- ACRONYMS AND DEFINITIONS
- APA - Antiphospholipid antibodies
- aPTT - activated Partial Thromboplastin Time
- dRVVT - dilute Russell Viper Venom Time
- DVT - Deep vein thrombosis
- ELISA - Enzyme-linked immunosorbent assays
- INR - International Normalized Ratio
- PCR - Polymerase chain reaction
- PE - Pulmonary embolism
- PT - Prothrombin Time
- TIA - Transient ischemic attack
- VTE - Venous thromboembolism
- VWD - Von Willebrand Disease
- VWF - Von Willebrand Factor
- Coagulation is a complex process that is not completely understood
- Coagulation involves a number of factors that interact with each other in differing ways depending on their environment
- Because coagulation is an integral part of some very important disease processes, a number of medications have been developed which affect the coagulation process
- The complexity of the process can be confusing for patients and healthcare providers alike
- Coagulation can be divided into two processes that interact
- Medications used to alter coagulation work on one of these two processes
- PLATELET ACTIVATION
- Unactivated platelets circulate freely in the blood
- When there is a breach in a blood vessel wall (ex. from a cut), platelets in the blood come in contact with collagen on the outside of the blood vessel
- Tissue factor on the surface of collagen activates platelets. Activated platelets also express tissue factor.
- Activated platelets interact with von Willebrand Factor (VWF) which causes the platelets to adhere to the blood vessel wall
- Activated platelets recruit more platelets to the area
- Tissue factor activates the coagulation cascade
- The coagulation cascade results in the formation of a fibrin mesh at the site of injury
- Activated platelets get caught in the fibrin mesh and a blood clot is formed
- COAGULATION CASCADE
- Coagulation factors
- The coagulation cascade is a multistep process that involves a number of coagulation "factors" that are denoted by Roman numerals I - XIII
- Coagulation factors circulate unactivated. Once they become activated, they are denoted by a small "a" (Ex. VIIa)
- The four factors in the table below are often referred to by other names
|Other names for Factors I, II, III, and IV|
- Vitamin K-dependent factors (II, VII, IX, and X)
- Factors II, VII, IX, and X are made in the liver and require Vitamin K for their synthesis
- Patients with significant liver disease may have deficiencies of Vitamin K-dependent factors. This can lead to bleeding disorders. The liver also makes protein C and protein S which are factors that inhibit coagulation. Deficiencies of protein C and protein S can lead to clotting disorders. Depending on the balance in deficiency of these factors, a patient with significant liver disease may have a bleeding or clotting disorder. (see coagulopathy of liver disease for more details)
- Warfarin (Coumadin®) is a widely used anticoagulant. Warfarin acts by inhibiting Vitamin K and thus inhibits the production of Factors II, VII, IX, and X.
- Coagulation cascade pathways
- The coagulation cascade can be divided into two different pathways:
- Intrinsic pathway - The intrinsic pathway is not as well understood as the extrinsic pathway. In the past, the intrinsic pathway was believed to propagate thrombosis, but it was not considered a factor in the initiation of thrombosis because patients with Factor XII deficiency do not have bleeding disorders. A recent study published in the NEJM challenged this hypothesis. The study found that inhibiting Factor XI significantly reduced the risk of postoperative thrombosis when compared to enoxaparin. The authors hypothesized that the release of DNA, RNA, and polyphosphates during surgical tissue damage leads to the initiation of the intrinsic pathway and subsequent thrombosis. [PMID 25482425]
- Extrinsic pathway - The extrinsic pathway is the main coagulation pathway initiated by tissue damage
- Factor VII circulates freely in the blood
- When there is a breach in a blood vessel wall (ex. from a cut), Factor VII in the blood come in contact with collagen on the outside of the blood vessel
- Tissue factor on the surface of collagen activates Factor VII (Factor VII → VIIa)
- Tissue factor-Factor VIIa complex then activates Factor X (Factor X → Xa). Factor IXa-VIIIa complex also converts Factor X to Xa, propagating the process.
