Laboratory and Molecular Diagnosis of Factor XI Deficiency

Abstract

The prevalence of factor XI (FXI) deficiency is 1 per 10 to 20,000 in the general population, much higher than that reported in most texts. The prevalence is higher in Ashkenazi Jews where it is about 1:20. Clinically, FXI deficiency presents as a mild bleeding disorder mostly associated with posttraumatic or postsurgical hemorrhages or unexplained minor bleeding. It is often discovered due to incidental finding of a prolonged activated partial thromboplastin time (aPTT) on routine laboratory screening. FXI deficiency is an autosomal recessive bleeding disorder with many causative F11 gene defects. Diagnosis is based on FXI activity, antigen levels, and molecular diagnostics. As FXI levels do not correlate with bleeding symptoms, identification of pathogenic genetic variants may be a more accurate predictor of bleeding risk and therefore aid in the clinical management of the patient. Two variants in the F11 gene account for most cases found in the Jewish and Arab populations. Patients with FXI deficiency can develop inhibitors to FXI although spontaneously acquired inhibitors are extremely rare. We will discuss laboratory and molecular assays used to diagnose FXI deficiency as well as interferences that can complicate diagnosis including new anticoagulants and acquired FXI inhibitors.

Keywords

factor XI deficiency - rare bleeding disorder - diagnosis - molecular - inhibitor

Factor XI (FXI) deficiency, also termed plasma thromboplastin antecedent deficiency or Rosenthal syndrome is a hemorrhagic disorder with variable bleeding diathesis that was first described in 1953 by Rosenthal.[1] Previously referred to as hemophilia C, that term is no longer used as the mode of inheritance and clinical features are very different from hemophilia A and B. As one would expect for an autosomal disorder, the prevalence is the same in males and females although women are more frequently symptomatic. Historical, and even more recent publications have estimated the incidence as approximately 1 per 1 million in the general population, but data from national registries indicate that it is much higher than this.[1] [2] [3] The UK National Haemophilia Database had nearly 4,000 registered cases of FXI deficiency in 2022 giving a population incidence of 1 in 17,000.[4] National registries that require patients to be registered for treatment purposes are more likely to be accurate than historical estimates. Using the same database, the incidence of hemophilia A is 1 in 5,000 males and hemophilia B is 1 in 25,000 males, consistent with global estimates. FXI deficiency is much more common than hemophilia B, rather belying its categorization as a “rare” bleeding disorder. The incidence in specific populations can now be inferred from genomic databases. The Genome Aggregation Database (gnomAD) gives a combined allele frequency for the two most common pathogenic variants of 1 in 24 in Ashkenazi Jews.[5] There are two main reasons why these more recent data sources give a much higher incidence than previously thought. Many patients with pathogenic F11 variants have mild bleeding symptoms or are asymptomatic in the absence of hemostatic challenges and would not have been identified in earlier studies. This is compounded by the poor availability of the diagnostic approaches discussed later in some health care systems. The high prevalence of a relatively rare disease in a specific population is a classic example of the “founder effect” that is associated with some variants.

Clinical Symptoms

Patients with FXI deficiency present with variable clinical phenotypes. Although this is an autosomal disorder with no difference in incidence between genders, women are much more frequently symptomatic than men. In the UK National Haemophilia Database, there are 50% more female registrants than males. Heavy menstrual bleeding, spontaneous bruising, and epistaxis are the most common symptoms, consistent with the role of FXI in hemostasis at mucocutaneous surfaces described earlier. Similarly invasive procedures at mucous surfaces, including dental extraction and those involving the gastrointestinal and urogenital tracts, are more frequently associated with bleeding than musculoskeletal operations. Postpartum hemorrhage and heavy menstrual bleeding occur in about 20% of affected women.[6] Although spontaneous bleeding is rare in this disorder, life-threatening bleeding after surgery or posttrauma may occur.[7] [8] [9]

FXI deficiency is usually detected after investigation of unexpected bleeding following hemostatic challenge, screening of relatives of an index case, or incidentally during routine preoperative blood tests.

