Introduction
Plasma protein coagulation factor XI (FXI) is the zymogen of the coagulation protease
FXIa, which contributes to physiological hemostasis and is involved in pathological
thrombosis.[1 ]
[2 ] Originally, FXI was considered part of the contact activation pathway. In presence
of negatively charged surfaces, prekallikrein, and high-molecular-weight kininogen,
FXII becomes activated (FXIIa) and activates FXI.[3 ]
[4 ] Activated FXI (FXIa) promotes thrombin generation (TG) and subsequent fibrin formation
through FIX activation. Contact activation is the basis for the activated partial
thromboplastin time (aPTT) assay. However, FXIa sustains in vivo TG independently
from FXIIa,[5 ] participating in the tissue factor (TF)-independent propagation phase of coagulation.[4 ] Here, FXI is activated in a positive feedback loop by small amounts of thrombin
generated in the initial phase of coagulation via TF/FVIIa complex. FXIa increases
TG through the activation of FIX ([Fig. 1 ]).[1 ]
[4 ]
[6 ] Of note, when FXII is activated in vivo by negative surfaces, such as polyphosphates,
collagen or nucleic acids, subsequent FXI activation promotes downstream coagulation.[2 ]
[3 ] FXI is primarily produced by hepatocytes[7 ]
[8 ] and circulates in human plasma as a complex with high-molecular-weight kininogen,[9 ] which helps the interaction of FXI with activated platelets[10 ] via glycoprotein (GP) Ib receptor.[11 ] FXI has a unique structure among the coagulation factors, since it is a disulfide-linked
homodimer consisting of two identical subunits of 80 kD. Each subunit is composed
of four apple domains (A1–A4) and one catalytic domain.[1 ] In addition to thrombin, FXI can be activated by FXIIa or by FXIa (autoactivation).[4 ]
[6 ] FXI is converted to the active form by cleavage after residue 369 (Arg369–Ile370),
which induces a conformational change in the catalytic domain, leading to the active
form FXIa.[12 ] The activation of FXI by thrombin or FXIIa goes through the formation of an intermediate
form, named 1/2-FXIa, in which only one of the two subunits is activated.[13 ] It has been proposed that the dimeric structure is necessary to localize FXI to
the platelet surface: the intermediate form 1/2-FXIa binds to GPIb receptor through
the A3 domain of the non-activated subunit, while the activated subunit would be free
to bind its substrate FIX.[13 ]
Fig. 1
Simplified scheme of coagulation cascade and fibrinolysis (adapted from Bouma et al[59 ] and Vandenbroucke et al[60 ]): Exposure of tissue factor (TF) to blood after vascular injury leads to TF–VIIa
complex formation. TF–VIIa complex activates factors X and IX to factors Xa and IXa
at the site of the injury, triggering the initiation stage of blood coagulation. Factor
Xa generates a small amount of thrombin and the first deposition of fibrin fibers
occurs (TF-dependent coagulation). Thrombin produced in this initial phase of coagulation
activates platelets and factors V, VIII, and XI with positive feedback loops, which
substantially increase thrombin production, leading to the second wave of thrombin
generation and fibrin deposition (amplification/propagation). This secondary burst
of thrombin is required for a normal hemostatic response and for regulation of the
fibrinolytic system. Thrombin stabilizes clots activating TAFI, which protects the
clot from fibrinolysis. Note that FXI can be activated either by FXIIa as part of
contact activation pathway (i.e., in vitro in the aPTT assay or in vivo in pathological
conditions[61 ]) or directly by thrombin in the TF-independent amplification/propagation phase of
coagulation in vivo. aPTT, activated partial thromboplastin time; TAFI, thrombin-activatable
fibrinolysis inhibitor; TF, tissue factor; TG, thrombin generation.
FXI has multiple in vivo roles in hemostasis, some of which are not fully elucidated.