- Factor Xa converts prothrombin to thrombin in the presence of Factor Va
- Thrombin converts fibrinogen to fibrin
- Fibrin links together to form a mesh which captures activated platelets and a blood clot is formed
- Factor XIIIa stabilizes the clot by cross-linking fibrin, and in the process, a degradation product called a D-dimer is released
- Factor XI, Factor IX, and Factor VIII propagate the pathway and are required for proper clot formation
- Genetically inherited disorders of these factors can lead to bleeding disorders (see bleeding disorders below)
- Illustration of coagulation cascade
- COAGULATION INHIBITION (ANTICOAGULANT PATHWAYS)
- Once the coagulation process has commenced, it is important to keep it at the site of vascular injury and not allow it to spread throughout the bloodstream
- A number of important factors play a role in containing coagulation
- Genetically inherited disorders of these factors can lead to thrombophilic disorders (see thrombophilic disorders below)
- Factors that inhibit coagulation
- Protein C - Vitamin K-dependent factor made by the liver. Activated by thrombin-thrombomodulin complex. Protein C works with protein S to inactivate Factor Va and VIIIa.
- Protein S - Vitamin K-dependent factor made by the liver. Protein S works with protein C to inactivate Factor Va and VIIIa.
- Antithrombin - Inactivates thrombin and Factor Xa
- Thrombomodulin - Found on the surface of normal vascular cells. Binds to thrombin and inactivates it. Thrombomodulin-thrombin complex activates protein C.
- Tissue Factor Pathway Inhibitor - Inhibits Tissue Factor-VIIa complex and Factor Xa
- BLEEDING DISORDERS
|Most common bleeding disorders|
|Von Willebrand Disease||0.5 - 2% of general population|
|Hemophilia A and B||1 in 5000 males
1 in 10,000 overall
Hemophilia A is 4 times more common than B
- BLEEDING DISORDERS | Von Willebrand disease (VWD)
- VWD is a genetically inherited bleeding disorder that affects the quantity or function of von Willebrand factor (VWF)
- VWD is the most common bleeding disorder affecting 0.5 - 2% of the general population
- VWF has two main functions
- It attaches platelets to the site of vascular injury
- It binds to Factor VIII in plasma and prevents its degradation and clearance 
- Because VWF is involved in both platelet function and the coagulation cascade (through Factor VIII), VWD affects both processes of coagulation
|Types of VWD and their characteristics|
Type I VWD
Type II VWD
Type III VWD
- The most common symptom of VWD is excessive bleeding from the following: nosebleeds, menses, bleeding after dental procedures, bruising, bleeding from minor cuts, gum bleeds 
- Bleeding will vary depending on the severity of disease
- VWF antigen - measures concentration of VWF protein in plasma. Does not measure VWF function.
- VWF ristocetin cofactor activity (VWF:RCo) - measures ability of VWF to interact with platelets. Ristocetin is an antibiotic that causes VWF to bind to platelets.
- Factor VIII coagulant assay - measures ability of VWF to bind and maintain Factor VIII in plasma. Patient's plasma is mixed with Factor VIII-deficient plasma and the clotting time is measured.
- aPTT - will be prolonged if Factor VIII is reduced, otherwise will be normal 
- Bleeding Time - the bleeding time test involves pricking a patient's skin and then measuring the amount of time it takes for bleeding to stop. The test measures platelet function. The test has limited utility because of high variability between individuals performing the test and the devices used to perform it.
- Patients with VWD are typically treated when they have bleeding or undergo surgery
- DDAVP (Desmopressin) - DDAVP is a synthetic analog of antidiuretic hormone (ADH). DDAVP stimulates the release of VWF from endothelial cells. Factor VIII levels also increase after DDAVP. Nasal administration is effective for minor bleeding. Intravenous administration is needed for major bleeding or surgery. Effect of daily administration diminishes after 3-5 days 
- VWF concentrate - Plasma-derived concentrates of VWF. Can be administered daily depending on need for surgery/bleeding. 
- Recombinant VWF (Vonvendi®) - In 2015, the first recombinant VWF was approved by the FDA. The drug is marketed as Vonvendi®, and it is approved for the as needed treatment of bleeding in patients with VWD.
- BLEEDING DISORDERS | Hemophilia A and B
- Hemophilia A and B are genetically inherited bleeding disorders
- Hemophilia A is caused by a deficiency of Factor VIII. Hemophilia B is caused by a deficiency of Factor IX.