One of the biggest issues in FXI deficiency is the poor correlation between factor IX (FIX) levels and bleeding symptoms. Those with variants in both alleles (homozygous or compound heterozygous) have a severe deficiency with FXI levels below 20 U/dL, while heterozygotes usually have FXI levels between 20 and 70 U/dL (mild deficiency).[10] While the bleeding tendency is more pronounced with severe deficiency, many patients are asymptomatic, including sometimes those homozygous for null variants with no detectable FXI. Heterozygotes are often asymptomatic throughout their life, even following major hemostatic challenges. The bleeding phenotype can vary significantly between individuals with the same level and within a pedigree.[8] There are various possible explanations for the poor correlation between FXI level and bleeding. FXI is not critical for hemostasis in the same way as thrombin, for example. This means that other hemostatic factors may ameliorate the phenotype. It may be that FXI in platelets, not measured by plasma assays, may rescue some patients with severe deficiency. Most likely a clot-based activity assay is not sensitive to all the functions of FXI in hemostasis. The effect on fibrinolysis, for example, is not measured by these tests. FXI antigen measurements and genetic variant detection do not correlate better with bleeding than activity assays. Attempts to find laboratory tests that correlate better with bleeding phenotype, particularly global hemostatic tests such as thrombin generation, have met with varying success as discussed below. For these reasons, utilization of all the available diagnostic tests, rather than relying on activity alone, in conjunction with an objective assessment of the personal and familial bleeding history offers the best opportunity for predicting future bleeding risk. Screening of relatives is therefore desirable. Nevertheless, it must be acknowledged that it remains difficult to predict clinical phenotype in those who have had a few hemostatic challenges such as children and younger males.

Laboratory Diagnosis of Factor XI Deficiency

FXI activation occurs via two routes. Firstly, contact activation via high-molecular-weight kininogen (HMWK) and factor XII (FXII). This route is now thought to be less important in vivo, although it forms the basis of FXI activation in the activated partial thromboplastin time (aPTT) assay. Secondly, and more importantly, from small quantities of thrombin generated by the tissue factor/factor VII (FVII) complex in the initiation phase of coagulation, the subsequent amplification phase enables thrombin to activate the intrinsic pathway through the conversion of FXI to FXIa. FXIa then cleaves FIX to FIXa, and then FIXa in complex with FVIIIa activates FX generating thrombin in higher concentrations.[11] FXIa therefore amplifies thrombin generation; however, it also reduces fibrinolysis via the activation of a thrombin-activated fibrinolytic inhibitor.[12] Deficiency of FXI reduces thrombin generation and does not limit fibrinolysis, both of which potentiate the bleeding disorder seen in these patients.

The normal range of FXI coagulant activity (FXI:C) is approximately 70 to 150 U/dL, although this can vary in the literature with some variance of ranges at the lower end of the normal range (e.g. 65–125 U/dL).[13] It is advisable to establish a normal range based on the local population, coagulation analyzer, and reagent configuration.

Diagnosis of Factor XI Deficiency

Diagnosis is made based upon unexplained bleeding episodes, family history, or presurgery laboratory work-up with an unexplained prolonged aPTT, correction in aPTT mixing studies with a normal prothrombin time (PT), thrombin time, and fibrinogen.

Some heterozygous patients have bleeding symptoms even though their FXI levels are in the normal range.[8] [14] In these cases, the diagnosis may only be made genetically.

Activated Partial Thromboplastin Time

The aPTT assay reflects the classical intrinsic coagulation cascade, namely activation of FXII via HMWK and pre-kallikrein, then FXII-mediated activation of FXI. Deficiency of any of these factors will therefore prolong the aPTT and FXII deficiency is relatively common in the general population of Europe.[15] The aPTT clotting end point in normal circumstances happens when approximately 5% of thrombin has been generated and does not incorporate feedback loops, therefore missing vital elements of further thrombin amplification.[16] In the current version of the coagulation cascade, the intrinsic system now forms a link between the initiation and amplification phases of coagulation via activation of FXI by thrombin generated via the tissue factor/VIIa complex.[1] [17]

