Inherited hypercoagulable states increasing risk of venous thromboembolism — Factor V Leiden, prothrombin G20210A, protein C/S, antithrombin deficiency.
Also known as: thrombophilia, Factor V Leiden, FVL, prothrombin gene mutation, protein C deficiency, protein S deficiency, antithrombin deficiency, inherited hypercoagulability
Overview
Group of inherited mutations affecting components of the coagulation cascade or natural anticoagulant pathways, predisposing to venous thromboembolism. The five most clinically relevant: Factor V Leiden (FVL), prothrombin G20210A, protein C deficiency, protein S deficiency, and antithrombin deficiency.
Epidemiology
Factor V Leiden — most common (3-8% of Europeans; rare in Asian/African populations); prothrombin G20210A — 1-3% of Europeans; protein C, S, and antithrombin deficiencies each <1% of general population but more frequent in patients with VTE and family history. Combined defects are not rare and multiply risk.
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Question 1HematologyMedium
A 34-year-old woman presents with a first episode of unprovoked left lower-extremity deep vein thrombosis confirmed on compression ultrasound. Her mother and brother both had pulmonary emboli before age 50. She has no prior thrombotic events and takes no other medications. She is started on apixaban and asks about being tested for an inherited clotting disorder. Which of the following is the most appropriate next step in evaluating her for a hereditary thrombophilia?
AOrder MTHFR genotyping and a fasting homocysteine level now as her initial screen
BOrder an APC-resistance assay after her first week of apixaban therapy
CSend a protein C and protein S activity panel before her first apixaban dose today
DDefer thrombophilia testing until 3 months after the event, off anticoagulation
EObtain an antithrombin activity level now while she is acute and untreated
Reveal answer & full explanation
Correct answer: D — Defer thrombophilia testing until 3 months after the event, off anticoagulation
AOrder MTHFR genotyping and a fasting homocysteine level now as her initial screen
BOrder an APC-resistance assay after her first week of apixaban therapy
CSend a protein C and protein S activity panel before her first apixaban dose today
DDefer thrombophilia testing until 3 months after the event, off anticoagulation✓
EObtain an antithrombin activity level now while she is acute and untreated
Why Defer thrombophilia testing until 3 months after the event, off anticoagulation is correct
Acute thrombus consumes natural anticoagulants and triggers an acute-phase response, while anticoagulants directly alter functional assays, so protein C/S activity, antithrombin activity, and APC-resistance testing are all unreliable in this setting, whereas DNA-based genotyping for factor V Leiden and prothrombin G20210A is unaffected by acute thrombosis or anticoagulation and can be sent at any time.
Standard guidance is to defer thrombophilia testing until at least 3 months after the acute event and at least 2 weeks off anticoagulation, and to test only when results would change management such as anticoagulation duration or family counseling.
Her young age, unprovoked event, and strong family history make her a reasonable candidate for eventual testing, but correct timing is what makes the result valid.
Why the others are wrong
Send a protein C and protein S activity panel before her first apixaban dose today — even off anticoagulation, the acute thrombotic and acute-phase state lowers these functional levels and produces false-positive deficiencies, so testing during acute VTE is discouraged.
Obtain an antithrombin activity level now while she is acute and untreated — initial anticoagulation for VTE is the same regardless of thrombophilia, so the result would not change her acute therapy, and an acute level is unreliable.
Order MTHFR genotyping and a fasting homocysteine level now as her initial screen — MTHFR polymorphisms are common in healthy people and do not independently predict venous thrombosis, so ACMG and hematology guidelines recommend against MTHFR testing; hyperhomocysteinemia has dropped out of the standard thrombophilia panel because lowering it does not reduce recurrence, and neither result would alter her anticoagulation or her family's counseling.
Order an APC-resistance assay after her first week of apixaban therapy — starting anticoagulation first can seem to get past the acute-state problem, but one week after the event the acute-phase response is still active and apixaban can interfere with this clot-based assay, so the result is unreliable and would not change her initial treatment; functional thrombophilia testing waits until the acute phase has passed and she is off anticoagulation.
Question 2HematologyMedium
A 34-year-old woman presents with a swollen, painful left calf 2 weeks after a long-haul flight. Compression ultrasound confirms a proximal deep vein thrombosis. She had an unprovoked DVT at age 28, and her mother had a pulmonary embolism at age 45. Three months after completing anticoagulation, genetic testing identifies a heterozygous G1691A point mutation in the factor V gene. Which of the following best explains the findings?
