HIT type II is thought to occur in approximately 0.3 to 3% of patients exposed to heparin for more than 4 days. Approximately 10% to 20% of patients receiving unfractionated heparin experience a fall in platelet count to less than the normal range or a 50% fall in platelet count within the normal range. However, most of these cases are thought to be secondary to HIT type I, or nonimmune-mediated disease.