These roles can be summarized in three different actions. (1) To sustain TG by enhancing
TF-independent propagation of the clotting cascade ([Fig. 1 ]).[4 ]
[6 ]
[14 ]
[15 ] (2) To downregulate fibrinolysis: the additional thrombin produced through the FXI
feedback-loop after clot formation activates thrombin-activatable fibrinolysis inhibitor
(TAFI), protecting clots from fibrinolysis.[16 ] (3) To increase procoagulant activity through inactivation of tissue factor pathway
inhibitor.[17 ]
FXI Deficiency and Bleeding
Reduced activity of FXI characterizes FXI deficiency, a mild to moderate bleeding
diathesis, previously known as hemophilia C. This rare autosomal bleeding disorder
affects both genders equally and has a frequency of 1:1,000,000 worldwide, with a
higher rate in people of Ashkenazi Jewish heritage (1:450).[18 ]
[19 ] In Europe, a recent large genetic study has reported a frequency of 12.9 affected
people in a million.[20 ] A large variety of mutations[21 ] has been reported within the F11 gene, which is localized on chromosome 4. The large majority of these mutations lead
to FXI deficiency type I, affecting both FXI activity and antigen, while few others
lead to FXI deficiency type II, affecting only FXI activity (FXI database: http://www.factorxi.org
[22 ]). FXI deficiency has a peculiar clinical interest: bleeding symptoms are very heterogeneous
among patients[23 ] and do not correlate with FXI residual plasma level or with genetic phenotype.[24 ] Differently from hemophilia A and B, in FXI deficiency spontaneous bleeding is uncommon
and mostly occurs after trauma or surgery, especially in tissues with high fibrinolytic
activity (oral, nasal cavity, and urinary tract).[24 ]
[25 ] Although bleeding tendency does not correlate with FXI coagulant activity (FXI:C,
normal range: 70–150 U/dL), FXI deficiency can be classified based on these values
as partial (FXI:C between 20 and 70 U/dL) or severe (FXI:C <20 U/dL).[26 ]
[27 ] Diagnosis of FXI is based on family history or presurgery laboratory workup and
completed by routine laboratory methods (prothrombin time [PT], aPTT, FXI:C) and in
addition by genetic analysis. Patients with FXI deficiency will usually have a prolonged
aPTT and normal PT.[28 ]
[29 ] However, patients with mild deficiency might have normal aPTT.[28 ] FXI-deficient patients are treated on demand or prior planned interventions and
current treatments consist of antifibrinolytics, desmopressin,[30 ] virus-inactivated plasma, and plasma-derived FXI concentrates.[31 ] The last one carries an increased risk of thrombosis and should be administered
with precaution.[29 ]
Laboratory Methods to Assess FXI Function
aPTT-based assays are not reliable for predicting bleeding tendency in FXI-deficient
patients[32 ] or thrombotic risk after FXI replacement therapy, because they provide a limited
view of the hemostatic system.[32 ] In fact, routine coagulation assays reflect only the amount of thrombin required
to signal the onset of clotting. The amount of thrombin needed to produce the initial
fibrin formation is very small (<5% of the total thrombin potential) and it is generated
during the initial phase of coagulation. The major burst of thrombin occurs in vivo
after the initial fibrin formation and it is used to increase and consolidate the
clot.[33 ] FXI coagulant activity (FXI:C) depends on the amount of FXI and—in case of preanalytical
coagulation activation—of FXIa present in plasma.[34 ]
[35 ] This amount is estimated by the ability of an unknown plasma to correct the aPTT
of a FXI-depleted plasma. The test is triggered by the contact activation pathway,
which generates FXIIa, leading to the activation of FXI to FXIa and TG. The endpoint
of the test is the formation of fibrin, which occurs rapidly, before achieving the
full TG potential. FXI plays a role in sustaining TG after clot formation as well,
increasing clot stability and resistance to fibrinolysis.[36 ] FXI activity as measured by aPTT-based assay does not reflect the physiological
activity of FXI and does not predict bleeding in FXI deficiency. This is explained
by the fact that in the aPTT assay FXI is exclusively activated by FXIIa as a part
of the contact activation pathway (a process which is not required in vivo for normal
hemostasis as demonstrated by the absence of bleeding diathesis in FXII deficiency[37 ]). In addition, the activation of FXI by thrombin, which occurs in vivo on platelet
surface, is not covered by this clotting test. Thus, it is unlikely that aPTT-based
FXI activity might reflect FXI's physiological role and correlate with bleeding phenotype.[32 ]
The lack of conventional assays able to discriminate FXI clinical phenotype has prompted
researchers to investigate whether global coagulation and/or fibrin clot formation
assays could be better tools for predicting bleeding risk ([Table 1 ]). Global coagulation assays measure physiological aspects of the coagulation process
(e.g., the amount of thrombin produced, the firmness/structure of fibrin clot or its
resistance to fibrinolysis) and consider several components of the coagulation cascade
rather than the plasma level of a single coagulation factor. Thus, global assays reflect
better organization of the hemostatic system in vivo, which reminds more of a network
than of separate pathways of activation.
