A diagnostic criterion for HIT includes a platelet count drop of 50% within how many days after heparin exposure?

• A. Within 3 days
• B. Within 7 days
• C. Within 14 days
• D. Within 1 day

Answer: (A)

Timing of the platelet drop is the key idea. HIT is caused by antibodies to the PF4–heparin complex and classically presents with a roughly 50% fall in platelets about 5–10 days after starting heparin in someone without prior exposure. If there has been recent heparin exposure, those antibodies can be present from before, and the platelet count can plummet more rapidly—often within a few days, with about a 50% drop possible within roughly 3 days. That rapid window is the one used in diagnostic considerations, so a 50% decrease within three days best fits HIT under the common criteria. Delays beyond a week or two are less characteristic of the initial HIT onset, and one day is usually too early unless there is known recent sensitization.