Thrombophilia and VTE — Written Clinical Reasoning

SAQ 1 — Recurrent VTE in triple-positive antiphospholipid syndrome (20 marks, 30 minutes)

Prompt: Outline your immediate assessment, the choice and duration of anticoagulation, your approach to future pregnancies, and the shared decision-making framework. Justify each decision with reference to evidence. [1]

Model Answer

Problem list (3 marks): [1]

1. Antiphospholipid syndrome (APS), triple-positive — the patient meets Sydney criteria: vascular thrombosis (two DVTs) plus pregnancy morbidity (two fetal losses beyond 10 weeks), with lupus anticoagulant (prolonged aPTT not correcting on mixing), high-titre anticardiolipin (55 GPL, above the 40 GPL threshold) and positive anti-beta-2 glycoprotein I, confirmed persistent [2].
2. Acute proximal DVT — requires immediate therapeutic anticoagulation.
3. Future pregnancy planning — APS with pregnancy morbidity requires a specific anticoagulation and obstetric strategy.
4. Psychosocial impact — recurrent thrombosis and pregnancy loss in a young woman; counselling and support are essential.

Step 1 — Immediate management (5 marks): [1]

The immediate priority is therapeutic anticoagulation. I would commence low molecular weight heparin (enoxaparin 1 mg/kg twice daily) as the initial agent. The reason for choosing LMWH rather than a DOAC upfront is that this patient has confirmed triple-positive APS, in which DOACs are inferior to warfarin. The TRAPS trial (Pengo et al, Blood 2018) compared rivaroxaban with warfarin in triple-positive APS patients and was terminated early because of an excess of thrombotic events in the rivaroxaban arm (12 per cent vs 0 per cent) [1]. The definitive agent is therefore warfarin, but warfarin cannot be started immediately because it takes 5 to 10 days to reach a therapeutic INR, and starting without heparin overlap risks warfarin-induced skin necrosis (especially in APS patients who may have concurrent protein C or S deficiency). I would overlap LMWH with warfarin for at least 5 days and until the INR is therapeutic (2.0 to 3.0) for 2 consecutive days.

I would also confirm the APS diagnosis meets the persistence criterion — the Sydney criteria require the laboratory tests to be positive on two occasions at least 12 weeks apart [2]. If this is the first documented positive panel, I would repeat at 12 weeks to confirm persistence before committing to lifelong warfarin.

Step 2 — Choice and duration of anticoagulation (5 marks): [1]

Agent: warfarin, target INR 2.0 to 3.0. [1]

For a first VTE in APS, warfarin with a target INR of 2.0 to 3.0 is the standard recommendation. The TRAPS trial specifically addressed the question of whether rivaroxaban could replace warfarin in triple-positive APS (the highest-risk subgroup), and the answer was definitively no — rivaroxaban was inferior [1]. The other DOACs (apixaban, dabigatran, edoxaban) have not been specifically tested in triple-positive APS in a randomised trial, but by extrapolation, warfarin is preferred across the class.

Duration: indefinite (lifelong). [1]

APS-associated VTE carries a very high recurrence risk after stopping anticoagulation (estimated at 20 to 30 per cent per year). This patient has already had a recurrence after a 6-month course of rivaroxaban — she is now on her second event. The recommendation is indefinite anticoagulation. [1]

For recurrent VTE despite a therapeutic INR (which this patient does not yet have — she was on a DOAC, not warfarin), the escalation strategy is to increase the target INR to 3.0 to 4.0, or to add low-dose aspirin (especially if there is an arterial component). [1]

Step 3 — Approach to future pregnancies (5 marks): [1]

This is one of the most complex aspects of managing APS in a woman of reproductive age. The strategy depends on her history — she has both thrombotic APS (two DVTs) and obstetric APS (two fetal losses beyond 10 weeks). This combination places her at very high risk of both maternal thrombosis and pregnancy loss. [1]

During pregnancy:

• LMWH (enoxaparin 1 mg/kg twice daily or 1.5 mg/kg daily, dose-adjusted by weight throughout pregnancy). Warfarin is teratogenic (first trimester) and causes fetal bleeding (third trimester) — it must not be used during pregnancy.
• Low-dose aspirin (100 mg daily) added to LMWH, as the combination has been shown to reduce pregnancy loss in APS with recurrent miscarriage. Start before conception or as soon as pregnancy is confirmed.
• Close obstetric and haematology collaboration — serial monitoring for fetal growth restriction, pre-eclampsia and placental insufficiency (all increased in APS), and planning of the mode and timing of delivery (to allow a window without anticoagulation for neuraxial anaesthesia if needed). [1]

Postpartum:

• Switch to warfarin for at least 6 weeks postpartum (warfarin is safe in breastfeeding; LMWH is also acceptable postpartum if preferred).
• Continue long-term warfarin for the thrombotic APS after the postpartum period. [1]