Table 1
Global coagulation assay and FXI clinical phenotype/FXI replacement therapy
Author
Objective of the study
Cohort size
Non-bleeders vs. bleeders phenotyping criteria
Type of assay
Type of samples
Experimental conditions
Results/main conclusions
Bertaggia Calderara et al[44 ]
Assess the clinical utility of TG assay for identifying FXI bleeding phenotype and
monitor FXI replacement
24
Clinical history performed by experienced hematologists. Bleeding events related to
surgery (including tooth extraction) and injury.
B were those who had excessive bleeding and required treatment.
NB were those who did not experience excessive bleeding after procedures; three patients
without surgical challenges
a. TGA-CAT
PPP
TF 1 pM
PLS 4 μM
-TG significantly decreased in B vs. C
-TGA could not discriminate between B and NB
b. TGA/Clot formation assay
Thrombodynamics
PPP
TF 100 pmol/m2
CTI 0.2 mg/mL
-Combinatorial ROC analysis of TG and fibrin formation parameters allows discriminating
FXI-deficient B
-TD can assess the correction of the hemostatic potential after FXI replacement
Gidley et al[50 ]
Study the ability of clot formation, structure, and fibrinolysis assay to predict
bleeding in FXI-deficient patients
71 (58 PTs with CTI; 69 PTs without CTI)
Note: Some of the 71 samples were not analyzed for clot formation or fibrinolysis
assay
Prediagnosis experience related to tonsillectomy and or dental extraction. B were
those requiring blood transfusion or return to surgery/dentist for resuturing or packing.
NB were those who underwent uneventful procedures
a. Clot formation and fibrinolysis assay (turbidimetric test, 405 nm)
PPP
TF 0.5 pM
With CTI in tubes or without CTI
PLS 4 μM
t-PA 0.5 μg/mL and thrombomodulin 5 nM
-CTI-treated plasmas of FXI B have slower clot formation rate and lower resistance
to fibrinolysis compared to NB
-Fibrin network correlates with bleeding tendency
-Combinatorial ROC analysis reveals parameters associated with bleeding tendency
b. Fibrin structure analysis (confocal microscopy)
Alexa Fluor-488–fibrinogen 80 μg/mL, TF 0.5 pM
PLS 4 μM
Pike et al[57 ]
Assess if TGA and ROTEM can be used to monitor FXI replacement
(FXI concentrate or SD-FFP)
11 (3 PTs had FXI concentrate;
8 PTs had SD-FFP)
Not applicable
a. TGA-CAT (ex vivo pre- and post-infusion)
PRP
TF 0.5 pM
With CTI in tube
PLS 4 μM
-TG and clot formation were improved after SSD-FFP or FXI concentrate
-Global hemostasis assays (TGA and ROTEM) can be used to monitor FXI replacement
with SSD-FFP or FXI concentrate
b. ROTEM
Whole blood
TF 0.12 pM
Pike et al[58 ]
Compare the effect of two FXI concentrates on TG in major FXI deficiency
10
3
Not applicable
TGA-CAT (in vitro pre- and post-spiking)
TGA-CAT (ex vivo pre- and post-infusion)
PRP
TF 0.5 pM
With CTI in tubes
