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. All three laboratory criteria of the Sydney classification are met [2].

4. The diagnosis is antiphospholipid syndrome. She has one clinical criterion (venous thromboembolism — pulmonary embolism plus DVT) and one laboratory criterion (lupus anticoagulant plus anticardiolipin plus anti-beta-2 GPI). 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 APS because the TRAPS trial showed higher event rates with rivaroxaban in high-risk APS patients [2].

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]

SAQ 2 — Interpreting an Abnormal Coagulation Screen in a Septic Patient (10 marks, 20 minutes)

Prompt: A 65-year-old man with acute pancreatitis is in the intensive care unit. He is bleeding from his nasogastric tube and venepuncture sites. Bloods: platelets 32 x 10^9/L, PT 26 seconds (prolonged), aPTT 58 seconds (prolonged), fibrinogen 0.7 g/L (low), D-dimer strongly positive. Outline the diagnosis, the scoring system you would use, and the management priorities. [1]

Model Answer

Diagnosis (3 marks): This is overt disseminated intravascular coagulation (DIC) secondary to severe acute pancreatitis. The lab pattern is the classic tetrad: thrombocytopenia (consumption), prolonged PT and aPTT (consumption of clotting factors), low fibrinogen (consumption and fibrinolysis), and elevated D-dimer (breakdown of cross-linked fibrin by plasmin). The bleeding from venepuncture sites and NG tube is characteristic — these are low-pressure sites where the consumption coagulopathy is most apparent [1].

ISTH DIC scoring (3 marks): The ISTH scoring system requires an underlying disorder known to be associated with DIC (present — acute pancreatitis), then assigns points: [1]

Total score: 8. A score of 5 or more is compatible with overt DIC. The score should be repeated daily to track the response to treatment of the underlying cause [1].

Management priorities (4 marks): [1]

1. Treat the underlying pancreatitis. This is the single most important intervention. DIC is a syndrome — it resolves when the trigger is removed. Supportive blood product replacement is futile if the pancreatitis is not controlled. Ensure aggressive fluid resuscitation, analgesia, nil by mouth, and surgical or endoscopic intervention for any underlying biliary obstruction or necrotic tissue. [1]

2. Support the coagulopathy with blood products — not prophylactically, but for active bleeding:

   • Cryoprecipitate 10 units (or fibrinogen concentrate) — the fibrinogen is critically low at 0.7 g/L. Give cryoprecipitate to raise the fibrinogen above 1.5 g/L (actively bleeding). This is the fastest way to provide fibrinogen.
   • Fresh frozen plasma 15 mL/kg — replaces consumed clotting factors and helps correct the prolonged PT and aPTT.
   • Platelet transfusion 1 adult dose — the platelets are 32 with active bleeding. Aim for a platelet count above 50 in a bleeding patient. [1]

3. Tranexamic acid 1 g IV may be considered as an antifibrinolytic for refractory bleeding, but use with caution as DIC has a thrombotic component — microvascular thrombosis may worsen. [1]

4. Monitor the ISTH DIC score daily, along with platelet count, fibrinogen, PT and D-dimer. Improving trends indicate that the underlying pancreatitis is coming under control. Worsening trends indicate escalation of treatment for the pancreatitis. [1]