Adverse Drug Reactions — DCE Clinical Case

DCE Long Case — Heparin-Induced Thrombocytopenia in a Complex Elderly Man

The patient

A 78-year-old man with atrial fibrillation (on apixaban), heart failure with reduced ejection fraction, type 2 diabetes, chronic kidney disease (eGFR 35) and osteoarthritis was admitted eight days ago with a proximal left leg deep vein thrombosis and a submassive pulmonary embolism. His apixaban was withheld on admission and he was started on therapeutic subcutaneous enoxaparin. Today, day 8, his platelet count has fallen from 260 to 120 times ten to the ninth per litre (a fall of 54 per cent). He has new swelling and pain in the right leg. A Doppler ultrasound confirms a new right iliofemoral deep vein thrombosis. He is afebrile, his other bloods are stable, and he has no bleeding. His medications are enoxaparin, metoprolol, frusemide, spironolactone, empagliflozin (held on admission), pantoprazole, and paracetamol. ^[1]

Candidate's opening statement (SASPOP format)

"A 78-year-old man with an acute, evolving and dangerous haematological complication of anticoagulation (symptoms and presentation), aged 78 (age), male (sex), presenting on day 8 of an admission for pulmonary embolism (presentation), who is a retired teacher (occupation), with the following problems: (1) heparin-induced thrombocytopenia — HIT — a severe Type B adverse drug reaction to enoxaparin, complicated by a new contralateral iliofemoral deep vein thrombosis; (2) the original submassive pulmonary embolism and left proximal DVT; (3) atrial fibrillation with an indication for long-term anticoagulation; (4) heart failure with reduced ejection fraction; (5) chronic kidney disease stage 3b with an eGFR of 35; (6) type 2 diabetes; and (7) osteoarthritis. The immediate threat is the prothrombotic state of HIT, which can produce limb-threatening or life-threatening thrombosis. My immediate plan is to stop all heparin, start an alternative non-heparin anticoagulant at a therapeutic dose, send a PF4 antibody, and arrange ongoing anticoagulation strategy with haematology." ^[1]

Structured problem list

1. Heparin-induced thrombocytopenia (Type B ADR) — confirmed by the 50 per cent platelet fall, the 5 to 14 day timing, the new thrombosis, and the high pre-test probability on the 4T score.
2. New right iliofemoral DVT — a thrombotic complication of HIT.
3. Submassive pulmonary embolism and left proximal DVT (the original presentation).
4. Atrial fibrillation with a long-term oral anticoagulation indication.
5. Heart failure with reduced ejection fraction on guideline therapy.
6. Chronic kidney disease stage 3b (eGFR 35), which influences the choice of alternative anticoagulant.
7. Type 2 diabetes and osteoarthritis. ^[1]

Integrated management plan

Problem 1 and 2 — HIT and the new DVT: ^[1]

I calculate the 4T score: Thrombocytopenia (fall of 54 per cent, nadir above 100 — score 1), Timing (day 8 of first heparin exposure, consistent with 5 to 14 days — score 2), Thrombosis (new iliofemoral DVT — score 1), and oTher cause (none — score 1). The total is 5, a high pre-test probability. This effectively confirms HIT clinically, though I will send a PF4 enzyme immunoassay and consider a serotonin release assay for confirmation. ^[1]

The immediate management is:

• Stop all heparin immediately — including the enoxaparin, any heparin flushes, and heparin-coated lines. I do not switch to unfractionated heparin because the antibody cross-reacts with all heparins.
• Start an alternative non-heparin anticoagulant at a therapeutic dose. Given his chronic kidney disease (eGFR 35), I would use argatroban (hepatically cleared, preferred in renal impairment) or fondaparinux (partially renal, dose-adjusted) or a direct oral anticoagulant such as rivaroxaban or apixaban (which are increasingly used and supported by recent guidance). Apixaban is reasonable here because he was already on it for atrial fibrillation and it covers both the HIT and the long-term indication.
• I do not give a platelet transfusion unless there is life-threatening bleeding, because platelets can fuel thrombosis in HIT.
• I monitor the platelet count daily and expect recovery over 5 to 7 days.
• Warfarin transition is delayed until the platelet count has recovered to at least 150, and is overlapped with the alternative anticoagulant because warfarin alone can cause skin necrosis in acute HIT from early protein C depletion. In this patient I would favour continuing apixaban rather than transitioning to warfarin. ^[1]

