CHAPTER 44 The Diagnosis and Management of Heparin-Induced Thrombocytopenia

Heparin-induced thrombocytopenia (HIT) can be defined as any clinical event (or events) best explained by platelet-activating antiplatelet factor 4 (PF4)/heparin antibodies (HIT antibodies) in a patient who is receiving, or who has recently received, heparin. A clinical scoring system, the “4 T's,” helps predict which patients have HIT, based upon the assessment of thrombocytopenia, timing, thrombosis, and the absence of other explanation(s). Evaluation of this scoring system suggests that HIT antibodies are unlikely (<5%) when a low score (≤3) is obtained, but very likely (<80%) with a high score (≥6). An intermediate score (4 or 5) usually indicates a clinical profile compatible with HIT, but also with another plausible explanation, such as sepsis. Rarely, HIT begins several days after heparin has already been stopped (delayed-onset HIT); this syndrome is associated with strong positive tests for HIT antibodies. Some sera activate platelets in vitro without the need to add heparin. Thrombosis is the most important complication of HIT and occurs in most patients. Both venous and arterial thrombi (or both) can occur. The odds ratio for thrombosis ranges from 20 to 40.