The patients with a severe FXIII deficiency usually present in the neonatal period compared to those with mild FXIII deficiency. A family history, particularly a history of consanguineous marriage, must be obtained.[11] Congenital deficiency of FXIII can result in severe bleeding in the neonatal period.[12] Umbilical cord bleeding is reported in up to 80% of neonates and can occur up to 3 weeks after birth.[6] Intracranial hemorrhage (ICH) is reported in up to 30% of neonates with severe FXIII deficiency, which is far more common compared to hemophilia A and B.[13] Post-traumatic intracranial hemorrhage (ICH) can often be the first sign of FXIII deficiency in older children and can recur in a third of cases. Recurrent ICH is associated with higher mortality in patients with FXIII deficiency.[14] Deep and superficial hematomas/ ecchymosis, post-operative bleeding, and prolonged bleeding after trauma or surgery are commonly seen in the older age group with FXIII deficiency.[11] Women with FXIII deficiency can suffer from recurrent spontaneous abortions early in the course of pregnancy. In addition to these, a general tendency to bleed, poor wound healing, and menorrhagia are also commonly noticed.  

Acquired FXIII deficiency is rare. The presentation depends on the etiology. The majority of cases are not immune-mediated.[15] The patients with immune-mediated acquired FXIII deficiency usually presents with spontaneous bleeding in the subcutaneous or intramuscular compartment.[16] The patients are older (age more than 70 years) and usually have underlying comorbid conditions listed in the etiology section.[16] 

Laboratory evaluation for FXIII deficiency starts after a thorough history and physical exam. The FXIII comes into the play after fibrin has been generated. Hence, the traditional coagulation tests- prothrombin time (PT), activated partial thromboplastin time (aPTT), and international normalized ratio (INR), are all normal. The laboratory evaluations mainly involve clot solubility test, FXIII activity assay, FXIII antigen assay, inhibitor assay, and molecular diagnosis. Thromboelastography (TEG) is not standardized and varies amongst different institutions. 

1. Clot solubility test: It is easy to perform, inexpensive, and does not require any specific instruments. The sensitivity of the test depends on the clotting agents (thrombin or Ca2+) and the solubilizing agents (2% acetic acid, 1% monochloroacetic acid, and 5 mol/L urea). Multiple conditions can yield a false-positive result:

• Hypofibrinogenemia or dysfibrinogenemia- especially if urea is used as a solubilizing agent. It must be ruled out with a thrombin time, reptilase time, fibrinogen assay, and fibrinogen activity.[17]
• High pepsinogen- seen in gastric disorders. It can cause false-positive results if acetic acid is used.[18]

The clot solubility test is fraught with several limitations. It is neither sensitive nor specific and highly underestimates FXIII deficiency. It cannot detect mild or moderate deficiencies. Heterozygous carriers may not be detected. It is not standardized amongst different laboratories; hence it is not used in developed countries. However, in developing countries, where alternative assays may not be present, this test is still widely popular due to low cost. No standard guidelines exist on the use of the clot solubility test. An alternative strategy is to use two different assays, with different clotting and solubilizing agents, and run both the tests in parallel. If one of them is positive, then further investigations should be undertaken to evaluate FXIII deficiency.[19] 

2. Quantitative assays (Functional activity assays): If available, then these are first-line screening tests recommended. 

• Ammonia release assay: Quick and efficient, this test reports FXIII activity by measuring the photometric absorbance at 340nm wavelength. The FXIII activity release ammonia, which is used to convert NADH or NADPH to NAD+ or NADP+. One caveat is that there is also an FXIII independent release of ammonia, which can lead to overestimation of FXIII activity and hence report falsely high levels. Therefore, a 'plasma blank' is supplied in the kit, which must be used to correct the falsely high FXIII activity.[7]
• Amine incorporation assay: These tests use fluorescent, radiolabeled, or biotinylated amines covalently bound to a glutamine residue of the substrate are measured after unbound amines are released from the protein fraction. Although a more sensitive test than ammonia release assay, they are not standardized, lack validity, and are time-consuming.[7] 
• Isopeptide assay: This test is based on the isopeptidase activity of FXIII under certain conditions. Although this test does not cause overestimation of the FXIII levels, it can only detect the FXIII level up to 5%, making it less sensitive compared to other tests.