- Factor VIII and IX play a role in propagating the coagulation cascade (see coagulation cascade) and are essential to normal clot formation
- Hemophilias are X-linked chromosome recessive disorders. This means mostly males will be affected, although it is possible for females to have hemophilia, but it is rare.
- The prevalence of hemophilia is 1 in 5000 males, and 1 in 10,000 overall. Hemophilia A is four times more common than hemophilia B.
- Hemophilias are divided into mild, moderate, and severe depending on the degree of factor deficiency. The type of genetic mutation that is inherited will determine this.
- Severe hemophilia often manifests as spontaneous bleeding into joints and muscles early in life (before 4 years of age). Easy bruising is also present.
- Moderate hemophilia manifests the same way as severe disease, but may appear later (by 5 years of age)
- Mild hemophilia may manifest as excessive bleeding after trauma or surgery later in life 
- A family history of hemophilia should raise suspicions, although a third of hemophiliacs have no known family history
- Levels of Factor VIII or IX are measured in the blood
- Factor VIII deficiency must be distinguished from Von Willebrand Disease ( see Von Willebrand disease above)
- Infusion of Factor VIII or Factor IX concentrate
- In severe disease, infusions can be given on a weekly basis for life
- In other cases, infusions may be given as needed for surgery or bleeding
- In hemophilia A, 30% of patients develop antibodies (inhibitors) to Factor VIII after repeated infusions making the infusions ineffective. These patients may be treated with immune tolerance protocols using low-dose Factor VIII or with bypassing agents (recombinant activated factor VII [factor VIIa] or activated prothrombin complex concentrate).
- A drug called emicizumab (Hemlibra®) is available for treating hemophilia A. Emicizumab is a recombinant bispecific monoclonal antibody that bridges activated Factor IX and Factor X to restore the function of missing activated Factor VIII. It's effectiveness has been demonstrated in several studies - PMID 28691557, PMID 30157389
- BLEEDING DISORDERS | Acquired disorders
- Acquired bleeding disorders occur when autoantibodies form against clotting factors (factor inhibitors). While antibodies may form to any factor, the most common factor affected is Factor VIII.
- These syndromes are rare with an estimated annual incidence of the most common syndrome, Factor VIII inhibitors, being only 1 - 2 persons per million
- Conditions that increase the risk for factor inhibitors include pregnancy, postpartum state, other autoimmune diseases, malignancy, advanced age, and certain medications. No underlying disorder is identified in up to 46% of patients.
- Treatment involves immunosuppression (e.g. steroids, cyclophosphamide) to eliminate the inhibitor and replacing the affected factor to control bleeding [13,14]
- THROMBOPHILIC DISORDERS
- Thrombophilic disorders (also called hypercoagulable disorders) are a group of diseases that increase a person's risk of forming a pathological blood clot
- Pathological blood clots include deep vein thrombosis, pulmonary embolism, and in rare cases, arterial thrombosis
- Some thrombophilic disorders are genetically inherited (ex. protein C and S deficiency, Factor V Leiden), while others are acquired (ex. antiphospholipid antibodies)
|Prevalence of thrombophilic disorders|
|Disorder||General population||Patients with VTE|
|High concentration of Factor VIII||11%||25%|
|Antiphospholipid antibodies||0 - 5%||24%|
|Factor V Leiden||5%||20%|
|Protein C deficiency||0.2 - 0.4%||3%|
|Protein S deficiency||<0.5%||2%|
- THROMBOPHILIC DISORDERS | High concentration of Factor VIII
- Factor VIII propagates the coagulation cascade
- Elevated levels of Factor VIII appear to be a risk factor for DVT
- This syndrome has only been recognized recently and has not been studied extensively
- Pregnancy and oral contraceptives may raise Factor VIII levels
- The use of oral contraceptives in patients with high concentrations of Factor VIII may increase their risk for blood clots 
- THROMBOPHILIC DISORDERS | Antiphospholipid antibodies
- Antiphospholipid antibodies (APA) are autoimmune antibodies
- The overall incidence of APA in the general population is 0 - 5%. APAs are found in up to 24% of patients with VTE, 20 - 40% of patients with systemic lupus erythematosus, and in 10 - 15% of women who have recurrent miscarriages.
