Residency · Residency · Hematology Thrombosis

Thrombophilia Evaluation

Introduction

Thrombophilia refers to inherited or acquired conditions that predispose individuals to venous thromboembolism (VTE). While thrombophilia testing is one of the most frequently ordered evaluations in hematology practice, it is also among the most commonly over-ordered and misinterpreted. The fundamental clinical question that should guide every thrombophilia evaluation is not "does this patient have a thrombophilia?" but rather "will knowing the result change my management?" In the majority of clinical scenarios, thrombophilia results do not alter anticoagulation decisions, as the duration and intensity of anticoagulation are determined primarily by whether the VTE was provoked or unprovoked, the bleeding risk, and patient preference. Testing should therefore be judicious, appropriately timed, and performed only when the results have a realistic probability of influencing clinical management.

Inherited Thrombophilias

Factor V Leiden (FVL)

Factor V Leiden is the most prevalent inherited thrombophilia, with a carrier frequency of approximately 5% in Caucasian populations. It is notably rare in individuals of African and Asian descent. The underlying mutation is a single nucleotide substitution (Arg506Gln) that alters the activated protein C (APC) cleavage site on factor Va, rendering it resistant to inactivation by the APC anticoagulant pathway. This resistance to APC prolongs the activity of factor Va in the prothrombinase complex, resulting in increased thrombin generation.

The thrombotic risk conferred by factor V Leiden is strongly dependent on zygosity. Heterozygous carriers have a 3- to 8-fold increased risk of VTE compared to the general population, while homozygous individuals face a 50- to 80-fold increased risk. The risk is further amplified when factor V Leiden is co-inherited with other thrombophilic defects; in particular, the combination of heterozygous factor V Leiden with heterozygous prothrombin G20210A mutation produces a multiplicative increase in VTE risk that substantially exceeds the sum of the individual risks. Laboratory diagnosis begins with the APC resistance assay as a screening test, which has been modified in modern iterations to remain specific for factor V Leiden even in patients receiving DOAC or heparin therapy. A positive screening result is confirmed by PCR-based genetic testing for the specific factor V Leiden mutation. It is important to recognize that factor V Leiden is primarily a risk factor for venous thrombosis; the association with arterial thrombotic events is weak and not considered clinically actionable.

Prothrombin Gene Mutation (G20210A)

The prothrombin G20210A mutation is the second most common inherited thrombophilia, with a carrier frequency of 2 to 3% in Caucasian populations. The mutation is located in the 3' untranslated region (UTR) of the prothrombin gene and results in elevated circulating prothrombin levels, approximately 30% higher than normal, which in turn leads to increased thrombin generation potential. Heterozygous carriers have a 2- to 5-fold increased risk of VTE, while homozygosity, which is rare, confers an approximately 20-fold increased risk. Unlike factor V Leiden, the prothrombin mutation cannot be reliably diagnosed by measuring prothrombin levels, as there is substantial overlap between the prothrombin levels of carriers and non-carriers. Diagnosis requires PCR-based genetic testing for the specific G20210A polymorphism.

Antithrombin Deficiency

Antithrombin deficiency is the most thrombogenic of the inherited thrombophilias, conferring a 10- to 50-fold increased risk of VTE. Despite this potency, it is rare, with a prevalence of approximately 0.02 to 0.1% in the general population and 1 to 2% among patients presenting with VTE. Two types are recognized: Type I represents a quantitative deficiency with proportionally reduced antigen and functional activity, while Type II represents a qualitative defect in which the antigen level is normal but functional activity is reduced due to mutations affecting the reactive site or the heparin-binding site.

Diagnosis is established through the antithrombin activity assay using either the heparin cofactor or chromogenic anti-Xa method. A critical caveat in interpretation is that antithrombin levels are reduced by numerous clinical conditions and therapeutic interventions, including acute thrombosis (consumption), heparin therapy (accelerated clearance through AT-protease complex formation), DIC, liver disease, nephrotic syndrome (urinary loss), and pregnancy. Testing during these states will produce falsely low results and should be avoided; the assay must be performed in a clinically stable state off heparin therapy.

A distinctive clinical feature of antithrombin deficiency is heparin resistance. Antithrombin should be suspected when the aPTT fails to prolong adequately despite escalating heparin doses, as the anticoagulant effect of heparin is entirely dependent on AT as its cofactor. In this situation, antithrombin concentrate (either recombinant ATryn or plasma-derived) can be administered to restore heparin sensitivity.