ADeficient protein S cofactor activity for activated protein C
CReduced antithrombin-mediated inhibition of thrombin and factor Xa
DLow protein C levels impairing inactivation of factors Va and VIIIa
EResistance of factor Va to inactivation by activated protein C✓
Why Resistance of factor Va to inactivation by activated protein C is correct
The factor V Leiden G1691A point mutation alters the cleavage site where activated protein C (APC) normally inactivates factor Va.
The mutant factor Va resists APC, so it persists and sustains thrombin generation, shifting hemostatic balance toward venous thrombosis.
This is the most common hereditary thrombophilia in people of European ancestry and is detected by an APC-resistance assay confirmed with factor V Leiden genetic testing.
Why the others are wrong
Increased prothrombin transcription elevating plasma prothrombin describes the prothrombin G20210A 3' UTR mutation, a different thrombophilia not identified by this factor V gene test.
Reduced antithrombin-mediated inhibition of thrombin and factor Xa describes antithrombin deficiency, which confers the highest absolute thrombosis risk but is unrelated to the factor V mutation here.
Deficient protein S cofactor activity for activated protein C describes protein S deficiency; protein S deficiency impairs APC function, but the defect here is in the factor V substrate, not the protein S cofactor.
Low protein C levels impairing inactivation of factors Va and VIIIa — protein C deficiency is tempting because it also leaves factor Va (and VIIIa) active and causes familial venous thrombosis, but there the enzyme itself is deficient; in factor V Leiden protein C levels are normal and the defect is the mutant factor Va substrate, which resists cleavage by activated protein C.
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Factor V Leiden — G1691A point mutation makes activated factor V resistant to cleavage by activated protein C → sustained thrombin generation. Prothrombin G20210A — 3' UTR mutation increases prothrombin transcription and plasma levels. Protein C and S deficiencies — reduce inactivation of factors Va and VIIIa. Antithrombin deficiency — reduces inhibition of thrombin and factors Xa, IXa, XIa. Net effect: shift of hemostatic balance toward thrombosis, primarily venous.
Clinical presentation
Symptoms
Deep vein thrombosis — unilateral leg swelling, pain, warmth
Pregnancy-associated VTE (antepartum and especially postpartum); links to late fetal loss, preeclampsia, FGR, and abruption are weak and inconsistent, and unlike antiphospholipid syndrome, inherited thrombophilias have no established causal role in recurrent early pregnancy loss — ACOG advises against testing for these outcomes
Neonatal/childhood purpura fulminans — severe protein C deficiency
HIT — Heparin exposure with platelet drop and thrombosis
Estrogen/OCP- or pregnancy-related thrombosis — Acquired transient hypercoagulability; resolves with discontinuation/postpartum
Diagnostic workup
Diagnostic criteria
Diagnosis is laboratory-based once acute thrombus and anticoagulation effects have resolved. Testing should only be pursued if results will alter management (duration of anticoagulation, family counseling) — many experts argue against routine thrombophilia screening because results rarely change management.
Thrombophilia testing — DEFER until ≥3 months after acute event and ≥2 weeks off anticoagulation (acute thrombus and anticoagulants alter functional assays)
Factor V Leiden — activated protein C (APC) resistance assay; confirm with FVL genetic testing
Prothrombin G20210A — PCR-based genetic test
Protein C activity (functional assay)
Protein S — free protein S antigen plus protein S activity
Antiphospholipid panel concurrently to exclude APS
Imaging
Compression ultrasound for suspected DVT
CT pulmonary angiogram or V/Q scan for PE
MR venography for cerebral sinus or splanchnic thrombosis
Diagnostic algorithm
Thrombophilia
Prevalence (general)
Relative VTE risk
Test
Factor V Leiden (heterozygous)
3-8% Europeans
~3-5x
APC resistance → FVL PCR
Factor V Leiden (homozygous)
0.02%
~10-80x
FVL PCR
Prothrombin G20210A (het)
1-3% Europeans
~2-3x
Targeted PCR
Protein C deficiency
~0.2%
~7-10x
Protein C activity
Protein S deficiency
~0.2%
~5-10x
Free protein S antigen + activity
Antithrombin deficiency
~0.02%
~10-20x
Antithrombin activity
Prevalence and VTE risk of the five major hereditary thrombophilias.
