A case of acquired FXIII deficiency with severe bleeding symptoms

Acquired factor XIII (FXIII) deficiency due to an autoantibody against FXIII is a very rare, yet potentially life-threatening bleeding disorder. As the standard coagulation tests (prothrombin time and activated partial thromboplastin time) are normal, the specialized tests are required to make an accurate diagnosis. Here, we report a case of acquired FXIII deficiency with severe bleeding symptoms. A 75-year-old man was referred to our hospital because of severe bleeding tendency after a tooth extraction. Laboratory findings showed that routine coagulation studies were normal, but factor XIII (FXIII) activity was low (3%). The presence of FXIII inhibitor was detected with dot blotting studies. Although the bleeding tendency was very severe, it was successfully controlled by infusion of FXIII concentrates combined with immunosuppressive treatment (oral prednisolone). Fibrin cross-linking study showed the significant delay of the γ-chain dimer and α-chain polymer formation. Western blotting revealed the marked decrease in FXIII-A level. The mixing study of FXIII activity measured using amine-incorporation assay showed the incomplete inhibition pattern. There seems to be little agreement as to the treatment strategy of acquired FXIII deficiency. In this patient, the use of FXIII concentrates was very useful in the initial treatment of bleeding symptom. The use of steroids was also effective in increasing FXIII activity without any serious complications.     

Hemorrhagic-acquired factor XIII deficiency associated with tocilizumab for treatment of rheumatoid arthritis

Factor XIII (FXIII) is the final enzyme in the coagulation cascade. Acquired FXIII deficiency is caused by inhibitors of FXIII or decreased synthesis and/or increased consumption of FXIII, which leads to severe bleeding. Recently, we experienced a case of hemorrhagic-acquired factor XIII deficiency that occurred during treatment with the IL-6 inhibitor tocilizumab for rheumatoid arthritis. A 48-year-old man was referred because of right hip pain due to a hematoma. Laboratory findings showed that routine coagulation tests were normal, while FXIII activity was slightly low (52.4 %). The patient was successfully treated with plasma-derived factor XIII concentrates. The time course of recovery suggests that tocilizumab might have inhibited FXIII production. To our knowledge, this is the first report of acquired factor XIII deficiency associated with administering of tocilizumab. When recurrent bleeding is seen during administering of tocilizumab, acquired factor XIII deficiency may have been induced, thus attending physicians should consider this disease in a differential diagnosis.     

Acquired FXIII inhibitors: a systematic review

Coagulation factor XIII (FXIII) is a protein that promotes fibrin stabilization by forming multiple covalent cross-links between fibrin monomers. Beside congenital FXIII deficiency, due to FXIII gene mutations, severe acquired FXIII deficiency has been described in association with autoantibodies against coagulation FXIII. These inhibitors, which occurs very rarely but may cause life-threatening bleeding complications, may arise spontaneously or in association with autoimmune and lymphoproliferative disorders or medications. The management of patients with acquired FXIII inhibitors is very demanding and treatment regimens must be focused on eradication of the inhibitor and to increase the plasma FXIII levels. In this systematic review, we analyse all the published case-reports on anti-FXIII autoantibodies focusing on the clinical features and treatment modalities of this acquired hemorrhagic condition.     

Defective α2antiplasmin cross‐linking and thrombus stability in a case of acquired factor XIII deficiency

Acquired factor XIII (FXIII) deficiency is a rare and life‐threatening condition that is often misdiagnosed or missed completely. A 72‐year‐old woman presented with symptoms of major unprovoked bleeding but routine coagulation screening tests and platelet count were normal. Low activated FXIII (FXIIIa) activity levels and abnormal urea clot stability led to diagnosis of acquired FXIII deficiency. A modified Bethesda inhibitor titre of 1.6 Bethesda units/ml indicated the presence of a FXIII inhibitor. Bleeding responded to a single dose of FXIII concentrate and immunosuppression with prednisolone induced remission. A subsequent relapse was treated with combined prednisolone and Rituximab resulting in a prolonged, ongoing remission. Here we analyse the mechanisms underlying this idiopathic case of acquired FXIII deficiency. Prospective analysis of patient plasma revealed minimal FXIIIa activity and antigen in presentation and relapse samples. Thrombi formed from these samples lysed rapidly and showed an absence of cross‐linked α₂AP. Western blotting revealed the presence of FXIII‐B, indicating only FXIII‐A and FXIII‐A₂B₂ were affected. FXIII activity and antigen levels normalised on remission. Our data suggest the presence of inhibitor‐induced clearance of FXIII from plasma. As a consequence, reduced thrombus stability was evident due to defective α₂AP cross‐linking, thereby explaining symptoms of excessive bleeding.     