-TG significantly impaired in individuals with major FXI deficiency (FXI:C < 15 IU
dL −1) vs. C
-TG improved after spiking or infusion
-Small doses of FXI concentrate are required to normalize hemostasis in vitro
-In vitro results could be used to estimate in vivo response
Pike et al[49 ]
To assess clinical utility of Thromboelastometry in detecting FXI bleeding tendency
93
Prediagnosis experience related to tonsillectomy and or dental extraction
ROTEM
Whole blood
PRP
TF 0.12 pM
With CTI in tubes
- On whole blood, no difference between B and NB, or C and all PTs
-On PRP, longer clot formation time and lower velocity of fibrin formation in B
Pike et al[42 ]
Testing different conditions to see if TGA could identify FXI clinical phenotype
97
Prediagnosis experience related to tonsillectomy and or dental extraction
TGA-CAT
PPP
PRP
TF 0.5 pM, 1 pM, 5 pM
With CTI in tubes or without CTI
PLS 4 μM (only for PPP)
-TGAs (ETP and PH) identify FXI clinical phenotype using a specific condition:
PRP + CTI + TF 0.5 pM
Livnat et al[43 ]
Assess if the use of global assays to predict bleeding risk
39
Questionnaire with bleeding history in absence of prophylactic treatment. B were those
who had excessive bleeding following surgery (including tooth extraction). NB not
specified
a. TGA-CAT
PPP
With TF 1 pM or without TF
With CTI in tubes or without CTI
PLS 4 μM
-TG parameters (ETP and PH) significantly decrease in B vs. in NB only if TG is induced
by recalcification in absence of TF
-TG induced by recalcification w/o TF in PPP can identify FXI B from NB
b. ROTEM
Whole blood
IN-TEM/NATEM assays without CTI
-No significant difference between B and NB
-Significant difference between C and PTs
Zucker et al[39 ]
Discriminate FXI B from NB
Characterize clot characteristic in patient with severe FXI deficiency
16
Clinical history performed by experienced hematologists. Bleeding events related to
tooth extraction. NB were patients who underwent at least two uneventful tooth extractions
without prophylaxis
a. TGA-CAT
PPP
TF 1 pM/with CTI in tubes, PLS 4 μM
-TG parameters: no statistical difference between B and NB
-PH decreased in B and NB vs. C (difference not reaching significance)
-Rate of clot formation was slower in B and NB vs. C
-B had decreased resistance to fibrinolysis compared to NB and C
-Reduced fibrin network in B vs. NB
-Abnormal fibrin network in FXI B
b. Clot formation and fibrinolysis assay (turbidimetric test, 405 nm
PPP
TF 0.5 pM/with CTI in tubes, PLS 4 μM
t-PA 0.5 μg/mL and thrombomodulin 5 nM
c. Fibrin structure analysis (confocal microscopy)
Alexa Fluor-488–fibrinogen 80 μg/mL, TF 0.5 pM, PLS 4 μM
Guéguen et al[40 ]
Find the biological determinants for bleeding risk in heterozygous FX-deficient patients
39
Blinded clinical anamnesis performed by two experienced hemostasis hematologists.
