Introduction

Although discovered in 1944, Factor XIII's (FXIII) role as a deficiency disorder became evident in the 1960s upon identifying other clotting factors.[1] Factor XIII, also referred to as fibrin stabilizing factor, plays a crucial role in the coagulation cascade by enhancing the stability of blood clot formation. The plasma form of Factor XIII is a protein heterodimer composed of A and B subunits expressed by bone marrow and mesenchymal lineage cells. Factor XIII functions as a transglutaminase, catalyzing peptide reactions responsible for cross-linking fibrin mesh. A deficiency can lead to life-threatening issues related to clot stability and hemostasis.[2]. 

Issues of Concern

Although exceedingly rare, congenital disorders of Factor XIII are associated with significant morbidity and mortality. Most commonly inherited in an autosomal recessive manner, immune or idiopathic causes account for acquired forms.[2] Factor XIII plays a critical role in normal hemostasis. However, there remains a substantial gap in research regarding its potential involvement in other processes. These processes encompass not only delayed postoperative bleeding but also delayed wound healing, recurrent miscarriages, and angiogenesis, as well as acquired coagulopathy resulting from reduced FXIII levels in the context of COVID-19 and Henoch-Schönlein purpura.[2] Additionally, factor XIII plays a role in immune function by killing bacteria and aiding phagocytosis by macrophages. Linked to inflammation, patients with COPD and a smoking history experience accentuated effects of FXIII.[2] More research is necessary on the involvement of FXIII in additional physiologic processes.

Cellular Level

In the presence of calcium, thrombin converts Factor XIII from a heterotetrameric proenzyme into a transglutaminase. This transglutaminase catalyzes the intertwining of fibrin monomers and forms connections between α2-antiplasmin and fibrin during the coagulation cascade.[3] This process serves to strengthen the blood clot.

FXIII has a plasmatic form and a cellular form carried inside platelet alpha granules. The cellular form consists of identical FXIII-A dimers, distinctive for its catalytic transglutaminase domain. The plasma form is a tetramer consisting of FXIII-A and FXIII-B subunits. FXIII-A contains a sandwich domain, a catalytic domain, an activation peptide, and 2 barrel domains. Ten repetitive domains are the defining characteristic of FXIII-B. To activate the plasma form of factor XIII, thrombin cleaves off the activation peptide of FXIII-A with the help of circulating calcium ions. FXIII-B detaches, and FXIII-A becomes the active transglutaminase involved in fibrin cross-linking and clot stabilization.[3] FXIII-B is a carrier protein that stabilizes subunit A, binds subunit A to fibrinogen, and helps stop FXIII activation. 

Function

FXIII is a transglutaminase that forms gamma-glutamyl-lysyl amide cross-linking of fibrin, stabilizing the insoluble clot against shear stress. As a result, it achieves further protection from clot degradation in the coagulation cascade.[4] Factor XIII also works on α2-antiplasmin to cross-link it with fibrin as part of its additional antifibrinolytic properties.[5] In the case of deficiency, there is a subsequent breakdown of fibrin mesh and recurrent bleeding after clot formation. 

Mechanism

Hemostasis is the body's way of preventing blood loss. The three main stages of hemostasis are vasoconstriction, platelet plug formation, and blood clotting. Within a clot, fibrin mesh strengthening occurs with the aid of Factor XIII, mainly by keeping the platelet plug together. The intrinsic and extrinsic pathways feed into the common pathway during clot formation. The common pathway involves thrombin-catalyzed cleavage of fibrinogen into fibrin, forming a mesh that strengthens the platelet plug. Subsequently, Factor XIII covalently bonds to fibrin to support fibrin's interaction with the platelets further and prevent the clot's dissolution. Clot retraction, which usually occurs a day after the initial clot formation, involves disruption of Factor XIII cross-links and fibrinolysis.[6]

Related Testing

Testing for FXIII deficiency is warranted when general hematology labs show no abnormality of PT, aPTT, INR, and bleeding time in the presence of delayed detachment and bleeding of the umbilical stump, hemorrhage, hemarthrosis, or hematomas. These findings indicate a defect in clot stabilization or fibrinolysis, particularly after trauma or surgical procedures.[7]

In the past, the initial testing for suspected FXIII deficiency involved conducting a clot solubility test. During this procedure, healthcare professionals placed the patient's anticoagulated blood specimen in a centrifuge, where calcium chloride induces plasma clot formation. Subsequently, the formed clot is suspended in 5M urea or 1% monochloroacetic acid solutions and incubated at either 37 ºC or room temperature.[8][9] The healthcare provider then regularly assesses the clot to observe for dissolution. Urea can break down the hydrogen bonds of the early clot but not the covalent bonds created by FXIII. Hypofibrinogenemia and dysfibrinogenemia will result in false-positive results. Urea is more specific, while monochloroacetic acid is more sensitive. In patients with FXIII levels <1%, the clots will dissolve. The clot solubility test is limited to detecting patients with severe FXIII deficiency and is not recommended as the initial diagnostic test if possible. Although not deemed sensitive, some labs still favor clot solubility due to its simplicity.