Acquired factor XIII inhibitor: clinical features,treatment, fibrin structure and epitope determination

Summary. Acquired factor XIII (FXIII) deficiency, arising from an autoantibody against factor XIII, is a rare bleeding disorder. This autoimmune disorder most commonly occurs in the elderly. Patients who develop such acquired FXIII inhibitors may present with catastrophic bleeding events and are hard to be diagnosed with the normal general coagulation tests. Though the disease is relatively rare, it is known to cause significant mortality. In this article we briefly describe a patient who presented with extensive bleeding and a normal activated partial thromboplastin time and prothrombin time (PT), but had an acquired inhibitor to FXIII; her primary disease was systemic lupus erythematosus (SLE). Also, we will focus on the clinical features, treatment modalities, fibrin structure and epitope identification for acquired factor XIII inhibitor with a review of the literature.     

Hemorrhagic disorder due to an isoniazid-associated acquired factor XIII inhibitor in a patient with Waldenstr?m's macroglobulinemia

A case is described of a 75-year-old woman with a history of pulmonary tuberculosis and Waldenstr?m's macroglobulinemia who developed an inhibitor of coagulation factor XIII while taking isoniazid. The patient presented with a subcutaneous hematoma of the abdominal wall that extended from the xiphoid process to the symphysis pubis and measured 20 cm in diameter. Results of routine coagulation studies were normal with the exception of an increased solubility of the patient's plasma clot in 5M urea consistent with a deficiency of factor XIII activity. Persistence of the deficiency following a 1:2 dilution of the patient's plasma in normal plasma indicated the presence of an inhibitor. A sample of the patient's plasma was depleted of IgG by streptococcal protein G adsorption. The IgG-depleted plasma did not inhibit factor XIII activity, indicating that the inhibitory activity was not attributable to the underlying IgM paraprotein. The patient's purified IgG, on the other hand, inhibited factor XIII activity and the inhibitory activity could be neutralized by anti-IgG antibody. The patient's IgG also inhibited factor XIII-mediated incorporation of fluorescent monodansylcadaverine into casein. Binding of the patient's IgG to factor XIII concentrate was demonstrated by enzyme-linked immunosorbent assay and the IgG that bound to the factor XIII was demonstrated to be polyclonal. Isoniazid was discontinued after the patient was admitted to the hospital. Cryoprecipitate infusion controlled bleeding and reduced the inhibitor titer by 50%. Treatment with cyclophosphamide and prednisone, followed by extracorporeal immunoadsorption over a staphylococcal protein A column, did not reduce the inhibitor titer further. Plasma exchange therapy reduced the inhibitor titer to undetectable levels but failed to restore factor XIII activity. Infusions of factor XIII concentrate reproducibly restored factor XIII activity and were not associated with an anamnestic rise in the inhibitor titer. This represents the seventh reported case of an acquired inhibitor to factor XIII associated with the ingestion of isoniazid.     

Successful delivery in patients with FXIII deficiency receiving prophylaxis: report of 17 cases in Iran

Summary. To prevent pregnancy loss in women with severe FXIII deficiency, prophylactic replacement therapy with a source of FXIII throughout pregnancy is essential. The aim of this study was to evaluate the bleeding score and rate of successful deliveries in FXIII‐deficient pregnant Iranian women receiving regular prophylaxis. Seventeen FXIII‐deficient women 18–35 years old (mean 24 years) were enrolled in the study. All patients except one had a history of at least one miscarriage. Patients received regular prophylaxis with 10 IU kg^?1 FXIII concentrate every 4 weeks before pregnancy and every 2 weeks during pregnancy for a period of 24–62 months. All bleeding episodes were recorded, and the bleeding score was determined on a standard form before and after the start of prophylaxis. After starting prophylaxis, monochloroacetic acid tests and 5 m urea tests were normal in all patients, and the bleeding score significantly decreased from 11–16 (mean 12 ± 1.5) to 23 (mean 2.2 ± 0.4) (P < 0.001). Thirteen minor bleeding episodes occurred during prophylaxis. All patients successfully delivered at 36 weeks’ gestation and there were no significant coagulation complications during or after delivery. In this study, successful pregnancy maintenance and delivery were achieved in Iranian women with severe FXIII deficiency. Precise detection and diagnosis of this condition in women with coagulation disorders is essential to enable implementation of appropriate prophylaxis to prevent pregnancy loss.     