History of spontaneous or provoked bleeding from any sites. Written questionnaire
using Tosetto score. Final bleeding score calculated by a third clinician
a. TGA-CAT
PPP
TF 1 pM without CTI, PLS 4 μM
-Global test (ROTEM and CAT) not helpful to distinguish FXI B
-Global fibrinolytic potential (ROTEM) not associated with bleeding
PRP
TF 0.5 pM/without CTI
b. ROTEM
Whole blood
IN-TEM/NATEM assays without CTI
Rugeri et al[41 ]
Assess correlation between FXI bleeding phenotype and TGA parameters
24
Bleeding symptoms that occurred in absence of any replacement drug during at-risk
situation (surgery/injury); one patient without challenge. Severe B had excessive
bleeding and required treatment
TGA-CAT
PRP
TF 0.5 pM/with CTI in tubes
-TG parameters LT, PH, VI significantly lower in B vs. in NB and C
-Correlation between FXI:C level and LT, PH, VI
-No significant difference between NB and C
Al Dieri et al[45 ]
Assess the correlation between coagulation factor concentration and parameters of
TG curve
7
Questionnaire of bleeding history
TGA chromogenic
PPP
TF 15 pM without CTI, PLS 4 μM
-ETP reduced of 55-65% only in PTs with FXI < 1%
Abbreviations: B, bleeders; C, controls; CAT, calibrated automated thrombogram; CTI,
corn trypsin inhibitor (inhibitor of the contact phase of the coagulation); ETP, endogenous
thrombin potential; IN-TEM assay, intrinsic thromboelastometry; LT, lag time; NATEM
assay, nonactivated thromboelastometry; NB, non-bleeders; PH, peak height; PLS, phospholipids;
PPP, platelet-poor plasma; PRP, platelet-rich plasma; PTs, patients; ROC, receiver
operating characteristic; ROTEM, rotational thromboelastometry; SD-FFP, solvent-detergent
fresh frozen plasma; TF, tissue factor; TG, thrombin generation; TGA, thrombin generation
assay; t-PA, tissue-type plasminogen activator; VI, velocity index.
Till this date different studies have used global coagulation assays to investigate
FXI clinical phenotype and/or FXI replacement therapy (summarized in [Table 1 ]). Different axes of research/methods investigating FXI functions using global assays
can be outlined.
Thrombin Generation Assay—Calibrated Automated Thrombogram
Thrombin is the key enzyme of the coagulation cascade, since it cleaves fibrinogen
into fibrin, which is needed for platelet clot stabilization and to prevent bleeding.
Thrombin also regulates the coagulation cascade through a set of positive and negative
feedback mechanisms ([Fig. 1 ]). TG assays allow to continuously monitoring thrombin production in clotting plasma.
The reference method is the calibrated automated thrombogram (CAT), a semiautomated
method developed by Hemker et al.[38 ] The test is performed in either platelet-poor (PPP) or platelet-rich plasma (PRP).
The coagulation reaction is triggered by TF, requires calcium and—when testing PPP—phospholipids.
TF concentration is variable depending on the condition/disease analyzed. TF is constantly
present and uniformly distributed in the sample mixture; thus, CAT is considered a
homogeneous model of coagulation. The assay uses a modified fluorogenic substrate
(Z-Gly-Gly-Arg-7-amino-4-methylcoumarin) which is cleaved by thrombin and produces
fluorescence proportionally to the amount of thrombin generated. By means of a thrombin
calibrator, the fluorescence of the unknown sample is converted to the concentration
of thrombin, thus constructing a TG curve ([Fig. 2A ]).
Fig. 2
Basic principles of different global coagulation assays . (A ) In calibrated automated thrombogram (CAT) assay, tissue factor (TF) is constantly
present and equally distributed in the sample mixture (measuring well). Thrombin generation
curve is described by (1) the lag time, which is the time between the addition of
the trigger and generation of thrombin; (2) the time to peak, which is the time needed
to reach the maximal amount of thrombin; (3) the peak height which is the maximal
amount of thrombin generated; (4) the velocity index which is the slope of the thrombin
generation curve, reflecting the rate at which thrombin is generated; and (5) the
endogenous thrombin potential, which is the area under the curve representing the
overall amount of thrombin generated. (B ) In rotational thromboelastometry (ROTEM), citrated whole blood is added in a disposable
cartridge together with calcium and an activator. A pin, suspended in the sample,
oscillates over a small arc. As the blood starts clotting, the raising clot firmness
increasingly restricts the rotation of the pin. This movement is detected and charted.
