Coagulation Disorders: Written Clinical Reasoning

SAQ 1 — Antiphospholipid Syndrome: Diagnosis, Investigation and Management (20 marks, 30 minutes)

Prompt: Interpret the coagulation studies, explain the paradox of the prolonged aPTT, construct a problem list, and outline the immediate and long-term management plan with justification for each decision. ^[1]

Model Answer

Interpretation of the coagulation studies and the aPTT paradox (5 marks): ^[1]

1. The aPTT is prolonged at 48 seconds but the patient is thrombosing, not bleeding. This is the hallmark paradox of the lupus anticoagulant. In the test tube, the lupus anticoagulant binds to phospholipid in the aPTT reagent, interfering with the clotting reaction and prolonging the time — an in vitro anticoagulant effect. In vivo, the same antibodies (directed against beta-2 glycoprotein I and prothrombin) activate endothelial cells, monocytes and platelets, tipping the haemostatic balance toward thrombus formation — a prothrombotic effect ^[3].

2. The mixing study does not correct. When the patient's plasma is mixed 1:1 with normal pooled plasma, the aPTT remains prolonged. This confirms that the problem is an inhibitor (something in the patient's plasma that neutralises the normal plasma), not a factor deficiency. A factor deficiency would correct because the normal plasma supplies the missing factor. An inhibitor does not correct because it neutralises the factors in both plasmas. ^[1]

3. The confirmatory tests are positive. The dRVVT (dilute Russell viper venom time) is prolonged — a phospholipid-dependent assay that is sensitive to the lupus anticoagulant. The anticardiolipin IgG is 45 GPL (high positive, above the 40 GPL threshold), and anti-beta-2 glycoprotein I is positive. She is triple-positive — any one of the three would satisfy the laboratory criterion, and triple positivity carries the highest thrombotic risk ^[2].

4. The diagnosis is antiphospholipid syndrome. She has the clinical criteria (venous thromboembolism — pulmonary embolism plus DVT — and pregnancy morbidity) and the laboratory criterion, met three times over by triple positivity. She also has a second clinical criterion — pregnancy morbidity with recurrent early miscarriage. The thrombocytopenia (95) is consistent with APS. The SLE association makes this secondary APS ^[2].

Problem list (3 marks): ^[1]

1. Antiphospholipid syndrome with acute pulmonary embolism and deep vein thrombosis — the immediate life-threatening problem requiring emergency anticoagulation.
2. Secondary APS to SLE — the underlying autoimmune condition requiring ongoing immunomodulation.
3. Recurrent pregnancy morbidity — two first-trimester miscarriages from placental microthrombosis, relevant to future pregnancy planning.
4. Thrombocytopenia (platelets 95) — mild, from APS-related platelet activation and consumption; does not preclude anticoagulation. ^[1]

Immediate management plan (5 marks): ^[1]

1. Anticoagulation for the acute pulmonary embolism. Start low-molecular-weight heparin (enoxaparin 1.5 mg/kg daily or weight-adjusted therapeutic dose) immediately. LMWH is preferred over unfractionated heparin in haemodynamically stable patients because it is more predictable and does not require monitoring. If she were haemodynamically unstable (hypotension, massive PE), the options would be systemic thrombolysis or catheter-directed thrombolysis — but she is currently stable on room air. ^[1]

2. Transition to warfarin when stable. Bridge with LMWH for a minimum of 5 days AND until the INR is therapeutic (2-3) for two consecutive days, then stop LMWH. The target INR for a first venous thrombotic event in APS is 2-3. Warfarin is preferred over DOACs in triple-positive APS because the TRAPS trial showed higher event rates with rivaroxaban than warfarin in high-risk patients ^[6].

3. Duration: lifelong anticoagulation. APS-related thrombosis recurs at high rates when anticoagulation is stopped. This is one of the few conditions where anticoagulation is truly indefinite. The patient must understand that this is lifelong treatment. ^[1]

4. Continue hydroxychloroquine. Hydroxychloroquine has been shown to reduce thrombotic risk in APS and SLE. It should be continued. ^[1]

5. Avoid oestrogen-containing contraception. Combined oral contraceptives increase thrombotic risk and are contraindicated in APS. ^[1]

Long-term management and surveillance plan (5 marks): ^[1]

1. Lifelong warfarin with INR 2-3 for venous thrombotic APS. Monitor INR regularly (weekly during initiation, then monthly once stable). The challenge in APS is that the lupus anticoagulant can interfere with some INR assays — if the INR is unreliable, consider monitoring with a chromogenic factor X assay or a specialised INR method. ^[1]

2. Pregnancy planning. If she wishes to conceive, warfarin must be switched to LMWH before conception (warfarin is teratogenic). The standard regimen is low-dose aspirin plus treatment-dose LMWH throughout pregnancy and for 6 weeks postpartum, then switch back to warfarin. This regimen significantly improves pregnancy outcomes in APS ^[2].

3. Screening for other manifestations of APS. Monitor for arterial thrombosis (stroke, myocardial infarction — which are more common in APS than in most thrombophilias), valvular heart disease (Libman-Sacks endocarditis), and catastrophic APS (a medical emergency with 30-50 per cent mortality). ^[1]

4. SLE management. Coordinate with rheumatology for ongoing SLE disease activity monitoring and immunosuppression. Hydroxychloroquine reduces both lupus flares and APS-related thrombosis. ^[1]

5. Patient education. The patient must understand: the risk of recurrence without anticoagulation, the importance of INR monitoring and medication adherence, the symptoms of new thrombosis and catastrophic APS, and the need for pregnancy planning with a haematology-obstetric team. ^[1]

Communication and shared decision-making (2 marks): Explain to the patient that her blood tests show a condition called antiphospholipid syndrome, where the immune system makes antibodies that paradoxically increase the risk of blood clots. The prolonged clotting test in the laboratory is misleading — in the body, her blood is actually too "sticky" and prone to clotting. She needs lifelong blood-thinning medication (warfarin) to prevent further clots, and if she plans to become pregnant, a carefully coordinated switch to heparin injections. Document the discussion about lifelong anticoagulation, the bleeding risk, and the pregnancy plan. ^[1]