Protein C Deficiency

Protein C deficiency occurs in approximately 0.2 to 0.5% of the general population and accounts for 3 to 5% of patients presenting with VTE. It is inherited in an autosomal dominant fashion and confers a 5- to 10-fold increased risk of VTE. Homozygous protein C deficiency is a devastating condition that presents in neonates with purpura fulminans, a life-threatening form of massive DIC with widespread skin necrosis that requires urgent protein C replacement.

Protein C deficiency is also the pathophysiologic basis for warfarin-induced skin necrosis, a complication that occurs during the initial loading phase of warfarin therapy. Because protein C has a short half-life of approximately 8 hours, its levels decline rapidly after warfarin initiation, well before the procoagulant vitamin K-dependent factors (particularly factor II, with a half-life of 72 hours) are adequately suppressed. This creates a transient window of hypercoagulability that can trigger microvascular thrombosis and skin necrosis, particularly in patients with underlying protein C deficiency. Prevention requires bridging with heparin when initiating warfarin and starting warfarin at a low dose not exceeding 5 mg daily.

Diagnosis is established by measuring protein C activity using a functional assay, with confirmation by antigen measurement if the activity level is low. Protein C levels are decreased by warfarin (which inhibits its synthesis), liver disease, DIC, acute thrombosis, and vitamin K deficiency, making timing of testing essential.

Protein S Deficiency

Protein S functions as the essential non-enzymatic cofactor for activated protein C in the inactivation of factors Va and VIIIa. It circulates in two forms: approximately 40% is free and functionally active, while the remaining 60% is bound to C4b-binding protein (C4b-BP) and is functionally inactive. This distribution is clinically important because conditions that increase C4b-BP levels will shift the equilibrium toward the bound fraction, reducing the free protein S available for anticoagulant function.

Protein S deficiency has a prevalence of approximately 0.03 to 0.1% in the general population and confers a 3- to 10-fold increase in VTE risk. The best screening test is the free protein S antigen level, as this directly measures the functionally relevant fraction. The protein S activity assay provides functional assessment. Protein S levels are decreased by an exceptionally wide range of clinical conditions and medications, including warfarin therapy, pregnancy (estrogen increases C4b-BP synthesis), oral contraceptive use, acute inflammation (C4b-BP is an acute-phase reactant), liver disease, nephrotic syndrome, DIC, and HIV infection. This extensive list of confounding factors makes protein S one of the most commonly falsely low results in thrombophilia panels, and a low value must always be confirmed by repeat testing in a stable clinical state.

Other Inherited Thrombophilias

Elevated factor VIII activity (above 150%) is an independent risk factor for VTE, conferring a 1.5- to 6-fold increased risk. When the elevation is sustained and confirmed on repeat testing in a non-acute-phase state, it appears to be heritable and is considered a thrombophilic trait. Elevated lipoprotein(a) is increasingly recognized as a risk factor for both arterial and venous thrombosis and is genetically determined. Dysfibrinogenemia is a rare inherited disorder that can cause either thrombosis or bleeding depending on the specific molecular defect. It is characterized by a prolonged thrombin time with low functional fibrinogen activity relative to a normal or elevated fibrinogen antigen level.

Acquired Thrombophilias

Antiphospholipid Syndrome (APS)

Antiphospholipid syndrome is the most clinically important acquired thrombophilia and the most consequential thrombophilia to diagnose because it directly changes both the choice of anticoagulant and the duration of therapy. APS is a systemic autoimmune disorder defined by the occurrence of vascular thrombosis or pregnancy morbidity in the presence of persistent antiphospholipid antibodies.

The diagnostic criteria, based on the revised Sapporo/Sydney classification, require at least one clinical criterion and at least one laboratory criterion. Clinical criteria include vascular thrombosis (arterial, venous, or small vessel, confirmed by imaging or histopathology) or pregnancy morbidity (one or more unexplained deaths of a morphologically normal fetus at or beyond 10 weeks of gestation; one or more premature births before 34 weeks due to eclampsia, severe preeclampsia, or recognized features of placental insufficiency; or three or more unexplained consecutive spontaneous pregnancy losses before 10 weeks). Laboratory criteria, which must be confirmed on at least two occasions separated by a minimum of 12 weeks to exclude transient positivity, include the lupus anticoagulant (LA), anticardiolipin antibodies (IgG or IgM at titers exceeding 40 GPL or MPL units, or above the 99th percentile), and anti-beta-2-glycoprotein I antibodies (IgG or IgM above the 99th percentile).