A depiction of highly pathological ROTEM output illustrates the parameters of the
measure: clotting time (CT), clot formation time (CFT), maximum clot firmness (MCF),
and maximum lysis (ML). (C ) A turbidimetric assay measures clot properties by reading the optical density of
the forming clot over time. An activation mix containing TF, calcium, and phospholipids
is added to a 96-wells plate containing plasma samples and coagulation is triggered.
The samples are measured in a plate reader at absorbance of 405 nm over time. The
optical density measures are plotted versus time. Onset: time to the inflection point
before turbidity increases; time to peak (TTP), is the time needed to reach the plateau;
peak turbidity change is the maximum clot turbidity less the initial turbidity; Vmax
is the rate of clot formation. The same principle can be used to monitor clot lysis,
after addition of t-PA to the plasma (not shown in the figure; [Fig. 2C ] was created using images from the Servier Medical Art, which are licensed under
a Creative Commons Attribution 3.0 Unported License; https://smart.servier.com). (D ) Thrombodynamics assay (TD) is a spatial heterogeneous system. TF is immobilized
to a surface and coagulation spreads from the TF-coated surface to the bulk of plasma
where the TF is absent. Differently from the CAT assay, the TD assay will measure
parameters informative of the thrombin generated during the TF-independent phase of
coagulation (Ast = amplitude of the thrombin moving peak; Vt = rate of thrombin propagation)
and parameters describing clot formation (clot growth and clot size). TF + , presence
of tissue factor; TF − , absence of tissue factor.
Identification of FXI clinical phenotype by CAT assay has produced conflictual results
(summarized in [Table 1 ]).[39 ]
[40 ]
[41 ]
[42 ]
[43 ]
[44 ]
[45 ] This may be explained by the different criteria used to classify patients in bleeders
or non-bleeders and by the various experimental conditions used to measure TG. Standard
bleeding scores are inadequate for predicting bleeding risk in FXI-deficient patients.[46 ] Thus, it is possible that in the studies that were unable to separate bleeders from
non-bleeders, the patients were inappropriately phenotyped, either because the classification
was based on a standard bleeding score or because the patients were not exposed to
an appropriate hemostatic challenge (e.g., interventions in high fibrinolytic area,
such as tonsillectomy or dental extractions). In addition, studies listed in [Table 1 ] used very different experimental conditions: (1) presence or absence of platelets;
(2) presence or absence of corn trypsin inhibitor (CTI); and presence of different
concentrations of TF. This variety makes data comparison among studies quite difficult.
Here, we summarize the main study characteristics and results (see also [Table 1 ]).
The studies from Zucker et al,[39 ] Guéguen et al,[40 ] and Bertaggia Calderara et al[44 ] found that TG measured by CAT assay in PPP using TF at 1 pM could not discriminate
between FXI-deficient bleeder and non-bleeder individuals. Of note, Zucker et al[39 ] used CTI in collecting tubes to prevent contact activation, while Guéguen et al[40 ] and Bertaggia Calderara et al[44 ] did not.
