Factor X Assay

Factor X is synthesized in the liver, and vitamin K is required for its production. The reference range of factor X is 45-155% 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). ^[2] Factor X deficiency may also occur in amyloidosis, in which factor X is adsorbed to the amyloid fibrils in the vasculature. ^{[3, 4]} 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. ^[5]

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

Description

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, 6]} 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. ^[7]

Factor X is activated to fully active factor Xa by factor VIIa/TF or factor IXa/VIIIa. Factor Xa in complex with factor Va (termed the prothrombinase complex) 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, ^[8] VIII, ^[9] and V. ^[10] 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. ^[11]

A study by Li et al suggested that the Cys22-Cys27 disulfide bond in the protease domain of factor X modulates factor X’s clotting activity. Absence of the bond hinders insertion, following activation cleavage, of the catalytic domain’s N terminal, interfering with the zymogen-to-enzyme conformational transition. ^[12]

A study by Bonde et al indicated that the factor X autolysis loop takes part in all elements of factor X regulation. Plasma-based assays demonstrated that the procoagulant response is apparently “knocked down” by even a small reduction in the factor X activation rate, even in light of a decrease in inhibitor-produced down-regulation of factor Xa activity. ^[13]

In addition to the procoagulant activity, factor X has mitogenic and pro-inflammatory activities. ^[3] It is reported to have mitogenic activity for smooth muscle cells and receptor-mediated proinflammatory activities. ^{[3, 14]}

Research indicates that factor X can be produced by malignant cells, with a study by Sierko et al reporting, for example, that it can be expressed by endometrial cancer cells; factor X showed medium expression in endometrial cancer cells but no expression in healthy endometrial cells. Moreover, protein Z and protein Z–dependent protease inhibitor, which work together to inhibit factor X, were strongly expressed in endometrial cancer cells, indicating involvement of these proteins in endometrial cancer. Investigations also suggest that gastric and colon cancer cells can express factor X. ^[15]

Indications

Factor X testing is indicated when a factor X deficiency is suspected.

Consideration

The letter "a" after 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 activated partial thromboplastin time (aPTT) are affected in a 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 (if it is more than a therapeutic dose; since factor X assay is based on PT, and PT is not affected by a therapeutic heparin dose, a therapeutic dose will not influence the test) or dilution of a collected sample if indwelling catheters are used

• Analytical errors such as lipemic, icteric, or hemolyzed plasma, which may interfere with photoelectric measuring instruments ^[16]