- The exact mechanism by which APAs stimulate clot formation and induce miscarriage is not completely understood, but it is believed to occur through the induction of inflammatory mediators
- The two most common APAs are anti-Beta 2 glycoprotein I antibodies and anticardiolipin antibodies. Antibodies to other phospholipids exist, but they are less common. Having multiple types of APAs present has been shown to confer a greater risk of thrombophilia. [7,8,18]
- Clinical features
- Many patients with APAs have no classic symptoms, and APAs are discovered during workups for autoimmune diseases, prolonged aPTT, and false positive syphilis tests
- Clinical features of the APA syndrome are listed in the table below
|Clinical features associated with APA syndrome|
|Miscarriages (see APA criteria for more)|
|Intrauterine growth restriction|
|Hematologic / Vascular|
|Recurrent VTE, especially at young age|
|Stroke and TIA, especially at young age|
|Acute and chronic renal thrombosis|
|Prolonged activated partial-thromboplastin time (aPTT)|
|False positive syphilis testing (RPR test)|
|Thrombocytopenia (< 100,000 per mm3)|
|Systemic lupus erythematosus|
|Cardiac valve vegetations|
|Livedo reticularis and livedoid vasculopathy|
- Laboratory tests
- Lupus anticoagulant
- The lupus anticoagulant test detects the presence of antiphospholipid antibodies in the blood
- The procedure for the test is explained here - lupus anticoagulant test
- The test should not be performed if the patient is anticoagulated
- Antibody tests
- Anticardiolipin antibodies (IgG and IgM) and anti-Beta 2 glycoprotein I antibodies (IgG and IgM) can be measured directly in the blood via ELISA
- These tests are not affected by anticoagulation
- Diagnostic criteria
- APAs are found in healthy individuals (up to 5%) and may be transiently present in certain conditions (e.g. rheumatological diseases, infections, cancer). Because of this, their presence alone does not establish a diagnosis of the APA syndrome.
- The criteria in the table below were developed to establish a more definitive diagnosis of APA syndrome based on laboratory and clinical criteria
|Diagnostic criteria for APA syndrome|
|Antiphospholipid antibody syndrome is present if at least one clinical criteria and one laboratory criteria are met|
Clinical criteria (one of the following):
Laboratory criteria (one of the following):
- Anticoagulation 
- A small study that compared rivaroxaban to warfarin in patients with high-risk APA syndrome was stopped early when rivaroxaban was found to be clearly inferior to warfarin for preventing thromboembolic events. [PMID 30002145]
- THROMBOPHILIC DISORDERS | Factor V Leiden mutation (activated protein C resistance)
- Factor Va is a coagulation factor that is part of the coagulation cascade
- Factor Va is deactivated by activated protein C
- The deactivation of Factor Va by protein C inhibits clot formation
- The Factor V Leiden mutation is a mutation in the gene that codes for Factor V which makes it resistant to deactivation by protein C
- Resistant Factor Va promotes clot formation because it cannot be deactivated as easily
- Factor V Leiden mutation is the most common thrombophilic disorder with a prevalence of 5% in the general population
- Factor V Leiden mutation is inherited in an autosomal dominant fashion with incomplete penetrance
- Patients who are heterozygous for the mutation are at a slightly increased risk for a blood clot (5 - 10 fold)
- Patients who are homozygous for the mutation are at a greatly increased risk for a blood clot (50 - 100 fold)
- Blood clots in the deep veins of the legs and/or lungs (DVTs and PEs)
- In heterozygous disease, clots often occur when patient is exposed to another risk factor (ex. oral contraceptives, pregnancy)
- Factor V Leiden mutation - PCR test that looks directly for the mutation; test is not affected by anticoagulation
- Activated protein C resistance assay - test measures Factor V resistance to inactivation by protein C; can be used as a screening test for Factor V Leiden mutation; anticoagulation will invalidate the test [6,9,10,21]
- THROMBOPHILIC DISORDERS | Hyperhomocysteinemia
- Homocysteine is an intermediate in amino acid metabolism
- Deficiencies in the enzymes that metabolize homocysteine and/or the vitamins involved in its metabolism (folic acid, vitamin B6, vitamin B12) can lead to elevated levels
- Mild-to-moderate elevations of homocysteine levels occur in 5 - 10% of the population
- Elevated levels of homocysteine are associated with an increased risk of both venous and arterial thrombosis
- The mechanism by which homocysteine promotes thrombosis is not exactly known, but may involve a number of processes including: decreased protein C activation; increased Factor V activity; induction of tissue factor; inhibition of thrombomodulin; decreased antithrombin 
- May cause arterial (strokes and TIAs) or venous (DVTs and PEs) blood clots
- Homocysteine levels can be measured directly in the blood
- High levels may be due to MTHFR (methylenetetrahydrofolate reductase) genetic mutations. This mutation can be measured directly with PCR.