The aPTT serves multiple roles in the coagulation laboratory, and it is therefore important to understand the sensitivity of the aPTT reagent to these competing needs. There is a wide variety of aPTT reagents available using different activators, ellagic acid, kaolin, or colloidal/micronized silica, and different phospholipid components varying between synthetic phospholipids and others from vegetable or animal sources and then these can be mixed in varying ratios within the reagent. These all influence the sensitivity of the aPTT reagent to factor deficiencies.[18] [19] The aPTT should be able to detect a factor level where it is clinically important, and bleeding has been observed.[20] For FXI deficiency, aPTT prolongation is suggested to be in the range of approximately 30 U/dL although partial deficiencies of approximately 20 to 60 U/dL may not be detected by different reagents. A study by Salloum-Asfar et al looked at two aPTT reagents, SynthASil and SynthAFax in patients defined as FXI heterozygous. The aPTT results for SynthASil were 41.9 ± 4.4 s (range 29.4–55.1 s) and FXI:C 42.8 ± 11.8% (range 11.4–73.1%). The results for SynthAFax were aPTT 32.9 ± 3.5 s (26.7–44.7 s) and FXI:C 48.4 ± 12% (range 12.0–83.3%). The same aPTT reagents were used in the FXI:C one-stage factor assays above.[21] They concluded that moderate FXI deficiency could be underestimated. This in part is due to the activator (SynthASil Colloidal Silica and SynthAFax Ellagic acid), and FXI and FXII levels that may contribute to a higher rate of false negatives. Their results suggest that between these two reagents, the Silica reagent was better at screening methods for FXI deficiency.

A study by Toulon et al looking at aPTT sensitivity to factor deficiencies showed that an aPTT for mild FXI deficiencies would need sensitivity in the range of FXI 40 to 50 U/dL.[18] This is based on previous observations of bleeding tendency in patients with FXI levels above 30 U/dL.[8] [14] This was achieved with the aPTT reagents used in the SynthASil, STA-CK Prest, and TriniCLOT aPTT HS automated aPTT assays.

A study by Lawrie et al, investigated aPTT reagents Actin FS and Actin FSL sensitivity to factor deficiencies using different coagulation-deficient plasmas from Technoclone, Siemens, and CryoCheck.[20] They found a range of cutoffs for Actin FS at FXI:C 52 to 71%. Actin FSL gave a lower range of FXI:C 25 to 50%, although Actin FSL is marketed specifically as a lupus-sensitive reagent. Another aPTT reagent with high sensitivity of detection for the intrinsic coagulation factors, and specifically the contact factors XI and XII, is Pathromtin SL (Siemens Healthcare GmbH). It is composed of vegetable phospholipids and a silicon dioxide activator. A correlation of r ²= 0.600 for STA-PTT Automate (Silica activator; STAGO) versus FXI activity was found across a range of FXI levels at 1 to 50% in a study by Kasonga et al.[22] This is not an exhaustive list of available aPTT reagents; however, it shows the variability of some commonly used reagents and their constituents and sensitivity to FXI.

The sensitivity of the local aPTT reagent can be assessed using the CLSI H47-A2 guidance document although some authors have argued against this due to the use of commercial deficient plasmas giving lower aPTT sensitivities than the use of patient plasmas.[23] [24] [25] Other methods to assess the sensitivity of aPTT reagents have been described by Favaloro et al[26] and Kershaw.[27] Essentially, they use the following methods. Determination of the factor deficiency sensitivity of an aPTT reagent by in vitro mixing studies (ex vivo can also be used if patient samples are available). Using the laboratory's current aPTT reagent(s), a commercial calibration plasma, and a factor-deficient plasma with <1 U/dL of the specific target factor (in this instance, FXI). Make a set of dilutions from 0 to 100% using the calibration plasma (100%) and the deficient plasma (0%), or a similar range, of FXI and perform aPTTs on them all to ascertain the factor level below which the aPTT is prolonged below the lower interval of the local reference range.

If the aPTT is prolonged, the normal routine screening tests including PT, aPTT, TT, and fibrinogen should be performed before FXI:C assays. This is to exclude any interference from heparin or other anticoagulants and lupus anticoagulants and exclude any consumptive coagulopathy. If these results are normal, an aPTT mixing study should be performed.

