/American College of Cardiology Foundation Guide to Warfarin Therapy. The prothrombin time (PT) test is sensitive to deficiencies of vitamin K-dependent factors and is commonly used to monitor warfarin therapy. In some cases, however, patients with lupus anticoagulants can have extended PT. Several studies have suggested that this can complicate the management of anticoagulant therapy with the prothrombin time. The chromogenic factor X activity test can be useful in monitoring patients where baseline PT is prolonged.
Blue-top (sodium citrate) tube
Citrated plasma samples should be collected by double centrifugation. Blood should be collected in a blue-top tube containing 3.2% buffered sodium citrate. Evacuated collection tubes must be filled to completion to ensure a proper blood to anticoagulant ratio. The sample should be mixed immediately by gentle inversion at least six times to ensure adequate mixing of the anticoagulant with the blood. A discard tube is not required prior to collection of coagulation samples, except when using a winged blood collection device (ie, “butterfly”), in which case a discard tube should be used. When noncitrate tubes are collected for other tests, collect sterile and nonadditive (red-top) tubes prior to citrate (blue-top) tubes. Any tube containing an alternate anticoagulant should be collected after the blue-top tube. Gel-barrier tubes and serum tubes with clot initiators should also be collected after the citrate tubes. Centrifuge for 10 minutes and carefully remove 2/3 of the plasma using a plastic transfer pipette, being careful not to disturb the cells. Deliver to a plastic transport tube, cap, and recentrifuge for 10 minutes. Use a second plastic pipette to remove the plasma, staying clear of the platelets at the bottom of the tube. Transfer the plasma into a PP transpak frozen purple tube with screw cap. Freeze immediately and maintain frozen until tested. To avoid delays in turnaround time when requesting multiple tests on frozen samples, please submit separate frozen specimens for each test requested.
Gross hemolysis; clotted specimen; frozen specimen thawed in transit; improper labeling
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