- Normal levels of homocysteine have not been completely standardized 
- Folic acid, Vitamin B12, and Vitamin B6 supplements can lower homocysteine levels, but they have not been shown to decrease the risk of blood clots [6, 11]
- THROMBOPHILIC DISORDERS | Prothrombin 20210A mutation (prothrombin gene mutation)
- Prothrombin 20210A is a genetic mutation of the prothrombin gene that is associated with an increase in prothrombin levels and a mild increase in the risk for VTE
- Prothrombin 20210A mutation is inherited in an autosomal dominant fashion. The mutation is more common in caucasians and rare in Africans and Asians.
- Patients who are heterozygous for the mutation have a 3 fold increased risk for VTE. Patients who are homozygous have a higher risk.
- The mutation can be detected directly with a PCR test. Anticoagulation does not affect the test. 
- THROMBOPHILIC DISORDERS | Protein C deficiency
- Protein C is a Vitamin K-dependent anticoagulant that is made in the liver. Unactivated protein C circulates freely in the blood.
- Protein C works to keep procoagulant (clotting) processes at the site of vessel injury by inhibiting their spread to normal tissue
- Protein C is activated by the thrombomodulin-thrombin complex
- Activated protein C inhibits coagulation by blocking Factor Va and VIIIa
- Protein C deficiency affects 0.2 - 0.4% of the population. People with protein C deficiency are at increased risk for VTE.
- Protein C deficiency may be inherited or acquired. Inherited protein C deficiency follows an autosomal dominant pattern, and it may be either quantitative (decreased levels) or qualitative (defective protein). Acquired protein C deficiency is more common and may be secondary to recent VTE, Vitamin K deficiency, warfarin, advanced liver disease, pregnancy, and sepsis.
- Blood clots in the deep veins of the legs (DVTs) or lungs (PEs)
- Clots typically occur in the late teenage years
- Protein C activity is measured first. If activity is low, an antigen test that measures protein C levels is then performed.
- Anticoagulation will invalidate the protein C activity test. Warfarin will invalidate the antigen test.
- Anticoagulation [6,21]
- THROMBOPHILIC DISORDERS | Protein S deficiency
- Protein S is a Vitamin K-dependent glycoprotein that is made in the liver. Protein S is a cofactor for protein C.
- Protein S and protein C work together to keep procoagulant (clotting) processes at the site of vessel injury by inhibiting their spread to normal tissue
- Protein S/protein C inhibit coagulation by blocking Factor Va and VIIIa
- Protein S deficiency affects < 0.5% of the population. People with protein S deficiency are at increased risk for VTE.
- Protein S deficiency may be inherited or acquired. Inherited protein S deficiency follows an autosomal dominant pattern, and it may be quantitative (decreased synthesis), qualitative (defective protein), or secondary to increased clearance of free protein S, the active form. Acquired protein S deficiency may be secondary to recent VTE, Vitamin K deficiency, warfarin, advanced liver disease, nephrotic syndrome, pregnancy, and sepsis.
- Blood clots in the deep veins of the legs (DVTs) or lungs (PEs)
- Blood clots in the superficial veins
- Clots typically occur in early adulthood
- Free levels of protein S antigen should be measured first. Total levels of protein S antigen are not typically helpful. Protein S activity is a difficult test to perform and is not generally recommended.
- Warfarin therapy will invalidate the free protein S antigen test
- Anticoagulation [4,6,10,21]
- THROMBOPHILIC DISORDERS | Antithrombin deficiency
- Antithrombin (also called antithrombin III) is an anticoagulant made in the liver and in endothelial cells. Antithrombin binds to thrombin and inactivates it.