Activated Partial Thromboplastin Time Mixing Studies

If the aPTT is prolonged, a standard mixing test can be performed using equal parts of the patient's plasma and normal pooled plasma. The aPTT mixing studies should generally be correct when the aPTT is tested immediately. If a coagulation factor deficiency is present, a lack of correction would be indicative of an inhibitor.[28] There are a variety of ways of interpreting the aPTT mixing studies such as the Rosner Index. This subtracts the clotting time of the pooled normal plasma (PNP) from the clotting time of the 1:1 mix. This result is then divided by the clotting time of the patient sample. The equation is as follows: Rosner Index = (1:1 mix clotting time result − PNP clotting time result)/initial prolonged clotting time of the patient sample.

With this method, a high index value represents the possibility of an inhibitor. A low index value would represent a possible factor deficiency. For example, an index of 10% or lower indicates correction, 15% and above indicates no correction. If after the calculation is performed and a value of 10 to 15% is obtained, it is recommended that the test is repeated.

aPTT mixing studies can also detect inhibitors in FXI deficiency. The lack of aPTT correction after mixing of patient and control plasma suggests the presence of an inhibitor. An inhibitor assay is performed in a similar way as that for factor VIII inhibitors. Incubation is carried out for equal parts patient and control plasma at 37 °C for 30 minutes and compared to a control sample mixed with equal parts normal plasma and incubated for the same time. Further incubation of 2 hours can be performed if a time-dependent antibody is suspected. In the literature, incubation of 2 hours is stated for factor VIII inhibitors and not for other inhibitors, although there is a lack of consensus.[29] However, many individual case studies in the literature have used longer incubation periods of 2 hours.[30]

The One-Stage Activated Partial Thromboplastin Time-based Factor XI Activity Assay

A one-stage aPTT-based FXI activity assay is used for the measurement of FXI activity in plasma. In this assay, the FXI activity level is measured based on the ability of test plasma to correct or shorten the aPTT of FXI-deficient plasma. The one-stage factor assay is normally fully automated even on small coagulation instrumentation.

A minimum of three or more serial dilutions (1/10, 1/20, 1/40, etc.) of the standard plasma (a value of 100% of FXI) are prepared and mixed with an equal volume of substrate plasma (FXI-deficient plasma) and the aPTT is measured for each dilution. Then the aPTT values are plotted against the dilutions via the automated analyzer software. The test plasma (patient plasma) is also treated the same way as standard plasma (i.e., preparation of serial dilutions followed by mixing with substrate plasma). Following the addition of phospholipids and an activating agent, calcium ions are added to start the coagulation reaction, and the aPTT is measured. The results are then read from the calibration curve.[31] Since the substrate plasma lacks FXI, the difference between the dilutions of standard plasma and the test plasma is determined as the FXI activity level. The automated analyzers can also support the use of parallelism where the dilutions of the patient when plotted against the calibration curve should run parallel to each other. If they do not, then this may be due to a lupus anticoagulant or another coagulation factor inhibitor. See [Fig. 1] showing a standard curve, a normal, a mild FXI deficiency, and a patient with an FXI inhibitor that does not run parallel to the standard curve.

Fig. 1 A standard curve, a normal, a mild FXI deficiency, and a patient with an FXI inhibitor that does not run parallel to the standard curve. FXI, factor XI.

Chromogenic Factor XI Activity Assay

The chromogenic assay measures FXI:C in plasma based on the two-stage assay and can be fully automated.[32] In this assay, plasma is treated with acetone to destroy inhibitors against FXIIa and FXIa in the plasma, incubated for 15 minutes at 2 to 8 °C, and then kept on ice before being assayed. Then the contact system is activated with an activating agent (kaolin) for 60 minutes at 37 °C which results in activation of FXI by FXIIa. Following the activation step, the FXIIa is inhibited with corn trypsin inhibitor. Then the FXI level in the plasma is determined through the ability of FXI to cleave the chromogenic substrate (substrate–p-nitroaniline [pNA]) and release pNA. This can be measured photometrically at 405 nm; the intensity of the measured substrate is proportional to the FXI concentration.

The method can be used as a rating method, the change in optical density per minute, or as an end point assay where acetic or citric acid is used to stop the assay.

The optical density results at 405 nm are plotted against the diluted standard plasma at values of 0 to 150 U/dL. Patient optical density results are read from the calibration curve. On most coagulation analyzers, this is automatically performed by the analyzer. The formula to convert % to U/dL is FXI (U/dL) = % activity × potency of the standard.