Pharmacokinetics of recombinant factor XIII in young children with congenital FXIII deficiency and comparison with older patients

Congenital factor XIII (FXIII) deficiency is a rare bleeding disorder, which in its severe form is associated with a significant bleeding phenotype, requiring regular prophylactic therapy. A recently developed recombinant FXIII (rFXIII) has demonstrated safety and efficacy in children aged ≥6 years and adults (mentor?1 trial). This article describes the mentor?4 trial, which has assessed the pharmacokinetics (PK) and safety of rFXIII in younger children (1 to <6 years) with congenital FXIII deficiency, and compares extrapolated PK parameters with the mentor?1 trial. Six children with congenital FXIII A‐subunit deficiency received a single, 35 IU kg^?1 rFXIII dose. PK properties were similar in all the children, with a mean area under the concentration vs. 30‐day time curve of 248.6 IU h^?1 mL^?1, maximal FXIII activity (30 min) of 0.67 IU mL^?1, and mean 30‐day trough of 0.21 IU mL^?1. All patients maintained FXIII activity above the lower target level (0.1 IU mL^?1). rFXIII half‐life was 15.1 days (range, 10–25). No safety findings of clinical concern were observed. PK properties of rFXIII were similar in patients from both trials. The study demonstrated that a single dose of 35 IU kg^?1 rFXIII maintained plasma FXIII levels above 0.1 IU mL^?1 over a 30‐day period in young children with congenital FXIII deficiency, and is, therefore, likely to provide adequate prophylaxis in this age group. The study extends the previous findings of the mentor?1 trial and confirms that no dose adjustment is required for different age groups with congenital FXIII deficiency.     

Acquired isolated factor VII deficiency associated with severe bleeding and successful treatment with recombinant FVIIa (NovoSeven).

Acquired isolated FVII deficiency not due to vitamin K deficiency or liver disease is rare and often associated with severe bleeding. We present a case of transient acquired factor VII deficiency associated with major bleeding, successfully treated with twice daily intermittent intravenous recombinant activated factor VII (rFVIIa) (NovoSeven; Novo Nordisk). The severe transient reduction in factor VII coagulant activity (FVII:C) levels, unresponsive to fresh frozen plasma and vitamin K administration, raise the possibility of an acquired inhibitor to factor VII. However, no inhibitor to factor VII could be demonstrated using protein G sepharose adsorption, or a Bethesda assay using IgG purified from patient plasma. There are few reports of the use of rFVIIa in this setting and this case suggests that rFVIIa is effective therapy, and should be considered early when acquired factor VII deficiency is associated with severe bleeding.     

A common F13A1 intron 1 variant IVS1+12(A) is associated with mild FXIII deficiency in Caucasian population

Mild factor XIII deficiency is an underdiagnosed coagulation disorder. Considering the large number of coding and non-coding polymorphisms identified in the F13A1 gene, there is a possibility that some of these might result in alterations of plasma FXIII levels and cause mild FXIII deficiency. Recently, a homozygous F13A1 gene intron 1 variant (IVS1+12C>A) was found in a patient with FXIII deficiency. In vitro expression studies for this variant demonstrated its lowering effect on FXIII levels. In order to determine the impact of this variant on a population level, we analysed the prevalence of this variant in three clinically and genetically defined population cohorts: an apparently healthy control cohort C1 (n?=?102), a mild FXIII deficiency cohort C2 with no detectable F13A1 or F13B gene mutations (n?=?183) and a mild FXIII-A deficiency cohort C3 exhibiting heterozygous F13A1 mutations (n?=?37). FXIII activity was determined using photometric assay on plasma samples. The F13A1 gene intron 1 variant was analysed by direct sequencing. The C1 cohort showed a normal distribution of FXIII activity (mean 114.1?±?20.86 %). Mean FXIII activity levels for the C2 and C3 cohorts were 54.45?±?11.12 % and 44.21?±?10.16 %, respectively. The frequencies of minor allele (A) were 0.07 in C1 cohort, 0.19 in C2 cohort and 0.11 in C3 cohort. The difference in minor allele frequencies for the C1 and C2 cohorts were highly significant (p?<?0.001). The greater frequency of the IVS1+12(A) variant among C2 cohort patients suggests that this polymorphism is associated with mild FXIII deficiency.     

