HAEMATOLOGY

Haematology

Blood coagulation

Björn Dahlbäck

• Under normal circumstances, the coagulation system is balanced in favour of anticoagulation.

• Thrombin is the key effector enzyme of the clotting cascade.
• Antagonists of vitamin K inhibit a vitamin-K-dependent post-translational modification of several coagulation
proteins, which is required for these proteins to attain a phospholipid-binding conformation.
• Heparin stimulates the activity of antithrombin, a serine-protease inhibitor.
• Analysis of knock-out mice has shown the relative importance of the coagulation factors in vivo.
• Gene therapy may soon be a therapeutic option for inherited deficiencies of factors VIII and IX.
Blood coagulation and platelet-mediated primary Prothrombin is activated to thrombin by the
haemostasis have evolved as important defence prothrombinase complex, which consists of the
mechanisms against bleeding. The coagulation system is phospholipid-bound complex between the enzyme,
triggered in response to rupture of endothelium, which factor Xa, and its cofactor, activated factor V (Va). The
allows exposure of blood to the extravascular tissue. The substances that activate factor V are factor Xa (on the
responses of the coagulation system are coordinated with phospholipid surface) and thrombin (both in solution and
the formation of the platelet plug that initially occludes on the surface). Thrombin feedback amplifies the system
the vascular lesion. Anticoagulant mechanisms ensure by activating not only factor V, but also factors VIII and
careful control of coagulation and, under normal XI. Factor VIII circulates bound to von Willebrand factor,
conditions, they prevail over the procoagulant forces. which is an adhesive protein important for the generation
Disturbances of the natural balance between the of the initial platelet plug.8 After activation, factor VIIIa
procoagulant and anticoagulant systems due to genetic or dissociates from von Willebrand factor and forms a
acquired factors may result in bleeding or thrombotic complex on the platelet surface with factor IXa; this
diseases. complex (denoted the tenase complex) then activates
factor X. Activation of factor XI by thrombin is another
Coagulation pathway amplification loop, resulting in the generation of
Thrombin is the key effector enzyme of the coagulation additional factor IXa, which in turn activates factor X.9
system, having many biologically important functions The initiation of coagulation via the exposure of tissue
such as the activation of platelets, conversion of fibrinogen factor (tissue-factor pathway) as described above, the
to a fibrin network, and feedback amplification of extrinsic pathway, is the mechanism by which coagulation
coagulation. The precise and balanced generation of is initiated in vivo in response to trauma. The intrinsic
thrombin at sites of vascular injury is the result of an pathway is an alternative mechanism by which the
ordered series of reactions collectively referred to as blood coagulation system can be initiated. It involves factor XII,
coagulation.1–3 The system is triggered on the surface of high-molecular-weight kininogen, prekallikrein, and factor
extravascular cells by the exposure of tissue factor to XI; it results in the activation of factor XI. The
blood (figure). physiological role of this pathway is not fully understood,
Tissue factor is a membrane protein abundantly present because it is not important in trauma-initiated
in cells surrounding the vascular bed. It binds both coagulation. Thus, inherited deficiencies of proteins of
zymogen and activated forms of factor VII (factor VIIa). A this system are not associated with bleeding problems,
fraction of factor VII in blood circulates as active enzyme except for deficiency of factor XI, which leads to a
and the binding of this form to tissue factor triggers moderately severe bleeding disorder.
coagulation by converting factors IX and X to their active Maximum thrombin generation occurs after the
forms (IXa and Xa).4,5 Feedback amplification is achieved formation of the fibrin clot.5 This thrombin is important
when factor VII bound to tissue factor is activated by for additional fibrin generation as well as for activation of
factors VIIa, IXa, and Xa. Factors IXa and Xa may factor XIII and the thrombin-activatable fibrinolysis
remain associated with the tissue-factor-bearing cell or inhibitor. Activated factor XIII (XIIIa) is a
diffuse into the blood and bind to the surface of nearby
transglutaminase that stabilises the clot by covalent cross-
activated platelets, which have formed the primary platelet
linking of fibrin.3 Thrombin-activatable fibrinolysis
plug.6 Activation of platelets is associated with the
inhibitor is a carboxypeptidase that releases carboxy-
exposure of negatively charged phospholipids, which have
terminal lysines from fibrin. Because these lysines are
high potential to bind coagulation factors and assemble
important for the binding of fibrinolytic enzymes to fibrin,
enzyme-cofactor complexes that are crucially important
activation of the inhibitor prevents further fibrinolytic
for efficient propagation of the system.7
attack.10
Phosphatidylserine is a negatively charged phospholipid
Lancet 2000; 355: 1627–32 required for the assembly of the tenase and
Department of Clinical Chemistry, Lund University, University prothrombinase complexes.1,7,11 Under normal conditions,
Hospital, Malmö, S-20502 Malmö, Sweden (Prof B Dahlback MD) phosphatidylserine is present in the inner-layer leaflet of
(e-mail: bjorn.dahlback@klkemi.mas.lu.se) the plasma cell membrane. During platelet activation, it is