The assay has a linear curve up to 150% FXI activity with a lower limit of detection at 5%.

The assay is registered for research use only.

Immunological Factor XI Assay

Although measurement of FXI antigen level is not a routine assay in most laboratories, it is necessary to distinguish between quantitative and qualitative defects of FXI deficiency. The concentration of FXI antigen in plasma, serum, or other biological fluids is estimated with an enzyme-linked immunosorbent assay (ELISA).

Current FXI:Ag ELISAs widely available are Affinity Biologicals, 5-Diagnostics, and Thermo Fisher (Invitrogen). The assays are for research use only and Affinity biologicals and 5-diagnostics calibrators are traceable to the World Health Organization (WHO) international standard for FXI antigen in human plasma and International Society on Thrombosis and Haemostasis, Scientific and Standardization Committee standard for FXI activity, respectively. There may also be other antigen assays available.

In the ELISA, a polyclonal antibody to FXI is adsorbed onto the wells of the 96-well microtiter plate. Following the dilution of the calibration plasma and the patient sample and addition to the microtiter wells, the FXI antigen in the plasma binds to the precoated antibody. After appropriate washing steps to remove any unbound material, the peroxidase-labeled detecting antibody is added and binds to the captured FXI. Following another washing step, a solution of TMB (the peroxidase substrate tetramethylbenzidine) is applied. TMB is catalyzed by Streptavidin–Peroxidase and the blue color products which change to yellow upon quenching the reaction by an acid, for example, 0.2 M sulfuric acid.

The color developed is measured spectrophotometrically at 450 nm. The absorbance at 450 nm is directly proportional to the quantity of FXI antigen captured in the plate.

Sample Requirements

Blood should be collected into trisodium citrate anticoagulant with a concentration of 105 to 109 mmol/L (3.1–3.2%) and a 1:9 ratio of blood:citrate. Samples taken into the wrong tubes or using the wrong sampling tube schedule can lead to a prolonged aPTT. For example, EDTA plasma contamination can reduce levels of factor V and VIII prolonging the aPTT and should be excluded. This also applies to serum and sodium heparin samples, which are also inappropriate.[33] [34] [35]

The samples should be processed within 4 hours of collection and centrifuged to obtain platelet-poor plasma with a residual platelet count of <10 × 10⁹/L. Centrifugation at 1,500 g for 15 minutes or 1,700 g for 10 minutes will achieve this. The samples can then be kept at −20 °C for 2 weeks or at −40 to −70 °C for 3 to 6 months. Frozen 1-mL aliquots should be thawed at 37 °C for 5 minutes and inverted gently before testing.[33]

Preanalytical Issues

The assays can be influenced by all the following preanalytical variables: hemolysis, lipemia, icterus, underfilling of the collection tube, the presence of clots, and the use of incorrect sample tubes.[34] Polycythemic samples with haematocrit >0.55 L/L can give spuriously prolonged aPTT results due to the incorrect ratio of citrate to plasma. Samples should be taken with the citrate concentration corrected for the plasma volume.[36]

Presence of Anticoagulants

Anticoagulants should be excluded in the investigation of an isolated prolonged aPTT. This includes vitamin K antagonists (although the PT would also be prolonged in this instance), unfractionated and low-molecular-weight heparin, direct oral anticoagulants including the direct thrombin inhibitor dabigatran and the anti-Xa agents rivaroxaban, edoxaban and apixaban.[37] In hospitalized patients, other anticoagulants should also be excluded, for example, fondaparinux which prolongs the aPTT by approximately 20% at standard therapeutic doses.

Differential Diagnosis

Differential diagnosis should exclude the presence of lupus anticoagulants, liver disease, disseminated intravascular coagulation, von Willebrand disease, and other coagulation factor deficiencies or factor inhibitors.[38]

The aPTT can be prolonged during inflammation by a raised C-reactive protein (CRP) and should be considered in inflammatory disorders. There is evidence that there is a direct interaction of CRP with the phospholipid in the aPTT reagent. Equally due to the inflammatory response raised factor VIII levels may confound some coagulation results when trying to interpret data.[39] [40]

Factor XI Inhibitors

Inhibitor formation against exogenous FXI is relatively uncommon in patients with FXI deficiency compared with hemophilia A. FXI inhibitors are polyclonal IgG alloantibodies against various epitopes of the FXI molecule and inhibit FXI activation. Importantly, the development of inhibitors is not associated with an increase in bleeding symptoms. Mostly the only impact is that replacement therapy is no longer effective.