Treatment
First-line
Acute VTE — therapeutic anticoagulation: LMWH (enoxaparin) bridge to warfarin, or DOAC (apixaban, rivaroxaban, edoxaban, dabigatran) — same as VTE without thrombophilia
Direct oral anticoagulants are first-line for most inherited thrombophilias; warfarin still preferred in severe antithrombin deficiency or triple-positive APS
Avoid combined oral contraceptives and estrogen-containing HRT in carriers
First unprovoked VTE + heterozygous FVL or prothrombin G20210A
3-6 months of primary anticoagulation, then offer extended/indefinite anticoagulation (DOAC; reduced-dose apixaban or rivaroxaban acceptable) unless bleeding risk is high — the unprovoked event, not the heterozygous genotype, drives this (CHEST 2021, ASH 2020)
If a second unprovoked VTE occurs: indefinite anticoagulation regardless of thrombophilia status
Homozygous FVL, homozygous prothrombin, or compound heterozygotes
Indefinite anticoagulation often recommended after first unprovoked VTE
Same approach for severe protein C, S, or antithrombin deficiency
Antithrombin deficiency with acute VTE
Heparin may have blunted response; consider antithrombin concentrate
Long-term anticoagulation; warfarin often preferred; DOACs increasingly used
Pregnancy in known thrombophilia
LMWH (enoxaparin, dalteparin) prophylactic or therapeutic dosing based on personal/family history; a prior VTE that was unprovoked or pregnancy/estrogen-associated calls for prophylactic LMWH throughout pregnancy (starting early, not just the third trimester) plus 6 weeks postpartum (ACOG, ASH 2018)
Avoid warfarin (teratogen) and DOACs (limited data) in pregnancy
Continue 6 weeks postpartum
Second-line / adjunct
Antithrombin concentrate for surgery/childbirth in antithrombin deficiency
Genetic counseling for family members; test asymptomatic first-degree relatives only when it would change management, mainly with a family history of antithrombin, protein C, or protein S deficiency (estrogen contraception/HRT decisions, thromboprophylaxis) or in pregnancy with a family history of high-risk thrombophilia (ASH 2023)
Complications
Recurrent VTE
Post-thrombotic syndrome (chronic leg pain, swelling, ulceration)
Pregnancy-associated VTE; associations with fetal loss, preeclampsia, FGR, and placental abruption are weak and inconsistent (contrast antiphospholipid syndrome)
Warfarin-induced skin necrosis in protein C/S deficiency
Neonatal purpura fulminans in homozygous protein C or S deficiency
PANCE pearls
Do not order thrombophilia panels during acute thrombosis or while on anticoagulation — functional assays will be inaccurate.
The strongest indication for testing is when results would alter management (treatment duration, family planning); for most provoked VTEs, testing changes nothing.
Factor V Leiden heterozygosity is common but a relatively weak risk factor — provoking factors (estrogen, surgery, immobility) drive most events.
Combined estrogen contraceptives raise VTE risk ~30-35x in heterozygous FVL carriers (higher still in homozygotes); estrogen-containing contraception is contraindicated (US MEC category 4) with any known thrombogenic mutation — use progestin-only or non-hormonal methods.
Antithrombin deficiency confers the highest absolute thrombosis risk among hereditary thrombophilias.
Warfarin initiation in protein C/S deficiency can transiently worsen the procoagulant state — always overlap with heparin/LMWH until INR therapeutic for ≥48 hours.
References
ASH 2023 Guidelines — Middeldorp S et al. American Society of Hematology 2023 guidelines for management of venous thromboembolism: thrombophilia testing. Blood Adv 2023.
NEJM review 2017 — Connors JM. Thrombophilia testing and venous thrombosis. N Engl J Med 2017; 377:1177-1187.
ACOG PB 197 — American College of Obstetricians and Gynecologists. ACOG Practice Bulletin No. 197: Inherited Thrombophilias in Pregnancy. Obstet Gynecol 2018;132(1):e18-e34
CHEST 2021 — Stevens SM, Woller SC, Baumann Kreuziger L, et al. Antithrombotic therapy for VTE disease: second update of the CHEST guideline and expert panel report. Chest 2021;160(6):e545-e608
ASH 2020 — Ortel TL, Neumann I, Ageno W, et al. American Society of Hematology 2020 guidelines for management of venous thromboembolism: treatment of deep vein thrombosis and pulmonary embolism. Blood Adv 2020;4(19):4693-4738
ASH 2018 — Bates SM, Rajasekhar A, Middeldorp S, et al. American Society of Hematology 2018 guidelines for management of venous thromboembolism: venous thromboembolism in the context of pregnancy. Blood Adv 2018;2(22):3317-3359
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