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HAEMATOLOGY

Activation and propagation of coagulation Thrombin

Tissue VIIa VIIIa Va

factor IXa
Tissue IX/X Xa Prothrombin
Xa IXa X
factor

Activation of protein C and inhibition of coagulation

Protein S
APC
Thrombomodulin Protein C V VIIIi Va Vi
VIIIa

T APC Protein S APC

Schematic models illustrating some of the phospholipid-bound reactions that are involved in the activation and regulation of
coagulation
Factor VIIa binds to tissue factor and activates factors IX and X. Factors IXa and Xa together with factors VIIIa and Va, respectively, form the tenase and
prothrombinase complexes that activate factor X and prothrombin, respectively. Thrombin-mediated activation of factor XI, factor V, and factor VIII, which
gives positive feedback amplification of the system, is not shown. Thrombomodulin is present on endothelial cells. Thrombin generated in the vicinity of
intact endothelial cells binds to thrombomodulin and efficiently activates protein C. Activated protein C (APC) and protein S form a complex on the plasma
membrane of endothelial cells and possibly also on other cells. This complex inactivates factors Va and VIIIa, which results in down-regulation of the
coagulation system. The degradation of factor VIIIa by APC is stimulated by protein S and by factor V, which in this context functions as an anticoagulant
protein.