- Antithrombin deficiency is rare, occurring in about 0.02% of the population. People with antithrombin deficiency are at increased risk for VTE.
- Antithrombin deficiency may be inherited or acquired. Inherited antithrombin deficiency follows an autosomal dominant pattern, and it may be either quantitative (decreased synthesis) or qualitative (defective protein). Acquired antithrombin deficiency is more common and may be secondary to sepsis, heparin therapy, advanced liver disease, and nephrotic syndrome.
- About 50% of patients with antithrombin deficiency will develop a blood clot during their lifetime
- Two-thirds of patients experience a clot by age 35 years
- Antithrombin activity is measured first. If activity is low, an antigen test that measures antithrombin levels is then performed.
- Anticoagulation invalidates the activity test. Heparin may invalidate the antigen test.
- Anticoagulation [4,6,21]
- THROMBOPHILIA TESTING
- When a patient is diagnosed with a VTE, the question often arises as to whether thrombophilia testing should be performed
- Currently, there are no consensus guidelines regarding when, or if, thrombophilia testing should be performed. In addition, there are no good studies that have evaluated the issue.
- Some practitioners routinely order thrombophilia testing on all patients with a VTE while others do not. Thrombophilia testing can be expensive.
- The decision to order thrombophilia testing must be made on an individual basis. Patient characteristics, treatment risks, and individual preferences need to be considered before ordering the testing.
- Since the issue has not been well-studied in prospective trials, the workup can raise more questions than it answers. Some important issues to consider before ordering thrombophilia testing are listed below
- Important considerations
- Studies have shown that patients with a VTE who have a negative workup have the same risk of recurrence as patients with a positive workup. Using negative testing to guide treatment decisions can be misleading.
- Many patients have multiple risk factors, and there is no good guidance for applying the data. For example, a woman on combined oral contraceptive pills who has a VTE is found to have a Factor V Leiden mutation (present in up to 5% of the population). The combined oral contraceptives should be stopped, but whether she would benefit from lifelong anticoagulation is unknown.
- Positive findings often lead to testing in relatives. It's unclear how relatives without a history of VTE who test positive should be managed. [18,20]
- Clinical features associated with thrombophilias
- Thrombophilia syndromes may be inherited genetically or they may be acquired over time
- Clinical features of each type are listed below
- Clinical features of inherited thrombophilias
- VTE at a young age (< 50 years)
- VTE brought on by weak provoking factor (e.g. oral contraceptives, minor surgery, immobility)
- Strong family history of VTE (first-degree family member affected at a young age)
- Recurrent VTE, especially at young age
- VTE in unusual site (e.g. cerebral veins, splanchnic veins) 
- Clinical features associated with acquired thrombophilias (APA syndrome)
- Testing recommendations
- There are no professional guidelines that give detailed recommendations for thrombophilia testing
- A review article on thrombophilia published in the NEJM in 2018 provided an algorithm that can help aid in decision making. The table below summarizes some information from the article.
|Testing for thrombophilia after first VTE|
Cerebral vein VTE
Splanchnic vein VTE
- Testing on anticoagulation
- Some thrombophilia tests are affected by anticoagulation, so in general, testing should not be performed until anticoagulation has been stopped
- All patients with VTE (except distal DVTs in some cases) should be anticoagulated for at least 3 months (see VTE treatment recs)
- Thrombophilia test results do not help guide acute treatment, so there is no reason to perform them at the time of VTE diagnosis
- Vitamin K antagonists like warfarin should be stopped 2 weeks before tests that are affected by anticoagulation. Factor Xa inhibitors and direct thrombin inhibitors should be stopped for 5 half-lives (typically 3 days) before testing.
- The table below lists common thrombophilia tests and whether they are affected by anticoagulation
|Activated protein C resistance
(screening test for Factor V Leiden mutation)
|Factor V Leiden mutation||
|Prothrombin gene mutation||
|Protein C deficiency||
|Protein S deficiency||
|Lupus anticoagulant tests
(aPTT-LA and dRVVT)
|Anticardiolipin antibodies (IgG, IgM)||
|Anti-Beta 2 glycoprotein I antibodies (IgG, IgM)||
- PREGNANCY AND THROMBOPHILIA
- During pregnancy and the 12 weeks following delivery (postpartum), a woman's risk for VTE is greatly increased
- In studies, the risk of VTE is 10-fold higher in pregnant women when compared to nonpregnant controls of similar age. The risk during the postpartum period is even greater with some studies showing a risk that is up to 35X higher. 