Livnat et al[43 ] performed TG assay in PPP with 1 pM TF as a trigger or without TF, in the presence
or absence of CTI. They found that TG parameters (endogenous thrombin potential [ETP]
and peak height) were both significantly decreased in citrated plasma, only when coagulation
was induced by recalcification (without addition of TF). The absence of significant
difference observed between FXI bleeders and non-bleeders in the presence of CTI might
be due to a concentration of CTI too high in the collection tubes, which could have
inhibited FXIa.[47 ]
Al Dieri et al[45 ] investigated TG in FXI deficiency and others rare bleeding disorders in PPP. In
a small cohort of seven patients, they found a 55 to 65% reduction of the ETP only
in patients with FXI <1%, while the others (FXI between 1 and 5%) had normal TG. However,
the very high TF concentration used (15 pM) probably limited the sensitivity of the
TG assay.[47 ]
[48 ]
Two studies[41 ]
[42 ] performed TG measurements in PRP. Blood was collected in tubes containing CTI and
TG assay was done using lower concentration of TF (0.5 pM). Rugeri et al[41 ] studied a cohort of 24 FXI-deficient patients and found that TG parameters (lag
time/peak height/velocity index) were significantly different in bleeders versus non-bleeders,
while there was no difference in non-bleeders versus controls. Pike et al[42 ] tested several sample conditions (i.e., PPP or PRP, with or without CTI, different
TF concentrations) in a larger cohort of patients (n = 97). Among the different conditions tested, TG measured in PRP in the presence
of CTI best differentiated between bleeders and non-bleeders, as it was confirmed
by receiver operating characteristic (ROC) curve analysis (specificity of 80% for
ETP and of 67% for peak height[42 ]).
While the use of PPP is very convenient because it is easy to standardize and allows
working with frozen samples from different laboratories, the use of PRP covers the
role of GPIb and the contribution of activated platelets in FXI activation.[6 ] Nevertheless, PRP samples cannot be stored and are more difficult to handle, since
platelets can be unintentionally activated. In sum, these studies have shown that
the use of a low TF concentration (<1 pM)[48 ] in the presence of CTI to inhibit the contact activation pathway (which enhances
TG via propagation loop coagulation of factors XI and IX/VIII) is important for increasing
the sensitivity of TG assay to FXI clinical phenotype.
Clot Formation and Fibrinolysis Assay
Further studies have used rotational thromboelastometry (ROTEM)[40 ]
[43 ]
[49 ] or turbidimetric assay[39 ]
[50 ] ([Fig. 2B, C ]). These studies focused on clot formation and fibrinolysis to investigate the hemostatic
role of FXI. Thromboelastometry is generally performed in citrated whole blood and
the assay provides a graphic representation of clot formation, stabilization, and
fibrinolysis. The few studies performed[40 ]
[43 ]
[49 ] concluded that ROTEM performed in whole blood was not able to discriminate FXI-deficient
bleeders from FXI non-bleeders, independently from the presence of CTI. Of note, Pike
et al[49 ] found that if the assay was performed in PRP + CTI, FXI-deficient bleeders had a
longer clot formation time with decreased velocity of fibrin generation, which is
in agreement with studies using other assays.[39 ]
[41 ]
[44 ]
Turbidimetric assay is a quite accessible technique and is performed in citrated CTI-treated
PPP, in the presence of low TF concentration and recalcification. The assay can be
done in the presence of tissue plasminogen activator (tPA) to monitor fibrinolysis;
readings are done in a plate reader with absorbance at 405 nm and monitoring of turbidity
allows building a clot formation/fibrinolysis curve, characterized by several parameters
(onset of clot formation, clot formation rate, time to peak, peak turbidity change,
and area under the curve). Zucker et al[39 ] in a small cohort of 16 patients found that FXI-deficient patients had a slower
rate of clot formation compared to controls and that clot from FXI bleeders was significantly
less resistant to fibrinolysis compared to that from non-bleeders and controls. Gidley
et al[50 ] in a larger cohort of patients (n = 71) confirmed these observations. The authors found that in FXI-deficient bleeder
plasma treated with CTI, the clot had a significantly slower formation rate and that