translocated from the inner layer of the membrane to the phospholipid surfaces increases the local concentration of
outer layer.7 In the tenase and prothrombinase complexes, the coagulation components and counteracts regulation
all participating proteins (ie, the enzymes factors IXa and by anticoagulant mechanisms.
Xa, the cofactors VIIIa and Va, and the substrates factor The relative importance of the various coagulation
X and prothrombin), have affinity for the negatively factors in vivo has been elucidated by knock-out mice
charged phospholipid surface. The enzymes and the technology.12 The crucial importance of the tissue-factor
substrates are vitamin-K-dependent proteins that interact pathway is shown by the finding that mice lacking tissue
with the phospholipid membrane via their amino-terminal factor die as embryos.13–15 Mice deficient in factor VII
domains, which contain -carboxyglutamic acid residues. develop normally in utero but die shortly after birth from
This post-translationally modified glutamic acid residue is severe bleeding.16 The difference in severity between
present only in the vitamin-K-dependent proteins. The deficiency of tissue factor and deficiency of factor VII
residues are involved in calcium binding, important for suggests a role for tissue factor during embryogenesis
the correct folding of the -carboxyglutamic acid domain. beyond fibrin formation. Deficiencies of factor V and
Inhibition of the -carboxylation reaction by antagonists prothrombin are both associated with fatal haemorrhage
of vitamin K results in defective calcium binding of the - and partial embryonic lethality,17–19 whereas animals
carboxyglutamic acid domains and loss of ability to deficient in factor IX or VIII develop normally during fetal
interact with the phospholipid membrane.2 This is the life, but show haemophilia-like disease after birth.20,21 As
molecular basis for anticoagulant therapy with vitamin K in human beings, fibrinogen deficiency in mice is
antagonists. associated with normal fetal development and a moderate
The concentrations of the various coagulation proteins to severe bleeding phenotype,22 which shows that
in circulating blood relate to their specific roles in the thrombin generation is more important than fibrin
pathway.2,3 The predominant clotting factor is fibrinogen deposition. This fact may be related to the multiple
(10 mol/L); this concentration is about 50 000 times functions of thrombin, including its ability to activate
higher than that of factor VIII (0·2 nmol/L). The high platelets—a reaction that is crucial for the haemostasis
fibrinogen concentration is required for the formation of response to vascular injury.
the fibrin network, whereas the low amount of factor VIII
is more than sufficient to support factor IXa in the
activation of factor X. There are also variations in the Regulation of blood coagulation by
concentrations of the vitamin-K-dependent proteins: anticoagulant pathways
factor VII (10 nmol/L) is the least abundant, factors IX Regulation of coagulation is exerted at each level of the
and X are present in intermediate concentrations (100 pathway, either by enzyme inhibition or by modulation of
nmol/L), and prothrombin circulates at the highest the activity of the cofactors. The tissue-factor-pathway
concentration (2 mol/L). Thus, early components of the inhibitor inhibits the reactions involving tissue factor and
pathway circulate at lower concentrations than the factors factor VIIa.23 This inhibitor is mostly bound to LDL in
that act at later stages, which is consistent with the plasma or to heparan sulphate when associated with
principal organisation of the system with multiple endothelial cells. The lack of tissue-factor-pathway
reactions and amplification potential. The assembly of inhibitor may not be compatible with life, since no
enzyme-cofactor complexes on negatively charged deficiency states have been described in human beings.

