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. 
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. 
Collection and Panels
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.  Complete deficiency of factor X resulted either in intrauterine death or fatal hemorrhage within 5 days after birth in a mouse model. 
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,  VIII,  and V.  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. 
In addition to the procoagulant activity, it also has mitogenic and pro-inflammatory activities.  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