Key Points
ÎHeparin-induced thrombocytopenia (HIT) is an adverse immune-mediated drug reaction that is associated with a high risk of venous and arterial thrombosis.
ÎRisk factors for HIT include duration and type of heparin exposure, patient population, severity of trauma, and gender.
ÎDiagnosis is based on the combination of a compatible clinical picture and the presence of platelet-activating anti-PF4 antibodies.
Î17% to 55% of untreated patients who present with thrombocytopenia develop deep venous thrombosis (DVT) and/or pulmonary embolism. Arterial thrombotic events, including limb artery thrombosis, thrombotic stroke, and myocardial infarction (MI), also occur, but less often (from 3%-10%).
ÎThrombocytopenia (defined as a platelet count < 150 x 109 /L) is the most
common clinical manifestation of HIT and occurs in 85% to 90% of patients. /L), this increases to 90% to 95%
Comment: If this definition is broadened to include a proportional fall in the platelet count (eg, 30%-50% fall even if the nadir remains > 150 x 109 of HIT cases.
ÎAlthough thrombocytopenia is the most common presenting feature of HIT, in ≤ 25% of patients with HIT the development of thrombosis precedes the development of thrombocytopenia.
ÎTypical-onset HIT: The characteristic onset of the platelet count fall in HIT is 5-10 days after initiation of heparin (first day of heparin = day 0), particularly when heparin is administered perioperatively.
ÎRapid-onset HIT: Sometimes the platelet count falls abruptly (within 24 h) in patients who already have circulating HIT antibodies because of recent exposure to heparin, usually within the past month but occasionally as long as 100 days earlier.
ÎDelayed-onset HIT: Occasionally, thrombocytopenia can occur as long as 3 weeks after cessation of heparin.
ÎThere is a 10-fold higher likelihood of HIT in patients who receive unfractionated heparin (UFH) compared with patients who receive low- molecular-weight heparin (LMWH) or fondaparinux.
ÎPatients who undergo cardiac or orthopedic surgery and receive UFH have a higher risk of HIT (1%-5%) than medical or obstetric patients (0.1%-1%). Women have approximately twice the risk of developing HIT as men.
ÎThe pattern of thrombocytopenia following cardiac surgery using heparin is worthy of special mention. Although approximately 50% of patients who undergo cardiac surgery will develop HIT antibodies, only 1% to 2% will develop clinical HIT (thrombocytopenia with or without thrombosis).
Comment: The following two patterns of thrombocytopenia should alert clinicians to the possibility of HIT following cardiac surgery: a fall in platelet count that begins > 4 days postoperatively (day of surgery = day 0), and thrombocytopenia that persists for > 4 days after surgery.