The incidence of FXI inhibitors in patients with inherited FXI deficiency has been reported as 5% (7/118 patients). However, in a subset of patients from one study, 7/21 patients homozygous for the Glu117Stop nonsense variant developed an inhibitor after receiving FXI plasma products.[41] Only 14 other patients, including those with spontaneous inhibitors, have been reported in the literature between 1958 and 2000.[30] [42] FXI autoantibodies are rare but have been reported in a variety of malignant hematological conditions including Waldenström macroglobulinemia, chronic lymphocytic leukemia, and plasma cell leukemia.[43] [44] Other disorders in which FXI autoantibodies have been described are autoimmune diseases and pregnancy.[30] [45] FXI autoantibodies have also been described in a patient with coronavirus disease 2019 (COVID-19); this patient also had underlying Crohn's disease which may have also contributed to these findings.[46] Alloantibodies have been described in FXI-deficient patients who have received plasma treatment.[41]

When investigating the presence of an FXI inhibitor following the routine investigation of a prolonged aPTT, aPTT mixing studies and inhibitor screens should be performed as described above. Lupus anticoagulant testing should also be performed, and the results interpreted in context with the patient's clinical history. Factor VIII and IX assays should be performed along with FXI to ascertain if other factors are reduced, in the case of rare cross-reacting antibodies or interference in the FXI assay from extremely high-potency antibodies to other coagulation factors.[47] [48] Factor VIII is commonly raised if there is an underlying inflammatory process in patients with inhibitors influencing the aPTT.

A specific inhibitor assay should be performed (e.g., modified Nijmegen Bethesda assay).[49] FXI inhibitor titers are calculated by using PNP mixed 1:1 with patient plasma and incubated at 37 °C for 30 minutes or 2 hours if the inhibitor seems to be time-dependent. FXI activity in the patient 1:1 mix is then measured and divided by the FXI activity in a normal plasma sample also incubated at 37 °C for 2 hours. An inhibitor unit is the amount inactivating one-half of the FXI activity in the patient mixture.

Global Hemostasis Assays for Monitoring Treatment in Factor XI Deficiency

The bleeding tendency in patients with FXI deficiency is unpredictable irrespective of the patient's baseline FXI activity level. Treatment guidelines suggest targeting FXI activity levels of about 40 U/dL for major surgery. The use of the one-stage factor assay may still not accurately reflect the potential for bleeding in these patients. Global assays of hemostasis, thrombin generation assays (TGAs) including the calibrated automated thrombogram, ST-Genesia (STAGO), Technothrombin TGA Ceveron (Technoclone), and viscoelastic assays thromboelastography/rotational thromboelastometry have had limited use in FXI-deficient patients.[50] However, they may give a better indication of a treatment's effectiveness as clotting factor assays do not measure the full extent of FXI function. Due to the small sample sizes, variable FXI activity levels, conflicting results in different studies, and a lack of standardization of measurement parameters used and sample conditions, their usefulness remains debatable.[51]

TGA and thromboelastometry can be used to measure the efficacy of treatment with solvent–detergent fresh frozen plasma (SD-FFP) or FXI concentrate in patients with FXI deficiency undergoing surgery and monitoring the treatment with recombinant activated FVII (FVIIa).[51] [52] [53]

Some of the assay conditions do vary and should be taken into consideration either when setting up the assay in-house or reviewing published data. For example, the use of tissue factor in the TGA tends to vary in the literature from 5 to 0.5 pM. However, at TF levels of 1 pM, some authors have not found this to discriminate between bleeders and non-bleeders.[50] The ISTH scientific subcommittee recommends the use of TF concentrations of <1 pM as they do detect hypocoagulable patients, particularly VIII and IX, however, they suggest even lower TF concentrations may be required for FXI deficiency of 0.5 pM.[54]