Ammonia is released during the transglutaminase reaction. Another testing mechanism is to measure FXIII activity by automatic ammonia release assay, which involves using ammonia as an indirect measurement of transglutaminase activity. 

The International Society for Thrombosis and Hemostasis recommends beginning with an FXIII activity level followed by an FXIII antigen level if the FXIII activity levels are low.[6][8] Factor XIII antigen levels will help differentiate acquired from congenital forms of FXIII deficiency. A mixing study then helps to determine the presence of an FXIII inhibitor. If an inhibitor to FXIII is suspected, clot-based inhibitor assays can only detect neutralizing antibodies. Binding assays, like the sodium-dodecyl sulfate-polyacrylamide ( SDS-PAGE), are necessary to determine the presence of non-neutralizing antibodies.[9]  

Pathophysiology

The two forms of FXIII deficiency are congenital and acquired.

Congenital Factor XIII Deficiency

Congenital FXIII deficiency is a rare autosomal recessive bleeding disorder. The cause of congenital factor XIII deficiency is a genetic mutation in subunit A of FXIII. Heterozygotes will likely show no symptoms. Homozygotes may present with spontaneous or provoked bleeding in the CNS, joints, or during surgery. Other possible indicators include recurrent pregnancy loss, heavy menstrual bleeding, and delayed detachment or bleeding of the umbilical cord. Due to consanguinuity, patients in Iran have a high rate of rare bleeding disorders, especially FXIII deficiency. While the ideal diagnostic method begins with factor activity measurement, the cost and small number of patients make these tests unavailable in many parts of the world. Patients undergo a PT, aPTT, INR, platelet count, and bleeding time first. If those are normal, then a clot solubility test is next. Molecular diagnosis is not available in many parts of the world. As aforementioned, severe, life-threatening bleeding is often the characteristic clinical presentation.[10] The prevalence of congenital FXIII deficiency in the general population is 1 per 2 million. Worldwide, FXIII deficiency is most prevalent in the Khash region of southeastern Iran.

Acquired Factor XIII Deficiency

Causes of acquired FXIII deficiency include immune-mediated inhibition, nonimmune FXIII heightened consumption, and lack of synthesis. Acquired deficiency can be due to idiopathic causes, autoimmune diseases like lupus, and malignant conditions.[11][12] Researchers have identified autoantibodies that inhibit the cross-linking of fibrin monomers.[13] Anti-IL6 receptor antibody treatment, like that received by patients with rheumatoid arthritis, has been shown to reduce FXIII activity.[14] The exact mechanism is unclear, but researchers believe suppressed FXIII-A expression is the cause.

The most common cause of death associated with FXIII deficiency is hemorrhage, often intracranial. Due to the rarity of acquired FXIII deficiencies, extensive literature and analytical information are lacking. As research continues to evolve, there have been reports of acquired deficiency in several medical conditions, such as stroke, IBD, liver cirrhosis, and disseminated intravascular coagulation. Factor levels in these medical conditions can drop by 20% to 70% through consumption of FXIII or decreased synthesis.[15]

Clinical Significance

The transglutaminase activity of FXIII is involved in wound healing, embryo and uterus interactions, angiogenesis, and bone extracellular matrix stabilization.[5] Deficiency of fibrinogen and FXIII can result in recurrent pregnancy loss, indicating their importance in placental stability.[12]

While intracranial hemorrhages are the most common overall cause of death, delayed umbilical cord bleeding, seen in up to 80% of neonates, may be the earliest presenting symptom of Factor XIII deficiency. Other common manifestations of FXIII deficiency are bruising, hematomas, muscle bleeds, and delayed post-surgical bleeding.

Common clinical signs of coagulation factor deficiencies include mucosal bleeding, bleeding before and after medical procedures, and hemorrhagic diathesis. Fairly specific to low levels of Factor XIII and Factor X and afibrinogenemia, more severe signs include intracranial bleeding and hemarthroses.[16] 

With the current advancement in treatment for rare factor deficiencies, acutely presenting FXIII-related hemorrhage treatment includes the administration of FXIII concentrates to stop blood loss. Prednisolone and cyclophosphamide combat anti-FXIII antibodies.[17] Plasma exchange can be beneficial in the short-term management of FXIII inhibitors.[18] Long-term prophylaxis includes platelet-derived or recombinant FXIII along with cryoprecipitate. The plasma half-life for FXIII is between 11 and 14 days, so regular administration of blood products is often necessary for long-term care.[19] Women with FXIII deficiency can experience recurrent miscarriages and menorrhagia, while in men, fertility can be affected. Oral contraceptives or antifibrinolytics can help manage menorrhagia.[20]

Other biological functions of factor XIII are under consideration beyond a "fibrin stabilizing factor."