A patient on unfractionated heparin therapy develops a marked drop in platelets by day 3. What is the most likely diagnosis?

• A. DIC
• B. Primary fibrinogenolysis
• C. Aspirin-induced thrombocytopenia
• D. Heparin-induced thrombocytopenia

Answer: (D)

A marked drop in platelets after starting unfractionated heparin points to heparin-induced thrombocytopenia, an immune-mediated reaction to the heparin-PF4 complex. In HIT, IgG antibodies form against this complex and bind to platelets via their Fc receptors, causing platelet activation and consumption. This produces thrombocytopenia while simultaneously putting the patient at high risk for new clots because activated platelets drive thrombosis. The day-3 timing fits the picture when there has been prior heparin exposure or rapid antibody formation, so the onset can occur within a few days.

DIC would usually come with abnormal coagulation studies (prolonged PT/aPTT, low fibrinogen, elevated D-dimer) and a bleeding tendency, not the isolated platelet drop seen with HIT. Aspirin-induced thrombocytopenia is rare and not directly tied to heparin exposure, and primary fibrinogenolysis isn’t the typical mechanism or pattern here.