Factor X Assay 

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Factor X is synthesized in the liver, and vitamin K is required for its production.

The reference range of factor X is 70%-150% of normal. ^[1]

Factor X levels are increased in pregnancy and in association with oral contraceptives. Defects in protein Z lead to increased factor Xa activity, which increases the risk for thrombosis.

Inborn deficiency of factor X is very uncommon (1 in 500,000 persons). Factor X deficiency may also occur in amyloidosis, in which factor X is adsorbed to the amyloid fibrils in the vasculature. Decreased factor X levels are also associated with vitamin K deficiency, warfarin therapy, severe liver disease, and disseminated intravascular coagulation.

Decreased factor X levels have also been found in myeloma, children with severe burns, Mycoplasma pneumonia infections, patients with lupus anticoagulants, leukemia, and other neoplastic diseases. ^[2]

Specimen: Plasma

Container: Blue-top vacuum tube

Collection method: Routine venipuncture

Other instructions: All samples must be sent in a sealed, leak-proof container marked with a biohazard sticker to comply with Occupational Safety and Health Administration (OSHA) safety standards

Panels: Quantitative functional assays of coagulation factors

Factor X is synthesized in the liver, and vitamin K is required for its production. The gene for human factor X is on a chromosome in close proximity to the factor VII gene. ^{[3, 4]} Factor X has a molecular weight of 59,000 daltons with a plasma half-life of approximately 34-40 hours. ^[3] Complete deficiency of factor X resulted either in intrauterine death or fatal hemorrhage within 5 days after birth in a mouse model. ^[5]

Factor X is activated to fully active factor Xa by factor VIIa/TF or factor IXa/VIIIa. Factor Xa in complex with factor Va on a phospholipid membrane surface activates prothrombin to thrombin by cleaving two peptide bonds. Factor Xa may also play a physiologic role in activation of factors VII, ^[6] VIII, ^[7] and V. ^[8] Although any membrane surface that expresses anionic phospholipid can support prothrombinase complex assembly, the activated platelet surface is especially well suited for this purpose. Prothrombinase assembly on platelets is not strictly a function of phospholipid composition, but is likely coordinated by one or more specific binding proteins. ^[9]

In addition to the procoagulant activity, it also has mitogenic and pro-inflammatory activities. ^[3] It is reported to have mitogenic activity for smooth muscle cells and receptor-mediated proinflammatory activities. ^{[3, 10]}

Factor X testing is indicated for the following:

When factor X deficiency is suspected

To measure blood heparin (particularly low molecular weight heparin) for factor Xa inhibition, indicating level of anticoagulation

When therapeutic inhibitors of Xa are used to find out the level of anticoagulation

The letter “a” in front of any coagulant factor indicates active form of the factor.

Newer anticoagulants directly inhibit activated factor X (Xa).

In biochemistry, factor Xa protease may be used to cleave off protein tags that improve expression of a protein of interest.

Both prothrombin (PT) and partial thromboplastin time (PTT) are affected in marked deficiency of factor X.

Limitations of factor X testing include the following:

Partially clotted specimens due to poor mixture of anticoagulant (3:2 sodium citrate, as per manufacturer’s blue-topped tube)

Overfilled or underfilled test tubes (altering the ratio of blood to anticoagulant [9:1])

Improperly stored plasma

Contamination with heparin or dilution of collected sample if indwelling catheters are used

Analytical errors such as lipemic, icteric, or hemolyzed plasma, which may interfere with photoelectric measuring instruments ^[1]

Williamson MA, Snyder LM, Wallach JB. Wallach’s interpretation of diagnostic tests. 9th ed. Wolters Kluwer/Lippincott Williams & Wilkins Health: Philadelphia; 2011.

Schwartz RA SC, Gascon P. Factor X. Medscape.

Monroe DM HM, Roberts HR. Molecular Biology and Biochemistry of the Coagulation Factors and Pathways of Hemostasis. Prchal JT KK, Lichtman MA, Kipps TJ, Seligsohn U, ed. Williams Hematology. 8th ed. McGraw-Hill: New York; 2010.

Scambler PJ, Williamson R. The structural gene for human coagulation factor X is located on chromosome 13q34. Cytogenet Cell Genet. 1985. 39(3):231-3. [Medline].

Dewerchin M, Liang Z, Moons L, Carmeliet P, Castellino FJ, Collen D. Blood coagulation factor X deficiency causes partial embryonic lethality and fatal neonatal bleeding in mice. Thromb Haemost. 2000 Feb. 83(2):185-90. [Medline].

Rao LV, Rapaport SI. Activation of factor VII bound to tissue factor: a key early step in the tissue factor pathway of blood coagulation. Proc Natl Acad Sci U S A. 1988 Sep. 85(18):6687-91. [Medline].

Neuenschwander PF, Jesty J. Thrombin-activated and factor Xa-activated human factor VIII: differences in cofactor activity and decay rate. Arch Biochem Biophys. 1992 Aug 1. 296(2):426-34. [Medline].

Monkovic DD, Tracy PB. Activation of human factor V by factor Xa and thrombin. Biochemistry. 1990 Feb 6. 29(5):1118-28. [Medline].

Bouchard BA, Catcher CS, Thrash BR, Adida C, Tracy PB. Effector cell protease receptor-1, a platelet activation-dependent membrane protein, regulates prothrombinase-catalyzed thrombin generation. J Biol Chem. 1997 Apr 4. 272(14):9244-51. [Medline].

Altieri DC, Edgington TS. Identification of effector cell protease receptor-1. A leukocyte-distributed receptor for the serine protease factor Xa. J Immunol. 1990 Jul 1. 145(1):246-53. [Medline].

George Ansstas, MD Assistant Professor of Medicine, Division of Medical Oncology, Washington University School of Medicine

George Ansstas, MD is a member of the following medical societies: American Medical Association

Disclosure: Nothing to disclose.

Bishnu Prasad Devkota, MD, MHI, FRCS(Edin), FRCS(Glasg), FACP Professor of Medicine, St Louis University School of Medicine

Bishnu Prasad Devkota, MD, MHI, FRCS(Edin), FRCS(Glasg), FACP is a member of the following medical societies: American College of Physicians, American Medical Informatics Association, Royal College of Physicians and Surgeons of Glasgow, Healthcare Information and Management Systems Society, Royal College of Surgeons of Edinburgh

Disclosure: Nothing to disclose.

Eric B Staros, MD Associate Professor of Pathology, St Louis University School of Medicine; Director of Clinical Laboratories, Director of Cytopathology, Department of Pathology, St Louis University Hospital

Eric B Staros, MD is a member of the following medical societies: American Medical Association, American Society for Clinical Pathology, College of American Pathologists, Association for Molecular Pathology

Disclosure: Nothing to disclose.

Factor X Assay 

Research & References of Factor X Assay |A&C Accounting And Tax Services
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