- Women who have a history of VTE or known thrombophilia pose a clinical challenge in pregnancy
- A meta-analysis published in 2017 looked at the risk of VTE during pregnancy and the postpartum period for a number of thrombophilias. The results are detailed below.
- A review that covers all the current guidelines for managing women in pregnancy who are at increased risk of VTE was published in 2016. A link to that review is available here - Treatment guidelines for women at increased risk for VTE during pregnancy
- Absolute risk of VTE during pregnancy and the postpartum period in women who are not anticoagulated:
- Controls (no known thrombophilia) - 0.1-0.5%
- Antithrombin deficiency - 16.6%
- Protein C deficiency - 7.8%
- Homozygous Factor V Leiden mutation - 6.2%
- Protein S deficiency - 4.8%
- Compound heterozygous factor V Leiden and prothrombin G20210A mutation - 2.5%
- Heterozygous Factor V Leiden mutation - 1.1%
- Heterozygous prothrombin G20210A mutation - 0.9% 
- COAGULOPATHY OF LIVER DISEASE
- The liver synthesizes clotting Factors II, VII, IX, and X. The liver also synthesizes the anticoagulant factors protein C, protein S, and antithrombin.
- Patients with significant liver disease (cirrhosis) are often deficient in all of these factors
- Traditionally, patients with cirrhosis were considered to have a bleeding disorder because their clotting assays (PT, INR) were often prolonged. However, these assays do not account for deficiencies in anticoagulant factors (protein C and S, antithrombin), and therefore, they do not reflect the overall coagulable state.
- Patients with cirrhosis may develop both bleeding and thrombophilic disorders depending on the imbalance in procoagulant and anticoagulant factors
- More research is needed in order to determine the optimal evaluation and treatment of patients with cirrhosis 
- COAGULATION LABS
- Prothrombin Time (PT) and International Normalized Ratio (INR)
- Factors measured
- II, V, VII, X, and fibrinogen (Factor I)
- In the PT assay, plasma is taken from the patient and stored in a tube containing citrate
- Citrate stabilizes calcium in the plasma and prevents the plasma from clotting
- In the lab, tissue factor and fresh calcium are added to the plasma
- The time it takes for a clot to form is then measured
- INR International Normalized Ratio
- The INR is typically measured with the PT
- Tissue factor used in measuring the PT comes from different manufacturers. Different batches of tissue factor have different activity levels.
- The INR is a calculation used to standardize PT results by taking variations in tissue factor activity into account
- Activated Partial Thromboplastin Time (aPTT)
- Factors measured
- I, II, V, VIII, IX, X, XI, XII
- In the aPTT assay, plasma is taken from the patient and stored in a tube containing citrate
- Citrate stabilizes calcium in the plasma and prevents the plasma from clotting
- In the lab, phospholipid and fresh calcium are added to the plasma
- The time it takes for a clot to form is then measured
- Bleeding time
- The bleeding time test involves pricking a patient's skin and then measuring the amount of time it takes for bleeding to stop
- The test is often ordered to measure platelet function
- The bleeding time has limited utility because of high variability between individuals performing the test and the devices used to perform it
- Lupus anticoagulant
- Lupus anticoagulant is a confusing term for a number of reasons
- The term "lupus anticoagulant" is used in different ways in the medical literature. It is used to refer to a lab test, a blood entity, and a syndrome.
- In its purest sense, "lupus anticoagulant" is a syndrome where antiphospholipid antibodies (APA) are present in the blood
- APAs that lead to a "positive lupus anticoagulant test" include anti-Beta 2 glycoprotein I antibodies, anticardiolipin antibodies, and several other APAs that are less common (ex. antiprothrombin antibodies)
- The term "lupus anticoagulant" comes from the fact that APAs are common in patients with lupus erythematosus
- Although APAs prolong clotting times (act as anticoagulants) in the lab, the presence of APAs is associated with thrombophilic disorders
- Testing for the lupus anticoagulant syndrome is reviewed in the illustration below
- Dilute Russell Viper Venom Time (dRVVT)
- The dRVVT is a test for antiphospholipid antibodies
- APAs are present in the lupus anticoagulant syndrome, and the dRVVT is often used to diagnose this syndrome
- In the dRVVT test, Russell's viper venom and phospholipids are added to the patient's plasma. Russell's viper venom activates Factor X, and in the presence of phospholipids, a clot will form.