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This idea is supported by the lethal phenotype found in disease and die during embryogenesis, even before
tissue-factor-pathway-inhibitor knock-out mice.24 These development of a functional cardiovascular system.30
animals show uncontrolled activation of coagulation with
consumption of coagulation factors. Inherited and acquired coagulation disorders.
Most of the enzymes generated during activation of Inherited deficiencies of factor VIII (haemophilia A) and
coagulation are inhibited by the serine-protease inhibitor factor IX (haemophilia B) are rare inherited bleeding
antithrombin, previously called antithrombin III. disorders with prevalence of about one in 10 000. The
Antithrombin preferentially inhibits free enzymes, whereas genes for both factors are on the X chromosome, which is
enzymes that are part of the tenase or prothrombinase why only males are affected, whereas females are carriers
complexes are less accessible for inhibition. The of the disease. Almost half of the disease-causing
physiological role of antithrombin is to limit the mutations are novel, and many boys with haemophilia are
coagulation process to sites of vascular injury and to born in families with no previous history of the disease.
protect the circulation from liberated enzymes. Severe haemophilia is associated with less than 1% of the
Antithrombin is, in itself, an inefficient serine-protease normal plasma concentration of either factor IX or factor
inhibitor, but heparin and the heparin-like molecules that VIII, whereas moderate and mild forms have plasma
are present on the surface of endothelial cells stimulate its concentrations of 1–5% and 5–20% of normal,
activity.25 This mechanism is the molecular basis for the respectively. The normal plasma concentrations of these
use of heparin as a therapeutic anticoagulant. components seem therefore to be higher than required for
The protein C anticoagulant system regulates a normal physiological response, which is noteworthy
coagulation by modulation of the activity of the two considering the very low normal concentrations of factor
cofactors, factors VIIIa and Va.26 Protein C, the key VIII in particular. The classic symptoms of haemophilia
component of the system, is a vitamin-K-dependent are bleeding episodes affecting joints, muscles, internal
zymogen to an anticoagulant protease. It is activated on organs, and the brain. Joint bleeding (haemarthrosis) is
the surface of intact endothelial cells by thrombin that has the most characteristic feature of severe haemophilia.
bound to the membrane protein thrombomodulin Repeated bleeds result in chronic arthropathy with loss of
(figure). Thus, thrombin has the capacity to express both joint movement, fixed flexion contracture, and severe
procoagulant and anticoagulant functions depending on muscle wasting. The first bleeding manifestations appear
the context under which it is generated. At sites of in early childhood, but not in the neonatal period.
vascular disruption, the procoagulant effects of thrombin Bleeding from the mouth caused by the eruption of the
are fully expressed. In contrast, in an intact vascular child’s teeth is a common early manifestation of
system, thrombin has anticoagulant function since it binds haemophilia. Haemorrhages in muscles and joints after
to thrombomodulin and activates protein C. Activated minor twists or knocks are associated with the early
protein C (APC) can cleave the phospholipid-membrane- crawling and walking efforts. Since primary haemostasis is