A child with acquired factor XIII deficiency: case report and literature review

Factor XIII (FXIII) deficiency is a rare bleeding disorder, which can result in life threatening hemorrhage. Rarer still is acquired FXIII deficiency, in which the disorder is due to autoantibodies that inhibit the factor. To describe one of the youngest reported patients with this condition. To discuss the challenges we encountered in monitoring response with the available assays. To review the literature and provide a review of all acquired FXIII cases. We present the case of our patient, a 9‐year‐old girl with acquired FXIII deficiency. We present a comprehensive review of all acquired FXIII deficiency cases reported globally in English, with focus on clinical presentation, diagnostic assays, treatment and prognosis. There is no current standard for therapy and measuring response to therapy can be complicated by limitations of assays in the presence of inhibitors. Clinicians should be aware of acquired FXIII deficiency as a potentially life threatening bleeding disorder even in young children. The case presented illustrates a young patient with acquired FXIII deficiency with a good clinical response to cryoprecipitate and difficulty in hemostasis monitoring utilizing clinically available assays.     

Congenital factor XIII deficiency in Switzerland: from the worldwide first case in 1960 to its molecular characterisation in 2005

Coagulation factor XIII (FXIII) has a major role in the final stage of blood coagulation, is important for wound healing and maintaining pregnancy. Severe congenital FXIII deficiency is a rare disorder with 1 patient in 1-3 million. Untreated, it causes bleeding events, with intracranial haemorrhage being the major cause of death, impaired wound healing, and abortion. FXIII deficiency was traditionally diagnosed using the clot solubility test, but quantitative FXIII activity and antigen assays are preferred today. Treatment consists of replacement therapy with FXIII concentrates administered every 4-6 weeks. The molecular-genetic causes of FXIII deficiency are mutations in the genes coding for the FXIII A- and B-subunits. More than 60 mutations distributed throughout the FXIII A-subunit gene have been identified so far and 4 mutations in the FXIII B-subunit gene. The first case of congenital FXIII deficiency was reported in Switzerland in 1960. In Switzerland we observed a disproportionately high incidence, which can be explained in part by a founder effect. In this article, we summarise general facts on severe congenital FXIII deficiency, and we characterise all FXIII deficient patients living in Switzerland, including the first case described in 1960 who is a member of a large family originating from the canton of Uri.     

Development of antibodies to thrombin and factor V with recurrent bleeding in a patient exposed to topical bovine thrombin

A 65 year old patient who was exposed to topical bovine thrombin during cardiac surgery developed markedly prolonged clotting times and a severe bleeding diathesis. Mixing studies with normal plasma failed to correct the clotting times. Platelet transfusions, immunosuppressive and immunomodulatory therapies were ineffective, but plasmapheresis was effective in decreasing clotting times and in the resolution of clinical bleeding events. The patient's purified IgG reacted with bovine thrombin by immunoblotting and enzyme-linked immunosorbent assay (ELISA). However, the IgG reacted minimally with human thrombin. In view of the severe bleeding, a coexisting inhibitor was sought. The patient's factor V activity was 1% of normal and was not corrected by mixing with normal plasma, demonstrating the presence of an inhibitor against factor V. The patient's IgG reacted with both bovine and human factor V. Immunoblotting localized the site of antibody binding to the light chain of activated bovine factor V. Detectable amounts of bovine factor V were found in commercial bovine thrombin preparations by ELISA. The data suggest that patients exposed to topical bovine thrombin may develop antibodies to thrombin and factor V. Anti-thrombin antibodies may mask coexisting factor V inhibitors responsible for clinical bleeding.     

