If the Platelets Are Low, Is It HIT?

Intraoperative heparin exposure and intra-/postoperative thrombocytopenia occur universally with cardiac surgery. Sometimes, heparin triggers formation of highly pathological platelet-activating antibodies that cause thrombocytopenia, hypercoagulability, and greatly increased risk of venous and/or arterial thrombosis beginning approximately 1 week post-surgery, a disorder called heparin-induced thrombocytopenia (HIT). The challenge is to distinguish HIT from the myriad of non-HIT thrombocytopenic disorders common to this patient population. The characteristic timing of HIT, and its hallmark of associated thrombosis, means that any otherwise unexplained platelet count fall of >40% (especially with associated thrombotic event) that begins during the characteristic day 5–10 “window” post-cardiac surgery should be presumed to be HIT unless proven otherwise. In contrast, early-onset and persisting thrombocytopenia following cardiac surgery is unlikely to be HIT. Growing recognition that HIT can begin or worsen after all heparin has been stopped (“autoimmune HIT”) indicates that this diagnosis should be considered even when thrombocytopenia or thrombosis is recognized up to 1 month following cardiac surgery.