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Difficulty: MediumFormulating a Differential Diagnosis

An inpatient consultation is requested for a 58-year-old man who developed sudden-onset right leg swelling and pain on postoperative day 6. He was admitted for recurrent diverticulitis and underwent an uncomplicated partial colectomy. Since admission, he has received subcutaneous unfractionated heparin for deep vein thrombosis (DVT) prophylaxis.

His temperature is 37.1°C (98.8°F), blood pressure is 130/80 mmHg, and heart rate is 92/min. Physical examination reveals a markedly edematous, erythematous, and tender right lower extremity extending from the calf to the mid-thigh.

Current laboratory studies are compared to admission baselines:
- Hemoglobin: 12.1 g/dL (Baseline: 13.5 g/dL)
- Platelet count: 55,000/mm³ (Baseline: 220,000/mm³)
- Leukocyte count: 9,500/mm³ (Baseline: 8,800/mm³)
- Prothrombin time (PT): 12 seconds (Normal: 11-15 seconds)
- Partial thromboplastin time (PTT): 32 seconds (Normal: 25-40 seconds)
- Fibrinogen: 400 mg/dL (Normal: 150-400 mg/dL)

Peripheral blood smear reveals decreased platelets with normal erythrocyte morphology and an absence of schistocytes. A venous duplex ultrasound of the right leg demonstrates an occlusive thrombus in the popliteal and femoral veins.

Based on the patient's presentation and laboratory findings, which of the following is the most likely diagnosis?

  1. Heparin-induced thrombocytopeniaAnswer
  2. B
    Disseminated intravascular coagulation
  3. C
    Thrombotic thrombocytopenic purpura
  4. D
    Immune thrombocytopenic purpura
  5. E
    Type 1 heparin-associated thrombocytopenia

Answer

Heparin-induced thrombocytopenia
The patient's clinical picture of a new-onset deep vein thrombosis coupled with a >50% drop in platelet count occurring 5 to 10 days after the initiation of unfractionated heparin is highly characteristic of Type 2 heparin-induced thrombocytopenia (HIT). HIT is an immune-mediated prothrombotic disorder caused by IgG antibodies against the heparin-platelet factor 4 (PF4) complex. These antibodies activate platelets, leading to consumptive thrombocytopenia and widespread thrombosis (venous more commonly than arterial). The normal PT, PTT, and fibrinogen, along with the lack of schistocytes on the smear, successfully rule out other consumptive coagulopathies and microangiopathic hemolytic anemias.

Step-by-Step Solution

1
Analyze the timing of the patient's presentation and the magnitude of the platelet drop.
The patient experienced a >50% drop in platelet count (from 220,000/mm³ to 55,000/mm³) exactly on postoperative day 6 following the initiation of unfractionated heparin.
The 5 to 10-day window is the classic timeframe for the development of IgG antibodies against the heparin-platelet factor 4 (PF4) complex in a previously unexposed patient.
2
Evaluate the associated clinical findings alongside the thrombocytopenia.
The patient developed an acute deep vein thrombosis (DVT) in the right lower extremity.
Identifying thrombosis rather than bleeding heavily narrows the differential diagnosis, as many common causes of profound thrombocytopenia (like ITP) present primarily with bleeding manifestations.
3
Interpret the supplementary coagulation and hematology laboratory values.
PT, PTT, and fibrinogen levels are completely normal, and there are no schistocytes on the peripheral blood smear.
Normal coagulation studies definitively rule out consumptive coagulopathies like DIC, while the absence of schistocytes rules out microangiopathic hemolytic anemias like TTP, confirming Type 2 HIT as the most likely diagnosis.

Key Concept

Differentiating the causes of thrombocytopenia in a hospitalized patient based on symptom timing, clinical presentation (thrombosis vs. bleeding), and targeted coagulation parameters.
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Formulating a Differential Diagnosis

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