3. Immunological assays: This assay is used "after" FXIII deficiency is confirmed. It is useful to distinguish between FXIII subunit A, B, and FXIII-A2B2 complex. The assays cannot detect the rare forms of FXIII defect. For instance, the assay cannot detect the 'type II' defect, where although FXIII A subunit is present, it is not functionally active. Enzyme-linked immunoassays (ELISA) is the most widely used immunoassay. R-ELISA (Reanal-ker, Budapest, Hungary) is a one-step sandwich ELISA with excellent sensitivity. Electroimmunoassays and radioimmunoassays are used infrequently due to a lack of standardization and cumbersome procedure. 

4. FXIII inhibitor assays: These are tested in a patient suspected of having antibodies. There are two types of antibodies.[20][21]

• Neutralizing autoantibodies against FXIII subunit A- usually detected by mixing studies. 
• Non-neutralizing inhibitors: ELISA assays are used to detect IgG or IgM antibodies.

5. Genetic analysis: These are done only in select countries and institutions where the tests are available. The A subunit gene is located on chromosome 6 and contains 15 exons and 14 introns. Missense mutations are more common. Nearly 150 mutations in the A-subunit have been reported so far.[3] The B subunit gene is located on chromosome 1 and contains 12 exons and 11 introns. Nearly 16 mutations are identified in the B-subunit.[4] More than 1000 polymorphism exists in both subunits of FXIII that makes it impossible to map the entire gene in all patients. Evidence supports that the polymorphisms are ethnicity dependent.[3] Many countries and institutions deploy testing for polymorphisms, which are most common in their population.[22][23] 

Thromboelastography (TEG) is not standardized amongst different institutions. However, in vitro, studies suggest that whole blood aggregometry can help in diagnosing FXIII deficiency.[24] Currently, the evidence is building up in favor of TEG being a more sensitive test compared to the solubility tests.[25] However, prospective studies are needed to confirm this finding. 

The treatment options depend on the presentation of the patient.

• Perioperative management: With FXIII levels above 5%, it is generally presumed that no spontaneous bleeding will happen. However, bleeding may still occur if significant stress is applied (trauma, surgery, etc.). If a 'prophylactic' dose was received within 7 days, then it is acceptable to proceed with the surgery; otherwise, a dose of FXIII concentrate would be needed
• Severe congenital deficiency: In patients with severe (FXIII level less than 1%) and moderate (level between 1% to 4%), prophylactic therapy is advocated, especially if there is a history of intracranial bleed.
• Pregnant patients with FXIII deficiency: Replacement should start by five weeks of gestation to prevent miscarriage. FXIII concentrate is administered more frequently (every week) during the pregnancy to maintain a target of 5% to 10%, and a higher boost is given at the onset of labor to achieve a target above 30%. 
• Patients with acquired FXIII deficiency: In addition to instituting factor replacement therapy, the underlying disorder must be addressed. Where a mild inhibitor may disappear with steroid therapy only; a stronger inhibitor may require B-cell directed therapy.

Several options are available to treat patients with FXIII deficiency. The most commonly available product is cryoprecipitate, which has approximately 20% to 30% of the original FXIII of plasma.[26] Although cryoprecipitate has been withdrawn from many European countries due to safety concerns (like pathogen transfer), it is still available in the U.S., Canada, and many other countries.[27] Along with plasma, cryoprecipitate may be the only source of replacing FXIII in countries where other products are not available. Fresh frozen plasma (FFP) can also be administered to an acutely bleeding patient. The mean content of FXIII in FFP is 1.0 U/ml (0.5 to 1.5 U/ml).[28] Compared to FFP, cryoprecipitate has a higher enrichment of FXIII (approximately 15% to 33%).[28][29] However, the yield of one bag of cryoprecipitate is lower than that of a bag of FFP.[28] The half-life of FXIII is 6 to 19 days.[14][2] Hence, prophylaxis with a single dose of FFP at 10ml/kg would last 4 to 6 weeks. Similarly, cryoprecipitate can be administered at 1 bag per 10 to 20 kg every 3 to 4 weeks.[2]

The USFDA approved Virus-inactivated FXIII concentrate derived from human plasma in 2011 for prophylaxis and perioperative management of patients with congenital FXIII deficiency.[30][31] The plasma-derived product has both subunit A and B. This makes it a universally acceptable product that can control bleeding in patients regardless of mutation in the subunit A or B.[32] It is dosed at 40 units/kg to achieve an FXIII concentration of 5% to 20%.