Molecular Basis of Factor XI Deficiency

Heritable FXI deficiency is caused by variation in the F11 gene which is located at the distal end of the long arm of chromosome 4 (4q35). It consists of 15 exons and 14 introns that span a 23-kb region. This gene is expressed in hepatocytes and regulated via transcription factor hepatocyte nuclear factor-4a (HNF4-α). Blood mononuclear cells, granulocytes, pancreas, and kidneys also express F11 to a lesser extent. FXI deficiency is an autosomal hemorrhagic disorder with variable penetrance.[55] FXI circulates as a dimer in vivo and heterozygosity for some missense variants results in a deficiency through a dominant negative effect of one monomer subunit on the dimer. Heterozygosity for a null allele is more likely to be recessive as increased expression of the normal allele can compensate.

Most variants result in a quantitative type 1 deficiency in which both FXI:C and FXI:Ag levels are decreased.[2] [3] [38] In this type, the variant protein level is low or absent due to reduced translation, secretion, or stability of the protein. The protein produced is functionally normal. There are three subgroups in type I.

1. Nonsense variants that affect transcription resulting in decreased production of the polypeptide. Glu117stop is the commonest example.

2. Missense variants that disrupt dimerization through the two Apple4 domains. The polypeptide remains in monomeric form and is mostly retained in the cytoplasm. Phe283Leu is the commonest example.

3. Variants that cause the production of nonsecretable homodimers. Ser225Phe and Trp569Ser are examples.

In the much less common qualitative type II deficiency, the FXI:C is disproportionately lower than FXI:Ag. The protein produced is dysfunctional, so the activity is reduced compared with the amount of protein. A total of eight type II variants; five in the catalytic domain and three in Apple domains have been identified up to now.[56] [57]

Alloinhibitors have only been reported in patients homozygous for a null variant that abolishes FXI protein production completely and are then treated with FXI concentrate or fresh frozen plasma (FFP). Most of these patients are homozygous for Glu135Stop.[55] Compared with the overall prevalence of 3 to 5% for antibodies in patients with FXI deficiency, the rate is as high as 30% in patients homozygous for Glu135Stop.[58]

FXI deficiency in the Ashkenazi Jewish population is mostly due to two common variants referred to as the Jewish type II (Glu135Stop) and III (Phe301Leu) variants, respectively. This is not to be confused with the quantitative type I and qualitative type II deficiency described above. There are two other founder variants in Ashkenazi Jews: the type I donor splice site variant at c.1716 G > A and the type IV variant which is a 14-bp deletion at the exon 14/intron N splice site. The type II and III variants account for >90% of causative mutations in the Jewish population.[59] The type II variant is also found in Iraqi, Arab, and other Middle Eastern Jews. The type III variant is more frequently found in European Jews. Cys88Stop in the French Basque region and Cys128Stop in the United Kingdom are examples of common variants in other populations.[60] [61] There are currently 403 unique variants curated in the European Association for Haemophilia and Allied Disorders FXI variant database.[62] FXI mutations include missense (58%), nonsense (10%), frameshifts including splicing abnormalities (9%) and indels (1%; [Fig. 2]). Fifteen percent of single nucleotide variants occur in the noncoding space. Forty percent of variants occur in the catalytic domain and the frequency of variants in the other domains is shown in [Fig. 3].

Fig. 2 Relative frequency of different effects of the 403 unique F11 variants listed in the EAHAD Factor XI database.

Fig. 3 Relative frequency of unique variants in different factor XI domains in the EAHAD Factor XI database.

The F11 gene is relatively straightforward to analyze with conventional techniques as there are no interfering sequences, such as pseudogenes or GC-rich regions, that interfere with probe-based techniques. Dideoxy nucleotide sequencing and next-generation platforms are well-established with very high sensitivity for single nucleotide changes. Larger deletions can be readily picked up with read-depth analysis in next-generation platforms or using multiplex ligation-dependent probe amplification. However, variants in noncoding sequences remain more challenging for detection and interpretation. To date, no complex rearrangements have been described in F11 but these would not be detectable using standard genetic analysis methods.