it was less resistant to fibrinolysis compared to non-bleeders and controls. They
proposed that using CTI-treated plasma, the association of aPTT with parameters of
fibrinolysis assays (clot formation rate and area under the curve) in a combined model
could help detect FXI-deficient bleeders. Interestingly, both studies[39 ]
[50 ] were completed by confocal images of fibrin network structure observed in plasma
clots. Zucker et al[39 ] observed that fibrin network in clots from bleeders was reduced by about 20 to 25%
compared with both controls (p < 0.05) and non-bleeders (p < 0.02). Gidley et al[50 ] found that fibrin network density in clots from FXI-deficient bleeders trended toward
a reduced density compared to controls and non-bleeders. These studies[39 ]
[50 ] highlight the role of clot density and its stability toward fibrinolysis in FXI-deficient
bleeders.[32 ] Of note, Colucci et al[51 ] studied TG and fibrinolytic resistance in a cohort of 18 patients exhibiting various
degree of FXI deficiency. Their work suggested that the reduced fibrinolytic resistance
observed in clots from FXI-deficient patients might be due to a defective TAFI-dependent
inhibition of fibrinolysis, described as “TAFIa resistance.”[51 ] While the underlying mechanism of TAFIa resistance needs to be further elucidated,
this activity may be important to understand the variety of the bleeding tendency
in FXI deficiency.[32 ]
[52 ]
Thrombin Generation and Clot Formation in a Spatial Heterogeneous Model of Coagulation—Thrombodynamics
Assay
Thrombodynamics (TD) assay ([Fig. 2D ]), a video microscopy system recently marketed, is an alternative experimental model
of coagulation, which considers the spatial and temporal dynamic of coagulation and
the biochemical reaction of the coagulation cascade. The test is performed on recalcified
PPP and the coagulation is triggered by a thin layer of TF immobilized on a coated
surface at low concentration (density = 100 pmol/m2 ) in the presence of CTI, phospholipids, and a modified fluorescent substrate cleavable
by thrombin. The test allows monitoring simultaneously TG and fibrin clot formation.[53 ]
[54 ]
[55 ]
[56 ] In this experimental model, coagulation is triggered by immobilized TF and propagates
into the bulk of plasma, where TF is absent, mimicking a blood vessel damage. Images
of TG and clot formation are recorded by a dedicated software, which calculates several
parameters of the TF-dependent and -independent phases of coagulation. Thus, this
system is suitable to analyze FXI's role in the coagulation. In fact, it is performed
in the presence of CTI, which increases the sensitivity of the test to the amount
of thrombin generated via the loop FXI and VIII/IX. Furthermore, with increasing distance
from the TF-coated surface, the influence of TF on the coagulation cascade becomes
null and the assay can measure parameters of TG and clotting formation of the TF-independent
phase of coagulation in which FXI plays a pivotal role. Using this assay in a cohort
of 24 FXI-deficient individuals, Bertaggia Calderara et al[44 ] could observed that FXI-deficient bleeders were characterized by a significant lower
TG and clot formation rate in the TF-independent phase of coagulation. An algorithm
based on combinatorial ROC analysis of TG and fibrin clot formation parameters could
identify all of the FXI-deficient patients with bleeding phenotype and 82% of FXI
patients without bleeding symptoms. In sum, the spatial heterogeneous model of coagulation
appears to improve the ability of traditional TG assays to recognize the FXI bleeding
phenotype, offering a promising tool for a tailored treatment of the FXI-deficient
patient.
Global Assay for Monitoring FXI Replacement
Global TG assays have been used to assess hemostatic potential after FXI replacement
with FXI concentrates.[44 ]
[57 ]
[58 ] In general, these studies showed that after in vitro spiking of FXI-deficient plasma
or after infusion of FXI concentrate in patients, TG was improved. Of note, after
the infusion of FXI concentrate, TF-independent parameters measured ex vivo by TD
assay shifted toward hypercoagulation at levels of FXI of about 30%.[44 ] These studies[44 ]
[57 ]
[58 ] agreed on the fact that low doses of FXI concentrate were sufficient to normalize
hemostasis in vitro and ex vivo[44 ]
[58 ] concluding that global TG assays are a promising tool for monitoring FXI replacement.