Another product, a recombinant FXIII-A (rFXIIIA) subunit (catridecacog), was approved by the FDA in 2013.[13] This formulation is specific to patients with FXIII subunit A deficiency, who comprise an overwhelming majority of all patients with FXIII deficiency. In a multinational, open-label, single-arm phase 3 trial, rFXIIIA was administered on demand to 41 patients with congenital FXIII-subunit A deficiency. The reported annual rate of bleeding was much lower compared to historical controls (0.138 vs. 2.91 bleeds/patient/year, respectively).[13] The efficacy of prophylaxis with rFXIIIA in surgical patients was tested in the MENTOR-2 trial, where it was administered prophylactically at a dose of 35 IU/kg.[33] The mean annual bleeding rates (ABR) were reported at 0.043/patient-year, and the mean spontaneous ABR was at 0.011/patient-year. No patient withdrew from the study, and the drug was very well tolerated.[33]

All bleeding disorders, including factor deficiencies and platelet dysfunction syndromes, can mimic FXIII deficiency.

1. Conditions that can be distinguished based on an abnormal coagulation profile (prolonged PT or aPTT)
   • Hemophilia A and B- must be ruled out in any patient presenting with recurrent and delayed joint bleeds. 
   • Type III von Willebrand disease- The bleeding is severe due to severe factor VIII deficiency 
   • All other factor deficiencies that are rarely seen in clinical practice
2. Platelet disorders with severe bleeding phenotype - Can be distinguished using platelet aggregation studies, platelet morphology, and platelet number. All these tests are normal in patients with FXIII deficiency. 
   • Glanzman thrombasthenia
   • Bernard Soulier syndrome
   • von Willebrand disease (type 1 and 2)
3. Rare coagulation defects which present with deranged coagulation profile
   • Alpha-2 plasmin inhibitor deficiency
   • Plasminogen activator inhibitor-1 deficiency
   • Inherited afibrinogenemia and dysfibrinogenemia

Acquired factor XIII deficiency is also in the differential diagnosis of FXIII deficiency. The etiology behind acquired FXIII deficiency is discussed above.

Congenital deficiency of FXIII is an extremely rare disorder, and acquired FXIII deficiency is even rarer. Patients who receive replacement factors live a normal life span. There are no large scale studies to determine the rate of mortality. Intracranial hemorrhage is the most common cause of death in untreated patients.[21] 

Complications from congenital FXIII deficiency is seen in untreated patients. Intracranial hemorrhage is the major cause of death in untreated patients with FXIII deficiency. In neonates with congenital FXIII deficiency delayed-type, umbilical stump bleeding is the represents the first classic clinical sign. Other common sites of bleeding are muscles, subcutaneous soft-tissues.

All patients and families must be educated regarding the nature of the disease. All patients must wear alert bracelets, clearly indicating their diagnosis. They must be registered with appropriate tertiary care centers that can provide expert care at all times in the event of uncontrolled bleeding. All patients with severe FXIII deficiency must have education regarding various prophylactic strategies, especially if they have had an intracranial bleed previously. All women of childbearing age must be counseled regarding pregnancy. Healthcare providers, including primary care providers, must be educated regarding the nature of the disease. All providers must remain in close communication in the event of a bleeding episode.

FXIII deficiency is a rare bleeding disorder where patients present with a normal coagulation profile. A high index of suspicion is needed to diagnose FXIII deficiency. A strong family history of bleeding disorders, history of consanguineous marriage, or a history of belonging to high prevalence areas (like Iran) may be present. There is a mismatch between the severity of factor deficiency and clinical presentation. A clot solubility test usually makes a diagnosis. However, this is not a sensitive or specific test, and a lot of variations exist between different institutions. Quantitative assays are preferred screening tests but may not be available in resource-limited settings. Immunologic assays are used only to classify the diagnosis. Cryoprecipitate is used widely in the treatment and prophylaxis for patients with FXIII deficiency. Now a recombinant FXIII is also available for the same purpose. Acquired FXIII deficiency is extremely rare. However, it has been described in elderly individuals and/or in those with numerous comorbidities, especially auto-immune diseases.

FXIII deficiency is a rare disorder, and a high index of suspicion is needed. The evaluation begins by eliciting proper history and performing a complete physical examination. All elective surgeries must be planned in consultation with a hematologist to help control the bleeding. In emergencies like traumatic hemorrhage, hematologists must be consulted to assist with the management of bleeding. Transfusion medicine must be involved in making blood products available in a timely fashion. In the event of pregnancy, prospective parents from a consanguineous marriage must be screened for being carriers of FXIII deficiency. If found positive, then they should be counseled on the risk of congenital FXIII deficiency in the child. All pregnant women must be managed in a high-risk obstetrics clinic in consultation with a hematologist. The women with severe FXIII deficiency who get pregnant must be supported with FXIII supplementation to prevent loss of pregnancy. 

FXIII is a rare disorder, hence the evidence available for this article is Level 3 at best.