An 18-year-old previously healthy man is placed on intravenous heparin after having a pulmonary embolism (PE) after exploratory laparotomy for a small-bowel injury following a motor vehicle collision. Five days later, his platelet count is 90,000/mL and continues to fall over the next several days. The patient’s serum is positive for antibodies to the heparin-platelet factor complexes. Which of the following is the most appropriate next management step?

**Core Concept**
The patient is experiencing heparin-induced thrombocytopenia (HIT), a serious adverse reaction to heparin therapy characterized by the formation of antibodies against the heparin-platelet factor 4 (PF4) complex, leading to platelet activation, aggregation, and thrombocytopenia.

**Why the Correct Answer is Right**
Heparin-induced thrombocytopenia is a distinct entity from other causes of thrombocytopenia and requires prompt recognition and management to prevent further complications. The correct answer involves discontinuing heparin therapy and initiating alternative anticoagulation to prevent further thrombosis while allowing the platelet count to recover. This is because the antibodies driving HIT are a result of heparin binding to platelet factor 4, making continued heparin use counterproductive and potentially worsening the condition.

**Why Each Wrong Option is Incorrect**
* **Option A:** Continuing heparin therapy would exacerbate the HIT reaction, leading to further thrombocytopenia and an increased risk of thrombosis.
* **Option B:** Administering platelet transfusions would not address the underlying cause of thrombocytopenia and may even worsen the situation by introducing more platelets that can be targeted by the antibodies.
* **Option D:** Switching to a different anticoagulant, such as warfarin, would not be sufficient as it may take several days for warfarin to take effect, and the patient is at risk of further thrombosis in the meantime.

**Clinical Pearl / High-Yield Fact**
In patients with suspected HIT, it's crucial to discontinue all forms of heparin (including low molecular weight heparin and heparin flushes) and initiate alternative anticoagulation with a direct thrombin inhibitor, such as argatroban or bivalirudin, to prevent further thrombosis.

**Correct Answer:** C. Discontinue heparin and start a direct thrombin inhibitor.

✓ Correct Answer: D. . Cessation of heparin and institution of lepirudin