- If APAs are present, clot formation will be inhibited, and the dRVVT will be prolonged
- Because Russell's viper venom activates Factor X, the test is not sensitive to factor deficiencies upstream of Factor X
- Mixing studies
- If antiphospholipid antibodies or other factor inhibitors are suspected based on a prolonged aPTT or dRVVT, then a mixing study may be performed
- Mixing studies involve mixing the patient's plasma with normal plasma
- If APAs or factor inhibitors are present, the normal plasma will not correct the clotting time
- If coagulation factors are deficient, the normal plasma will correct the clotting time 
- See lupus anticoagulant illustration for more information
- Anticardiolipin antibodies
- Cardiolipin is a phospholipid found on mitochondrial membranes
- Anticardiolipin antibodies are one of the main antiphospholipid antibodies that are found in the APA syndrome
- Anticardiolipin antibodies (IgG and IgM) can be measured directly in the blood via ELISA
- Anti-Beta 2 glycoprotein I
- Anti-Beta 2 glycoprotein I is a phospholipid found on the surface of cell membranes
- Anti-Beta 2 glycoprotein I antibodies are one of the main antiphospholipid antibodies that are found in the APA syndrome
- Anti-Beta 2 glycoprotein I antibodies (IgG and IgM) can be measured directly in the blood via ELISA
- Factor XIIIa stabilizes a clot by cross-linking fibrin in the fibrin mesh (see coagulation cascade for an illustration)
- A degradation product called a D-dimer is released when cross-linking occurs
- D-dimer levels can be measured in the blood
- An elevated D-dimer level is a sign of ongoing clot formation
- This test is often used to rule out a DVT or PE
- A number of medications affect coagulation
- Medications typically work on either platelet activation or the coagulation cascade
- Antiplatelet medications
- Dipyridamole (Persantine ®, Aggrenox ®)
- GP IIb/IIIa inhibitors (eptifibatide, Integrilin®, tirofiban, Aggrastat®, abciximab, ReoPro®)
- COX inhibitors - NSAIDS (ibuprofen, naproxen, etc.)
- P2Y12 inhibitors (Plavix®, clopidogrel, prasugrel, Effient®, ticagrelor, Brilinta®)
- Vorapaxar (Zontivity®)
- 1 - 15306670 - VWD review
- 2 - NHLBI GL on VWD
- 3 - 12781551
- 4 - 10209995 Lancet hypercoagulable review
- 5 - 20822807 Lancet APA review
- 6 - 11700155 AIM hypercoagulable review
- 7 - 23484830 NEJM APA review
- 8 - 20822807 Lancet APA review
- 9 - 10821379 Lancet coag review
- 10 - 10926920 Clin chem review
- 11 - 17470822 homocysteine trial
- 12 - 21751907 Liver dx review
- 13 - 27406351 Acquired deficiency review
- 14 - 27893111 Mixing Study for Evaluation of Abnormal Coagulation Testing, JAMA (2016)
- 15 - 26780741 Guidance for the treatment and prevention of obstetric-associated venous thromboembolism, J Thromb Thrombolysis (2016)
- 16 - 29074563 Pregnancy, thrombophilia, and the risk of a first venous thrombosis: systematic review and Bayesian meta-analysis, BMJ (2017)
- 17 - 29490924 Antiphospholipid antibodies and recurrent thrombosis after a first unprovoked venous thromboembolism, Blood (2018)
- 18 - 28930509 Thrombophilia Testing and Venous Thrombosis, NEJM (2017)
- 19 - 29791828 Diagnosis and Management of the Antiphospholipid Syndrome, NEJM (2018)
- 20 - 15900005 Thrombophilia, clinical factors, and recurrent venous thrombotic events, JAMA (2005)
- 21 - CPL labs website
- 22 - 16420554 International consensus statement on an update of the classification criteria for definite antiphospholipid syndrome, Journal of Thrombosis and Haemostasis, (2005)