bound cofactors factors Va and VIIIa, which results in unaffected, the bleeding time is normal and the patients
inhibition of the coagulation system. A vitamin-K- do not experience major problems with bleeding from
dependent cofactor protein, protein S, supports the mucous membranes and minor skin lesions.
anticoagulant activity of APC. In human plasma, about von Willebrand’s disease is caused by quantitative or
30% of protein S is free, the remainder being bound to the qualitative defects in von Willebrand factor. This results in
complement regulatory protein C4b-binding protein.26 a primary haemostasis defect, which is caused by deficient
APC and free protein S form a membrane-bound adhesion of platelets to exposed subendothelial collagen.
complex, which can cleave factors VIIIa and Va, even Bleeding from skin and mucous membranes is common,
when they are part of fully assembled tenase and and the bleeding symptoms begin soon after birth in many
prothrombinase complexes. cases. This disorder occurs in both male and female
In vivo, APC does not cleave intact factor VIII because children. The disease is clinically heterogeneous, and the
the binding of factor VIII to von Willebrand factor severity of the symptoms depends not only on the nature
prevents it from interacting with the phospholipid of the disease-causing mutation, but also on whether or
membranes. In contrast, factor V binds phospholipids as not both alleles are affected. Three major categories of von
well as factor Va does, and APC is able to cleave the intact Willebrand’s disease are distinguished. Type I refers to
form of factor V. The consequence of APC-mediated partial deficiency (heterozygous) and is inherited as an
cleavage of factor V is the generation of anticoagulant autosomal dominant trait. Type II (several subtypes
factor V that functions in synergy with protein S as an distinguished) is associated with a qualitative defect in von
APC cofactor in the degradation of factor VIIIa. Thus, Willebrand factor, which commonly affects the multimeric
factor V can function as a procoagulant and an structure of the protein. Type III refers to total deficiency
anticoagulant cofactor, procoagulant factor Va being (homozygous or compound heterozygous) and is inherited
formed after limited proteolysis by thrombin or factor Xa, as an autosomal recessive disease. Severe forms of von
whereas the anticoagulant factor V activity is expressed by Willebrand’s disease have a secondary deficiency of factor
factor V that has been proteolytically cleaved by APC.27,28 VIII, since von Willebrand factor is a carrier of factor VIII
The anticoagulant potential of factor V may be in blood.
particularly important in the regulation of the tenase Acquired bleeding disease may be caused by an
complex by APC and protein S. The physiological autoantibody directed against a coagulation factor, the
importance of the protein C system is shown by the severe most common being antibodies directed against factor
thromboembolic disease that is associated with VIII or V. Unlike inherited haemophilia, the acquired
homozygous deficiency of protein C in both human beings bleeding disorders mainly affect elderly people. The
and mice.29 In both cases, the severe lethal thrombotic molecular mechanisms involved in the generation of the
disease manifests shortly after birth. Mice lacking a autoantibodies are not known. The associated bleeding
functional thrombomodulin gene have even more severe tendency may be severe and occasionally life-threatening.