Management of Factor XI Deficiency

Most patients with FXI deficiency have few bleeding problems during their life, while a few have major bleeding, determining whom to treat, when, and with what is challenging. The main therapeutic strategies in FXI-deficient patients are to raise FXI levels with replacement therapy or use adjunctive treatments to improve hemostasis without affecting levels. Factor levels can be elevated using FXI concentrate or FFP, preferably SD-FFP.[1] [63] Both are plasma products that have been associated with the transmission of bloodborne pathogens and can result in allergic reactions. The safety profile has been much improved with enhanced purification and viral inactivation steps. There have been no confirmed cases of viral transmission with FXI concentrate since the 1990s.[4] The advantages of FXI concentrate over FFP are shorter infusion times, a more predictable rise in FXI level, other coagulation factors not increased, and a lower rate of transfusion-related reactions. The half-life in vivo is about 45 hours and levels remain elevated for several days after a single infusion.[64] However, concentrate use has also been associated with thrombotic complications. This has been mitigated by the addition of anticoagulant proteins during concentrate manufacture and the use of lower doses aiming to raise the level into the range of a mild deficiency rather than the normal range.[65] There are no recombinant concentrates in development and so other ways of elevating FXI have been tried. Desmopressin has also been reported to slightly increase levels in some reports but not others, and so it is not used in clinical practice as it does not have a reliable effect.[66]

Improving the hemostatic defect without affecting the FXI level can be achieved with adjunctive or bypassing therapy. Recombinant factor VIIa is the main bypassing therapy that has been tried, particularly in patients with inhibitors. It has also been used in patients with severe deficiency without inhibitors wishing to avoid exposure to blood products.[53] Although being a recombinant product removes the risk of transfusion-associated infection, the thrombosis risk is higher than with FXI concentrate. The much shorter half-life of 2.5 hours means that dosing is required every 2 to 6 hours and the hemostatic effect is likely to fluctuate with peaks and troughs.

Antifibrinolytic agents such as tranexamic acid or ε-aminocaproic acid are the main adjunctive options. There has been the greatest experience with tranexamic acid which is effective in a wide variety of mild bleeding disorders and patients with bleeding without a preexisting hemorrhagic tendency. It has an excellent safety profile and does not increase the risk of thrombosis. Moreover, it can be given orally as well as parenterally and may be applied topically which is useful for treating oral cavity bleeding. It can be safely used in pregnancy and childhood.[67] [68] Although there have been reports of ureteric obstruction if used to treat bleeding in the kidney, this is not a problem with bleeding distal to the ureter.[1] Perhaps its main drawback is the short half-life of about 2 hours which means that optimal treatment requires dosing every 6 hours.[68]

The most common symptom that requires treatment is probably heavy menstrual bleeding. Tranexamic acid is effective but nonhemostatic treatments such as contraceptive pills are often tried first and may control the bleeding without any other treatment.[69]

It is now clear that if treatment is required for bleeding or to prevent bleeding during a hemostatic challenge, tranexamic acid alone is sufficient in the majority of cases including patients with severe deficiency. If the bleeding phenotype or procedural risk indicates a need for replacement or bypassing therapy, then FXI concentrates or FFP is administered. Recombinant FVIIa is mostly reserved for patients with inhibitors but it should be noted that tranexamic acid alone may also be effective in this group.[58]

Conclusion

FXI deficiency is a rare disorder with a variable bleeding tendency that is not correlated with factor activity levels. Identification of the causative genetic variant similarly does not predict the bleeding risk but can indicate the risk of inhibitor formation. Laboratory diagnosis is made primarily by prolonged aPTT with correction upon mixing with normal plasma. aPTT reagent sensitivity to FXI levels vary with different manufacturers reagents and this should be taken into consideration when selecting such screening tests. FXI one-stage factor assays can indicate the severity of FXI levels although they do not necessarily correlate with the bleeding phenotype. FXI antigen assays and two-stage chromogenic assays are also available; however, their use is limited in certain parts of the world due to availability or research use only status. Global hemostatic assays may correlate better with the bleeding phenotype but are not standardized. Effective and safe therapeutic options are available for the treatment and prevention of bleeding including replacement therapy with blood products. Most patients can avoid exposure to blood products by receiving antifibrinolytics.

Conflict of Interest

None declared.

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Publication History

Article published online:
04 November 2024