Severe bleeding complications caused by an autoantibody against the B subunit of plasma factor XIII: a novel form of acquired factor XIII deficiency

Acquired factor XIII (FXIII) deficiency due to autoantibody against FXIII is a very rare severe hemorrhagic diathesis. Antibodies directed against the A subunit of FXIII, which interfere with different functions of FXIII, have been described. Here, for the first time, we report an autoantibody against the B subunit of FXIII (FXIII-B) that caused life-threatening bleeding in a patient with systemic lupus erythematosus. FXIII activity, FXIII-A(2)B(2) complex, and individual FXIII subunits were undetectable in the plasma, whereas platelet FXIII activity and antigen were normal. Neither FXIII activation nor its activity was inhibited by the antibody, which bound to structural epitope(s) on both free and complexed FXIII-B. The autoantibody highly accelerated the elimination of FXIII from the circulation. FXIII supplementation combined with immunosuppressive therapy, plasmapheresis, immunoglobulin, and anti-CD20 treatment resulted in the patient's recovery. FXIII levels returned to around 20% at discharge and after gradual increase the levels stabilized above 50%.     

Bleeding tendency caused by IgG inhibitor to factor XIII, treated successfully by cyclophosphamide

A case exhibiting bleeding tendency caused by an acquired inhibitor to factor XIII is reported. The patient, a hitherto healthy 87-year-old Japanese man, presented with a massive subcutaneous bleeding, leading to severe anaemia. The routine coagulation study was normal except for a decreased plasma factor XIII level, which was 3% of the control level by the dansylcadaverine incorporation assay. An inhibitor of factor XIII was demonstrated to be present in the IgG fraction of the patient's plasma; by immunoblotting this inhibitor was shown to bind specifically the a and a' subunits of factor XIII. The IgG fraction suppressed the transglutaminase activity of activated factor XIII, but did not inhibit the molecular transformation of subunit a to a' in the activation process. Massive infusion of plasma and factor XIII concentrate was effective for controlling the bleeding temporarily. In the long-term prednisolone was ineffective for suppressing the plasma inhibitor level and bleeding episodes recurred. A small daily dose (50 mg) of cyclophosphamide, however, effectively decreased the inhibitor level and controlled bleeding.     

Efficacy and safety of prophylactic treatment with plasma‐derived factor XIII concentrate (human) in patients with congenital factor XIII deficiency

Congenital factor XIII (FXIII) deficiency is an extremely rare, potentially life‐threatening bleeding disorder. Routine prophylactic management is recommended for individuals with clinically relevant FXIII deficiency. This prospective, multicentre, open‐label study evaluated the long‐term efficacy and safety of prophylactic infusions of FXIII concentrate (human) 40 IU kg^?1 in patients with congenital FXIII deficiency. FXIII concentrate (human) was administered every 4 weeks for 12 months. Dosing was adjusted to maintain trough FXIII activity levels of 5–20%. Logistical and ethical constraints precluded use of a placebo control group. Annualized incidence of spontaneous bleeding was compared with historical rates; safety was assessed as a secondary objective. Forty‐one patients were enrolled and completed the study. The annualized rate for spontaneous bleeding episodes requiring FXIII treatment was 0.000 episodes per patient‐year (95% CI: 0.000; 0.097). The study met its primary endpoint: the upper limit of the 95% CI was substantially below the historical rate of 2.5 bleeding episodes per patient‐year. Five spontaneous bleeding episodes (involving three patients; none requiring FXIII treatment) and eight trauma‐related bleeding episodes (two requiring FXIII treatment) occurred. Five patients had surgery during the study, only one of whom required FXIII treatment for post‐surgical bleeding. Most patients (≥85%) had trough FXIII activity levels ≥10%. No patient discontinued treatment due to an adverse event. No adverse events related to thromboembolism or viral transmission were reported. Prophylactic treatment with FXIII concentrate (human) was well tolerated and prevented spontaneous bleeding episodes that were serious enough to require treatment with FXIII‐containing product. Clinical trial registration: www.clinicaltrials.gov/ct2/show/NCT00885742 .     

Arg77His and Trp187Arg are the most common mutations causing FXIII deficiency in Iran

The aim of this study was to review the literature for the genetic mutations causing inherited factoe XIII (FXIII) deficiency in patients from Iran, where the consanguineous marriage is common. Data were collected from 30 patients (18 males and 12 females) with FXIII deficiency, from 26 unrelated families. Data of mutation analysis were obtained from 2 previously published studies. A total of 7 mutations consisting of 5 new mutations and 2 previously reported mutations were identified. Of the 5 novel missense mutations, 2, Arg77His and Trp187Arg, were the most common in Iranian FXIII-deficient patients. In regions like Iran with high rate of consanguineous marriages, the identification of common mutations in disease like severe FXIII deficiency increases the capacity to make a precise screening and diagnosis assays to screen and diagnose families with high risk of FXIII deficiency for prevention of clinical complications in them.     

