Factor XIII, an enzyme that cross-links fibrin, belongs to the blood coagulation system. Screening for factor XIII, also known as fibrin-stabilizing factor, is performed when its absence is suspected.
Qualitatively, factor XIII levels are referred to as decreased or normal. Quantitation of the enzyme's levels is carried out in research laboratories. 
Decreases in factor XIII levels occur in association with the following:
Inheritance (severe bleeding in homozygous carriers)
Acute promyelocytic leukemia
Autoantibodies against factor XIII (presence of circulating inhibitors)
Collection and Panels
Collection details are as follows:
Container: Blue-top vacuum tube
Collection method: Routine venipuncture
To comply with Occupational Safety and Health Administration (OSHA) safety standards, all samples must be sent in a sealed, leak-proof container with a biohazard sticker attached.
Panel: Qualitative functional assays of factor XIII
The enzyme factor XIII, which cross-links fibrin, belongs to the blood coagulation system. Also known as fibrin-stabilizing factor, the 320,000-Dalton glycoprotein is activated by thrombin in the presence of calcium and has a plasma half-life of approximately 10 days. 
Plasma, platelets, monocytes, and monocyte-derived macrophages also contain factor XIII. In plasma, it exists as a heterotetramer made up of paired A and B subunits (A2, B2). In platelets and other cells, factor XIII lacks the B domain, existing instead as an A2 dimer.
Monocytes/macrophages can synthesize factor XIII,  while megakaryocytes are probably responsible for synthesizing the factor XIII found in platelets. [4, 5] The primary site for the synthesis of subunit A in plasma factor XIII seems to exist in cells that originate in the marrow. Synthesis of the B subunit occurs in the liver.  The factor XIII A chain gene is found on chromosome 6. 
Circulation of plasma factor XIII occurs in association with its substrate, fibrinogen. Thrombin cleavage of the Arg37 -Gly38 bond in the A chain, causing the release of an Mr 4500 activation peptide, is the key step in the activation of plasma factor XIII. As a result of this step, the A and B subunits become dissociated and the active site on the free A subunits is exposed.
In platelets, a nonproteolytic process activates cellular factor XIII. In the absence of the B-chain, the elevation of intracytoplasmic Ca2+ during platelet activation causes the zymogen to assume an active configuration. [2, 5]
Stabilizing the fibrin plug through cross-linkage of the alpha and gamma chains of fibrin is plasma XIIIa's main physiologic function.  Although a clot will form if factor XIII is absent, it will not be adequate for hemostasis. Components of the clotting and fibrinolytic system, along with multiple adhesive and contractile proteins, are additional factor XIIIa protein substrates. By crosslinking fibrin to alpha 2-antiplasmin, factor XIIIA also protects fibrin from fibrinolysis. 
Screening for factor XIII is carried out when the enzyme's presence is suspected.
Neither prothrombin time (PT; international normalized ratio [INR]) nor partial thromboplastin time (PTT) is affected by factor XIII deficiency. Clots are soluble in 5-molar urea.  Plasma factor XIII also plays a role in wound healing and tissue repair, and the enzyme is essential in maintaining pregnancy.
When activated by thrombin, factor XIIIa forms an insoluble clot by causing cross-linkage between fibrin molecules. If a clot has not been stabilized by factor XIIIa, it is soluble in 5 mol/L urea; a stabilized clot is resistant to this. 
Although measurement of factor XIII levels is not considered routine, patients with an unexplained tendency to bleed may be considered for this screen. The enzyme is very specific for monocytes and macrophages, so identification and classification of malignant diseases involving these cells can be carried out by determining whether or not factor XIII is present.