Triple-positive APS, defined by the simultaneous presence of all three antibody types, identifies the highest-risk subset of patients, with an annual thrombotic recurrence rate approaching 10% even on anticoagulation. The treatment of APS-related VTE requires warfarin with an INR target of 2.0 to 3.0, continued indefinitely. DOACs are inferior to warfarin in triple-positive APS, as demonstrated definitively in the TRAPS trial, which compared rivaroxaban to warfarin and was terminated early due to excess thrombotic events in the rivaroxaban arm. For arterial thrombosis in APS, warfarin with an INR target of 2.0 to 3.0 is standard, though some experts advocate for a higher target of 3.0 to 4.0 based on limited data. For obstetric APS, treatment consists of LMWH combined with low-dose aspirin. DOACs may be considered only for low-risk, non-triple-positive APS patients with isolated venous events, though this remains an area of ongoing investigation.

Catastrophic APS (CAPS) is a rare, life-threatening variant characterized by rapid-onset multi-organ thrombosis developing within less than one week. Treatment requires a multimodal approach including anticoagulation, high-dose corticosteroids, plasma exchange and/or IVIG, and in refractory cases, eculizumab (a complement C5 inhibitor). Despite aggressive treatment, mortality remains approximately 50%.

<image>A comprehensive thrombophilia evaluation guide. At the top, show a decision framework: "When to Test for Thrombophilia" with two columns: "Testing May Be Useful" (unprovoked VTE in patients <50, VTE in unusual sites, family history of VTE with known thrombophilia, recurrent unexplained pregnancy loss, warfarin-induced skin necrosis, neonatal purpura fulminans) vs. "Testing Generally NOT Useful" (provoked VTE, elderly first VTE, will not change management, during acute thrombosis or anticoagulation). Below, display a table of all thrombophilias (Factor V Leiden, prothrombin G20210A, AT deficiency, protein C deficiency, protein S deficiency, antiphospholipid syndrome) with columns for: prevalence, relative VTE risk, test of choice, factors that affect test results (acute phase, anticoagulation, pregnancy), and clinical implications. Include arrows showing which tests are affected by which medications (warfarin → protein C/S, heparin → AT, DOACs → LA assays). At the bottom, list the APS diagnostic criteria with the triple-positive high-risk designation highlighted. Medical education reference card format.</image>

When to Test (and When NOT to Test)

Indications for Thrombophilia Testing

Thrombophilia testing is most likely to be clinically useful in specific, well-defined scenarios. These include unprovoked VTE in young patients under 50 years of age, where the result may genuinely influence the decision regarding anticoagulation duration. VTE occurring in unusual anatomic sites, such as cerebral venous thrombosis, splanchnic vein thrombosis, or unprovoked upper extremity DVT, may warrant evaluation given the higher probability of an underlying thrombophilic state. Recurrent unprovoked VTE may prompt testing, though these patients are generally committed to indefinite anticoagulation regardless of the result. Testing is valuable when a strong family history of VTE has been identified with a known thrombophilia in a proband, allowing cascade testing of at-risk family members to guide prophylaxis during high-risk situations. APS testing specifically is indicated for recurrent pregnancy loss. Protein C deficiency testing is relevant when warfarin-induced skin necrosis has occurred, and homozygous protein C or S deficiency should be investigated in neonates presenting with purpura fulminans. Antithrombin deficiency should be considered in patients demonstrating heparin resistance.

Testing is Generally NOT Indicated

Thrombophilia testing is generally not indicated and adds cost without clinical benefit in several common scenarios. Provoked VTE with a clearly identified transient risk factor does not warrant testing because the duration of anticoagulation is determined by the provoking factor, not by the thrombophilia status. A first VTE in elderly patients rarely leads to management changes based on thrombophilia results. Patients who are already committed to indefinite anticoagulation gain nothing from testing. Testing during acute VTE, while on anticoagulation, during pregnancy, or during acute illness yields unreliable results due to the numerous confounders that affect functional assay levels in these states. Population-level screening before oral contraceptive prescription is not cost-effective and should be reserved for individuals with a strong family history of VTE.

When to Test (Timing)

*Some DOAC interference reported with certain functional platforms.