Counselling: I would counsel her that the combination of aspirin and LMWH during pregnancy reduces the risk of fetal loss from about 60 per cent (untreated) to under 20 per cent, but that pregnancy in APS remains high-risk and requires a planned, supervised pregnancy with a maternal-fetal medicine specialist. I would also advise against oestrogen-containing contraception permanently (it increases VTE risk further in APS). [1]

Step 4 — Communication and shared decision-making (2 marks): [1]

I would sit with the patient and her partner in a quiet room. I would explain APS in plain language — an acquired condition in which the immune system makes antibodies that increase the tendency to clot, affecting both veins and the placenta. I would frame the management as a long-term partnership: lifelong warfarin with regular INR monitoring, a specific pregnancy plan when she wishes to conceive, and avoidance of oestrogen. I would acknowledge the emotional impact of recurrent thrombosis and pregnancy loss. I would give written information, a named contact in the haematology and obstetric teams, and arrange follow-up. I would document the shared decisions and review them as her circumstances evolve. [1]

SAQ 2 — Timing of thrombophilia testing in a young man with unprovoked VTE (10 marks, 20 minutes)

Prompt: A 28-year-old man presents with an unprovoked proximal DVT. His father had a DVT at 40 and his sister had a PE at 32. He is commenced on apixaban. Outline your approach to thrombophilia testing — what to test, when to test, and why — and justify with reference to the assay characteristics and the guidelines. [1]

Model Answer

Step 1 — Whom to test and why (2 marks): [1]

This patient is an appropriate candidate for thrombophilia testing. The indications are: unprovoked VTE at a young age (under 50), and a strong family history (two first-degree relatives with VTE at a young age). The yield of inherited thrombophilia in this group is higher than in the general VTE population, and the result may identify a high-penetrance deficiency (antithrombin, protein C, or protein S) that affects family counselling, future pregnancy planning for female relatives, and potentially the duration of anticoagulation. The ASH 2018 guidelines and NICE NG158 both support testing in this scenario [4].

Step 2 — What to test and the assay characteristics (4 marks): [1]

I would test for: [1]

1. Factor V Leiden (genotyping by PCR) — a DNA test that is NOT affected by the acute thrombotic event or by anticoagulation. It can be sent at any time.
2. Prothrombin G20210A (genotyping by PCR) — similarly a DNA test, unaffected by the acute event or anticoagulation.
3. Antithrombin activity (functional assay) — a clot-based or chromogenic assay. It is LOWERED by acute thrombosis (consumption) and by heparin (including LMWH). It must be tested off heparin and after the acute event has settled.
4. Protein C activity (functional assay) — LOWERED by warfarin (it is vitamin K-dependent) and by acute thrombosis. It must be tested off warfarin and after the acute event.
5. Protein S activity and free antigen (functional and immunologic assay) — LOWERED by warfarin (vitamin K-dependent), acute thrombosis, pregnancy, and oestrogen therapy. It must be tested off warfarin, off oestrogen, and when not pregnant.
6. Lupus anticoagulant, anticardiolipin, and anti-beta-2 glycoprotein I (APS panel) — the APS laboratory tests. They can be distorted by acute thrombosis and by DOACs (which can cause false positive LA results in clot-based assays). They require confirmation on a second occasion at least 12 weeks apart (Sydney persistence criterion) [2].

Step 3 — When to test — the timing strategy (3 marks): [1]

The correct timing is the critical issue: [1]

• Send NOW: Factor V Leiden and prothrombin G20210A genotyping (unaffected by anticoagulation or the acute event). These can be done immediately.
• DEFER the functional assays (antithrombin, protein C, protein S): until the acute event has settled (at least 2 to 4 weeks) AND the patient has stopped anticoagulation for at least 4 to 6 weeks. Testing during the acute event or while on apixaban will give falsely low results, leading to misdiagnosis and unnecessary anxiety.
• DEFER the APS panel: ideally until the patient is off DOACs for at least 2 to 3 days (DOACs can cause false positive lupus anticoagulant results). If the APS panel is sent urgently during the acute event, a positive result must be repeated at 12 weeks to confirm persistence. [1]

The practical approach: send the genetic tests now, and arrange an outpatient thrombophilia clinic appointment at 3 to 6 months to repeat the functional assays and the APS panel after a planned pause in anticoagulation (if the clinical situation allows). The duration of anticoagulation is NOT changed by the immediate test results — this patient with unprovoked VTE gets at least 6 months regardless of the thrombophilia screen, so the testing does not need to be rushed. [1]

Step 4 — Common errors and pitfalls (1 mark): [1]

The most common error is ordering a full thrombophilia screen (including antithrombin, protein C, protein S) during the acute event or while the patient is on anticoagulation, and interpreting the low results as a true deficiency. The second most common error is failing to repeat the APS panel at 12 weeks to confirm persistence. The third is over-interpreting a heterozygous Factor V Leiden or prothrombin G20210A result as a mandate for indefinite anticoagulation — the duration is driven by the clinical event (unprovoked), not by the genotype. [1]

The principle: test the right patient, at the right time, for the right reason — and interpret the result in clinical context. [1]