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Another type of acquired bleeding disorder is that related low. High concentrations of fibrin degradation products,
to the requirement for vitamin K in the biosynthesis of including D-dimers, result from activation of the
many of the coagulation proteins. Conditions associated fibrinolytic system.
with malabsorption of vitamin K can lead to deficient -
carboxylation of the vitamin-K-dependent coagulation Treatment of haemophilia and von Willebrand’s
proteins. In severe cases, this process results in an disease
increased bleeding tendency. However, a more common
Concentrates of factor IX or VIII, derived either from
disorder is deficiency of vitamin K due to excessive intake
plasma or produced by recombinant techniques, are used
of antagonists, for example warfarin, used as anticoagulant
in the treatment of haemophilic patients. Treatment can
therapy. Severe liver disease may be associated with
be given on demand or as prophylaxis. Drugs that inhibit
bleeding tendency due to deficient synthesis of
platelet function, such as aspirin, should be avoided.
coagulation factors. The acquired bleeding tendency
Intramuscular injections can trigger severe bleeding
associated with disseminated intravascular coagulation is
episodes and should therefore not be used. In most cases,
due to consumption of platelets and coagulation factors,
the administration of factor concentrates is uneventful but
which is the result of widespread pathological proteolysis.
may be complicated by virus transfer (plasma-derived
In this disorder, many proteolytic enzyme systems are
products) or the induction of antibody formation by the
activated, including both coagulation and fibrinolysis,
administered products. The management of haemophilic
which result in microvascular thrombosis and major
patients who have inhibitory antibodies is difficult and
disturbances of the capillary circulation. Disseminated
may include the use of high doses of factor VIIa or efforts
intravascular coagulation is commonly caused by severe
to induce immune tolerance.31,32 Gene therapy has not so
infections with septicaemia or malignant disease, but it
far been established as a therapeutic modality, although
can also complicate traumatic injury, surgery, or
pregnancy. research in this area is very intense. In animals, gene
therapy of factor IX in dogs has shown promising results.
Bleeding episodes in patients with severe forms of von
Laboratory investigation of bleeding disorders Willebrand’s disease are treated with concentrates of
The initial investigation of patients with bleeding von Willebrand factor. DDAVP (D-amino-D-arginine
symptoms includes measurements of platelet counts, vasopressin) increases the release of von Willebrand factor
bleeding time, and global clotting tests such as the from endothelial cells. It is therefore useful in the
activated partial thromboplastin time (intrinsic pathway) treatment of type I von Willebrand’s disease. However,
and the prothrombin time (extrinsic or tissue-factor because an efficient response to DDAVP requires that the
pathway). Patients with primary haemostatic disorders
von Willebrand factor can be synthesised, it is not effective
who have normal results of clotting tests but long bleeding
in type III von Willebrand’s disease. Type II shows
times are further investigated with functional platelet tests,
variation in the response to DDAVP, which should
and immunological and functional assays for von
therefore only be used after individual testing.
Willebrand factor. When a coagulation disorder is
suspected, specific functional and immunological testing
of coagulation factors is possible. Severe haemophilia Inherited and acquired thrombotic disorders
shows long clotting times in laboratory tests that are Venous thrombosis is common, each year affecting one in
sensitive to the tenase complex (the activated partial 1000 individuals, with higher rates among elderly people
thromboplastin time but not the prothrombin time, which than in the young. Pulmonary embolism or post-
is insensitive to the tenase complex owing to the high thrombotic syndrome may complicate the disease, but in
tissue-factor concentrations used in the assay). Specific most cases the recovery is uneventful. Inherited and
functional assays for factors VIII and IX are used to acquired risk factors are involved in the pathogenesis of
confirm the diagnosis. Identification of causative thrombosis. The inherited risk factors are life-long,
mutations has so far been carried out only in research whereas most acquired risk factors are of short duration
laboratories and has generated large databases with many (eg, pregnancy, surgery, and immobilisation). An acquired
different disease-generating mutations. Moderate and risk factor may seem to be the cause of a thrombotic
mild forms of haemophilia have either normal or only episode that is in fact due to a combination of genetic and
slightly longer than normal activated partial acquired risk factors.33 Most inherited risk factors for
thromboplastin times, and the diagnosis relies on the thrombosis affect the natural balance between
specific tests for the respective factor. Diagnosis of procoagulant and anticoagulant forces, and most are
acquired bleeding disorders caused by autoantibodies is found in the protein C system.26 The most common
based on inhibition in clotting tests by the antibodies. inherited risk factor is a single point mutation in the gene
Identification of which specific factor is recognised by the for factor V (G1691A), which results in phenotype called
antibody involves specific coagulation or immunological APC resistance, found in 20–40% of patients with
tests (eg, western blotting, which analyses the reactivity of thrombosis.34–36 The factor V mutation predicts the
the antibodies with various coagulation proteins). The replacement of arginine 506 with a glutamine residue,
laboratory diagnosis of vitamin K deficiency is based on which results in the loss of one of the APC cleavage sites
the use of clotting tests examining the tissue-factor in factor V/Va.37 Mutant factor V (VR506Q, V:Q506, or
pathway (prothrombin time). The activated partial factor V Leiden) has full procoagulant capacity. Dual
thromboplastin time is normal in most patients with this mechanisms cause the hypercoagulable condition that
disorder. Disseminated intravascular coagulation is characterises APC resistance.27 The factor V mutation is
characterised by consumption of both platelets and associated with impaired degradation of factor Va by
coagulation factors. Thus, platelet counts are low, the APC, since the arginine 506 site is one of three APC
activated partial thromboplastin time long, and the cleavage sites in factor Va. In addition, the factor V
concentrations of fibrinogen, factor V, and factor VIII mutation affects factor VIIIa degradation, because the