Problem 3 — the original PE and DVT: managed by the alternative anticoagulant above; the duration will be extended (lifelong) because of the HIT and the atrial fibrillation indication. ^[1]

Problem 4 — atrial fibrillation: the long-term anticoagulation will be apixaban (resuming his pre-admission therapy), which covers both the AF and the venous thromboembolism. ^[1]

Problem 5 — heart failure: continue the guideline therapy (metoprolol, frusemide, spironolactone), resume empagliflozin once the acute phase has settled, and avoid fluid overload during the alternative anticoagulation. ^[1]

Problem 6 — chronic kidney disease: the dose of fondaparinux, argatroban or DOAC must be adjusted to the eGFR; argatroban is preferred if the renal function deteriorates further. ^[1]

Problem 7 — diabetes and osteoarthritis: routine; resume the empagliflozin when stable. ^[1]

Probing questions the examiner would ask

Examiner: Why is HIT thrombotic rather than haemorrhagic? ^[1]

"HIT is caused by IgG antibodies against the complex of platelet factor 4 and heparin. The antibody binds to platelets and activates them, producing both thrombocytopenia (from platelet clearance) and a striking prothrombotic state, because the activated platelets generate thrombin. The patient is therefore at risk of venous and arterial thrombosis — including limb gangrene, stroke, myocardial infarction and pulmonary embolism — rather than of bleeding. This is why we give a therapeutic dose of an alternative anticoagulant even before the platelet count has recovered, and why we do not give prophylactic platelets." ^[1]

Examiner: What is the difference between typical-onset and rapid-onset HIT? ^[1]

"Typical-onset HIT occurs 5 to 14 days after first heparin exposure — the antibody is being formed de novo. Rapid-onset HIT occurs within 24 hours of heparin re-exposure in a patient who has circulating antibody from a heparin exposure within the previous 30 to 100 days. A delayed-onset form also exists, in which the thrombocytopenia and thrombosis continue or worsen after the heparin has been stopped, driven by the antibody alone." ^[1]

Examiner: How would you classify HIT in the ABCDEF system, and why does that matter? ^[1]

"HIT is a Type B — bizarre — adverse drug reaction: it is idiosyncratic, dose-independent, rare, and not predictable from heparin's known pharmacology (which is anticoagulation, not thrombosis). The management implication is that heparin must be stopped permanently and never re-challenged, and the patient must carry an alert. The irony — a drug used to prevent thrombosis causing thrombosis — is the hallmark of a Type B reaction." ^[1]

Examiner: How would you communicate this to the patient and family? ^[1]

"I would explain in plain language that the blood-thinning injection he has been receiving has, paradoxically, caused a reaction that made his blood more likely to clot, that his platelet count has fallen, that he has developed a new clot in the opposite leg as a result, and that I have stopped that injection and started a different blood thinner. I would tell him he must never receive heparin again, that this has been recorded in his chart, and that he should carry an alert card or medical identification. I would report the reaction to the Therapeutic Goods Administration via the Blue Card and tell him I have done so. I would set out the plan for the next days — daily blood counts, monitoring of the leg, and the transition to long-term apixaban — and I would invite questions." ^[1]

Communication and shared decision-making

The shared decision here is about the long-term anticoagulation strategy: apixaban (resuming his pre-admission therapy, covering both the AF and the VTE, with no INR monitoring and a lower bleeding risk than warfarin) versus warfarin (the traditional choice post-HIT, with INR monitoring and the need for a prolonged overlap with a parenteral alternative). For most patients, including this one, apixaban is the preferred and more convenient option; I would discuss the bleeding risk, the irreversibility (no specific antidote at the time of the consultation, though andexanet is increasingly available), and the importance of adherence. I would also discuss the lifelong heparin avoidance and its implications for future surgery, dialysis, or line placement. ^[1]