PF4 and deep vein thrombosis: In susceptible patients, heparin treatment generates antibodies against the heparin-PF4 complex, leading to platelet activation and aggregation.[1] For heparin-exposed patients, HIT incidences range from 0.5% to 5% with a mortality rate of >20%.[2] The most common clinical symptom of HIT is the marked decrease in platelet counts with thrombosis (HITT) or without thrombosis (isolated HIT).