Optimal timing for thrombophilia testing is at least 2 to 4 weeks after completing anticoagulation, in a clinically stable, non-pregnant, non-acutely ill patient. However, two important exceptions exist. Factor V Leiden and prothrombin G20210A are genetic tests performed by PCR and are not affected by anticoagulation, acute thrombosis, pregnancy, or any clinical condition; they can therefore be performed at any time. This distinction between genetic tests and functional assays is one of the most important practical points in thrombophilia evaluation.

For the functional assays, specific anticoagulant effects must be considered. Antithrombin should not be measured on heparin (which lowers AT levels through accelerated complex formation) but can be tested on warfarin or DOACs. Protein C and protein S should not be measured on warfarin (which suppresses their synthesis) but can be tested on heparin or DOACs, though some DOAC assay interference has been reported with certain functional platforms. Lupus anticoagulant testing should not be performed on heparin or DOACs (which interfere with the phospholipid-dependent clotting assays used for LA detection) but can be assessed on warfarin with careful interpretation.

Impact on Management

Does Thrombophilia Testing Change Anticoagulation Duration?

For the majority of patients, the answer is no. Heterozygous factor V Leiden or prothrombin gene mutation alone does not independently mandate extended anticoagulation. The decision for extended therapy is based primarily on whether the VTE was provoked or unprovoked, the individual's bleeding risk, D-dimer levels after stopping anticoagulation, patient sex (males have higher recurrence rates), and patient preference. Adding a common, low-risk thrombophilia to this equation rarely tips the balance.

Important exceptions exist where thrombophilia testing genuinely influences management. Antithrombin deficiency carries such a high recurrence risk that there is a strong argument for indefinite anticoagulation after a first unprovoked VTE. Homozygous factor V Leiden or combined thrombophilic defects (such as concurrent FVL and prothrombin mutation) confer a sufficiently elevated recurrence risk to favor extended anticoagulation. Triple-positive APS mandates indefinite anticoagulation with warfarin specifically, with DOACs being contraindicated. Protein C deficiency, when diagnosed, requires awareness that warfarin initiation must be performed with heparin bridging to prevent skin necrosis.

When Thrombophilia Testing IS Clearly Useful

Cascade testing of family members of a proband with a high-risk thrombophilia (antithrombin, protein C, or protein S deficiency) represents one of the most valuable applications of thrombophilia testing. Identifying an at-risk family member allows for targeted thromboprophylaxis during high-risk situations such as surgery, immobilization, or pregnancy, potentially preventing a first thrombotic event. The diagnosis of APS is clearly useful because it changes the anticoagulant choice (warfarin rather than DOACs), mandates indefinite duration, and in obstetric APS, requires specific prophylaxis with LMWH and aspirin during pregnancy. For pregnancy management more broadly, identification of antithrombin, protein C, or protein S deficiency guides decisions regarding thromboprophylaxis during pregnancy and the postpartum period.

Key Clinical Pearls

• The most common error in thrombophilia testing is ordering tests during acute thrombosis or on anticoagulation, leading to false results and misdiagnosis
• Factor V Leiden and prothrombin G20210A are genetic tests that can be performed at any time regardless of anticoagulation status; protein C, protein S, and antithrombin are functional tests affected by anticoagulation and acute illness
• Heterozygous Factor V Leiden or prothrombin mutation alone does NOT change anticoagulation duration for a provoked VTE; testing adds nothing to management in this scenario
• Antiphospholipid syndrome is the most clinically impactful thrombophilia to diagnose: it changes the choice of anticoagulant (warfarin, NOT DOACs for triple-positive) and mandates indefinite treatment
• Testing all patients before starting OCPs is NOT cost-effective; reserve for those with strong family history of VTE
• When in doubt about whether to test, ask: "Will the result change what I do?" If the answer is no, do not test

References

1. Connors JM. Thrombophilia Testing and Venous Thrombosis. N Engl J Med. 2017;377(12):1177-1187.
2. Pengo V, et al. Rivaroxaban vs warfarin in high-risk patients with antiphospholipid syndrome (TRAPS). Blood. 2018;132(13):1365-1371.
3. Middeldorp S, et al. ASH 2023 Guidelines for Management of Venous Thromboembolism: Thrombophilia Testing. Blood Adv. 2023;7(22):7101-7138.
4. Stevens SM, et al. Antithrombotic therapy for VTE disease: CHEST Guideline. Chest. 2021;160(6):e545-e608.
5. Garcia D, Erkan D. Diagnosis and management of the antiphospholipid syndrome. N Engl J Med. 2018;378(21):2010-2021.

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