Factor XIII deficiency: complete phenotypic characterization of two cases with novel causative mutations

Coagulation factor XIII (FXIII) exists as heterotetramer (FXIII‐A₂B₂) in the plasma and as dimer (FXIII‐A₂) in cells. Activated FXIII mechanically stabilizes fibrin and protects it from fibrinolysis by cross‐linking fibrin chains and α₂‐plasmin inhibitor to fibrin. FXIII is essential to maintaining haemostasis, and its deficiency causes severe bleeding diathesis. Due to improper laboratory practices, FXIII deficiency is considered the most under‐diagnosed bleeding disorder. The aim of this study was to demonstrate in two cases how FXIII deficiency is properly diagnosed and classified, and to compare results of laboratory analysis and clinical symptoms. FXIII activity from plasma and platelets was measured by a modified ammonia release assay, while FXIII‐A₂B₂, FXIII‐A and FXIII‐B antigens were determined by ELISA. The exon–intron boundaries and the promoter region of F13A1 gene were amplified by PCR and the amplified products were analysed by direct fluorescent sequencing. FXIII‐A mRNA in platelets was determined by RT‐qPCR. Two children with severe bleeding symptoms were investigated. In both cases FXIII activity and FXIII‐A antigen were undetectable in the plasma and platelet lysate. In the plasma no FXIII‐A₂B₂ antigen was found, while FXIII‐B antigen was >30% in both cases. Proband1 was a compound heterozygote possessing a known missense mutation (c.980G>A, p.Arg326Gln) and a novel splice–site mutation (c.1112+2T>C). Proband2 was homozygote for a novel single nucleotide deletion (c.212delA) leading to early stop codon. The discovered mutations explain the severity of clinical symptoms and the laboratory data. Methods precise in the low activity/antigen range are required to draw valid conclusion on phenotype–genotype relationship.     

An unusual clinical presentation of factor XIII deficiency and issues relating to the monitoring of factor XIII replacement therapy

Congenital factor XIII deficiency is a very rare bleeding disorder. Patients with severe FXIII deficiency usually exhibit severe bleeding diatheses. Factor XIII is also involved in maintaining pregnancy, and women with factor XIII deficiency have a high risk of spontaneous abortions. We report here the case of a patient with a mild bleeding history before her pregnancy but who had three spontaneous haemorrhagic miscarriages. The patient was homozygous for G 501 R mutation of the factor XIII A subunit gene. We also detected a coinherited heterozygous factor V Leiden mutation, probably leading to a milder bleeding tendency. The patient had successful factor XIII replacement therapy throughout her fourth pregnancy. The efficacy of the factor XIII infusions was monitored using thromboelastometry and routine factor XIII measurements. This case report shows that factor XIII deficiency should be ruled out in patients with recurrent fetal loss but with a normal miscarriage workup, even in the absence of a history of severe bleeding since childhood. We also showed that thromboelastometry could be a valuable tool for the monitoring of factor XIII replacement therapy.     

Safety and pharmacokinetics of recombinant factor XIII-A2 administration in patients with congenital factor XIII deficiency

Congenital factor XIII (FXIII) deficiency is associated with a tendency for severe bleeding, a risk for spontaneous abortion, and a high rate of spontaneous intracranial hemorrhage. This phase 1 escalating-dose study was developed to evaluate the safety and pharmacokinetics of a single administration of human recombinant FXIII-A2 (rFXIII-A2) homodimer in adults with congenital FXIII deficiency. Pharmacokinetics and activity of rXIII and changes in endogenous B subunit levels were assessed. Recombinant FXIII-A2 homodimer were complexed with endogenous FXIII-B subunits to form an FXIII-A2B2 heterotetramer with a half-life of 8.5 days, similar to that of endogenous FXIII. The median dose response was a 2.4% increase in FXIII activity based on unit per kilogram rFXIII administered. After the administration of rFXIII-A2, clot solubility normalized as measured by clot lysis in urea. Clot strength and resistance to fibrinolysis, as assessed by thromboelastography, also improved. Safety reviews were conducted before each dose escalation; no serious adverse events, including bleeding or thrombosis, were noted during the study. In addition, there was no evidence of the generation of specific antibodies to rFXIII or yeast proteins. Recombinant FXIII appears to be a safe and potentially effective alternative for FXIII replacement in patients with FXIII deficiency.