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anticoagulant activity of factor V is stimulated by the the blood are APC resistant, even though DNA analysis
arginine 506 cleavage. The factor V mutation is found suggests heterozygosity. Diagnosis of the prothrombin
predominantly in populations of caucasian origin, which is mutation (G20210A) relies on DNA testing, whereas
explained by a single mutational event that took place deficiencies of protein C, protein S, and antithrombin are
around 30 000 years ago and a subsequent founder best diagnosed by functional or immunological assays. For
effect.38 The prevalence of the factor V mutation varies protein S, assays for the free form of the protein are
between different countries of Europe and, with some preferable to those measuring the total protein S, since
exceptions, a north-south gradient is apparent with they have higher predictive value for deficiency. Global
highest prevalence (10–15%) in the north and lowest in clotting tests, such as the activated partial thromboplastin
the south (about 2%). In populations with mixed ethnic time, are useful for the initial identification of patients
backgrounds, such as the USA, the prevalence is about with antiphospholipid syndrome: in many cases of this
5%. APC resistance due to the factor V mutation is disorder the clotting times in all tests are long because the
associated with a slightly increased risk of thrombosis (five antibodies disturb the interaction between the coagulation
to ten fold) in its heterozygous state and a greatly factors and the phospholipids. Further investigation of a
increased risk in the homozygous state (50–100 fold).33 On lupus anticoagulant includes the use of other clotting
the other hand, APC-resistant women have a reduced tests, such as the dilute Russell’s viper venom time, a test
bleeding tendency after delivery, which, during evolution, in which a prothrombinase complex is generated due to
may have provided a survival advantage explaining the activation of factors X and V by the venom.
high prevalence of the mutation.39 Demonstration of inhibitory activity in mixtures between
The second most common genetic risk factor for normal plasma and the patient’s plasma is used to assess
thrombosis (found in 6–8% of patients with thrombosis) is the potency of the lupus anticoagulants. The laboratory
a single mutation (G20210A) in the 3 untranslated region investigation may also include neutralisation tests with
of the prothrombin gene.40 The mutation does not affect excess phospholipids and immunological tests that
prothrombin function but is associated with slightly measure the binding of the patient’s antibodies to
increased concentrations of prothrombin in plasma. It is immobilised phospholipids.43
found in around 2% of white people and is associated with
slightly increased risk of thrombosis (three to five fold). Treatment of patients with thrombosis
Heterozygous deficiency of protein C, protein S, or Venous thrombosis is initially treated with a combination
antithrombin also increases the risk of thrombosis (found of heparin and vitamin K antagonists. Either
in 1–3% of thrombosis patients), but these deficiencies are unfractionated or low-molecular-weight heparin can be
uncommon in the general population (protein C and used; the latter is prepared from unfractionated heparin by
protein S deficiency in about one in 300 and antithrombin chemical or enzymic cleavage methods. Low-molecular-
deficiency in one in 2000).41,42 Many different mutations in weight heparin has better pharmacokinetic properties than
each of these genes cause the deficiency states. The risk of unfractionated heparin, and adequate haemostatic control
thrombosis in deficiency of protein C or protein S is is achieved with a single daily subcutaneous injection. In
similar to that in APC resistance, whereas antithrombin addition, laboratory monitoring is not needed with low-
deficiency is a somewhat stronger risk factor. Since the molecular-weight heparin. After a few days of the
risk of thrombosis associated with the inherited disorders combined treatment, the concentrations of functional
is low, most individuals with a single genetic risk factor vitamin-K-dependent coagulation proteins fall into the
will not have thrombosis. Individuals with more than one therapeutic range, and heparin is discontinued. Treatment
risk factor, either genetic or acquired, have a higher risk. with vitamin K antagonists, which should be regularly
Venous thromboembolism is now considered to be a monitored by prothrombin time (international normalised
typical multigenetic/multifactorial disease. ratio), is generally continued for 3–6 months. The risk of
The antiphospholipid syndrome (lupus anticoagulant) bleeding complications must always be weighed against
is an acquired risk factor for thrombosis, which can be of the benefits of the anticoagulation effect, especially if an
long duration and can cause both arterial and venous oral anticoagulant is used for periods exceeding 3–6
thrombosis. The antibodies in the plasma of patients with months, when the risk of thrombotic recurrence probably
this syndrome are directed against a protein-lipid declines. Whether the presence of a genetic risk factor is
complex. In most cases, the protein is 2-glycoprotein 1, associated with an increased risk of recurrence is not
known, though several studies of APC resistance suggest
but antibodies against prothrombin have also been
this association.44 Patients with combined genetic defects,
identified. The antiphospholipid syndrome is associated
and probably also patients with single gene defects, may
with increased risk of pregnancy-related complications
be at increased risk of recurrence, and long-term
including miscarriages.43
anticoagulation beyond 6 months can be considered, even
after an isolated thromboembolic event. Prophylactic
Laboratory investigation of risk factors for treatment with low-molecular-weight heparin or oral
thrombosis anticoagulants is recommended for individuals with
Simple functional clotting-based APC resistance tests are multiple genetic defects (including homozygous APC
available with close to 100% sensitivity and specificity for resistance) in situations known to be associated with a
the factor V mutation. Positive results are generally high risk of thromboembolic complications. This
confirmed by DNA-based tests, which distinguish recommendation holds even if the patient neither has
heterozygous from homozygous forms. In rare cases, the experienced thrombosis nor has any family history of such
APC resistance test suggests a more severe phenotype complications. In symptom-free carriers of single genetic
than the DNA test, which may be due to risk factors lacking a family history of thrombosis, short-
pseudohomozygosity with one mutant factor V allele and term prophylaxis may be considered in high-risk
one null allele. In this condition, all factor V molecules in situations.

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1632 THE LANCET • Vol 355 • May 6, 2000