A 65-year-old man is evaluated in the hospital for a new, acutely swollen and painful righ… — USMLE Step 2 CK

A 65-year-old man is evaluated in the hospital for a new, acutely swollen and painful right leg. He was admitted $7$ days ago for a massive pulmonary embolism and was started on a continuous intravenous infusion of unfractionated heparin. Over the past few days, his respiratory status has improved, and he currently requires no supplemental oxygen. His temperature is $37.0^\circ\text{C}$ ( $98.6^\circ\text{F}$ ), blood pressure is $132/82\text{ mmHg}$ , pulse is $88/\text{min}$ , and respirations are $14/\text{min}$ . Physical examination today reveals a swollen, tender right calf. Duplex ultrasonography demonstrates a new, noncompressible thrombus in the right popliteal vein. Laboratory studies on admission showed a platelet count of $240,000/\mu\text{L}$ . Today, his platelet count is $95,000/\mu\text{L}$ . Which of the following is the most appropriate next step in management?

A
Continue unfractionated heparin and transfuse platelets
B
Discontinue unfractionated heparin and start warfarin
Discontinue unfractionated heparin and start argatrobanAnswer
D
Discontinue unfractionated heparin and start enoxaparin
E
Continue unfractionated heparin and start intravenous vancomycin and cefepime

Answer

Discontinue unfractionated heparin and start argatroban

The correct answer is to discontinue unfractionated heparin and start argatroban. This patient's presentation is classic for Heparin-Induced Thrombocytopenia (HIT) type 2. HIT is a life-threatening, immune-mediated disorder caused by IgG autoantibodies directed against complexes of platelet factor 4 (PF4) and heparin. It typically presents

5

10

days after heparin initiation with a platelet count drop of

>50\%

and a paradoxical, significantly increased risk of arterial and venous thrombosis (e.g., the new popliteal deep vein thrombosis in this patient). The initial management of suspected HIT requires the immediate cessation of all heparin products and the initiation of a non-heparin anticoagulant. Direct thrombin inhibitors (e.g., argatroban, bivalirudin) or fondaparinux are the drugs of choice to prevent further thrombotic complications. Confirmatory testing (PF4-heparin antibody ELISA, followed by a Serotonin Release Assay) should be ordered, but treatment must not be delayed while awaiting these results.

Step-by-Step Solution

Calculate the 4T score for suspected Heparin-Induced Thrombocytopenia (HIT).

The score is high, driven by a >50% drop in platelets, occurrence between days 5-10 of heparin exposure, and a new proven thrombosis.

A high pretest probability (4T score) necessitates immediate clinical intervention prior to waiting for confirmatory laboratory testing.

Identify the appropriate first intervention for HIT.

All forms of heparin must be immediately discontinued.

Continued exposure to heparin will fuel the immune-mediated formation of prothrombotic platelet factor 4 (PF4)-heparin immune complexes.

Select a suitable alternative anticoagulant.

A direct thrombin inhibitor such as argatroban (or an indirect inhibitor like fondaparinux) is selected.

Patients with HIT are in a highly prothrombotic state (as evidenced by the new DVT) and require continuous anticoagulation with an agent that does not cross-react with circulating HIT antibodies.

Key Concept

Diagnosis and management of Heparin-Induced Thrombocytopenia (HIT) type 2

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