In a patient receiving unfractionated heparin, what laboratory value raises suspicion for heparin-induced thrombocytopenia (HIT)?

Heparin-induced thrombocytopenia (HIT) is a significant immune-mediated reaction to heparin that results in a decrease in platelet counts. The most critical laboratory finding that raises suspicion for HIT is a significant drop in platelet count, especially a decrease of more than 50% from baseline.

This definition is crucial because HIT can occur even in patients with initially normal platelet counts, as the condition is characterized by a sudden and significant decline in platelet levels after the initiation of heparin therapy. If a patient who started with a platelet count of 300,000/mm³ experiences a drop to 150,000/mm³, that would indicate a decrease of more than 50%.

In contrast, a stable platelet count or a drop to below the traditional cutoff of 100,000/mm³ could indicate other issues unrelated to HIT. Although a platelet count below 100,000/mm³ might raise some concern, it does not specifically indicate HIT without the context of a preceding drop. Similarly, a significant decrease after day 7, while potentially concerning, does not capture the hallmark of HIT as effectively as the percentage drop does.

In summary, a platelet count that falls by more than 50% is a key indicator of potential