Heparin-Induced Thrombocytopenia - Medscape Reference

In the United States, approximately 12 million individuals, or one third of hospitalized patients, have some heparin exposure yearly. A study by Smythe and colleagues estimated the frequency of heparin-induced thrombocytopenia (HIT) to be 0.76% in patients receiving therapeutic doses of intravenous unfractionated heparin (UFH) and less than 0.1% in patients receiving antithrombotic prophylaxis with subcutaneous UFH, with an overall risk of HIT of about 0.2% in all heparin-exposed patients. [20]

Other studies in the literature quote frequencies as high as 1-5%. [22, 23, 24] High frequencies of HIT are especially common in surgical patients receiving prolonged postoperative thromboprophylaxis (eg, for 10-14 days following orthopedic surgery [25] or after coronary artery bypass and/or valve replacement surgery [20] ).

Mortality/morbidity

HIT is a severe prothrombotic condition, with affected individuals having a greater than 50% risk of developing new thromboembolic events. [26] The mortality rate is approximately 20%, and approximately 10% of patients require amputations or suffer other major morbidity. [13, 27, 28]

A study of 108 consecutive hospitalized patients diagnosed with HIT showed that thrombotic complications occurred in about 29%. Early, severe falls in platelet counts in elderly patients receiving heparin appear to be associated with the development of thrombotic complications. [27]

Thrombosis associated with HIT can involve the arterial system, the venous system, or both. Thrombotic complications may include deep venous thrombosis, stroke, myocardial infarction, limb ischemia, and, rarely, ischemia of other organs. The thrombotic complications are fatal in about 29% of patients, and an additional 21% have to undergo limb amputations. [29]

Although HIT is a hypercoagulable disorder, patients remain at risk for major bleeding. A review by Pishko et al found that over a third of patients with HIT who were exposed to an alternative anticoagulant experienced a major bleeding event. Factors associated with increased risk of major bleeding included intensive care unit admission, platelet count < 25 × 109 /L, and kidney dysfunction. [30]

Race-, sex-, and age-related differences in risk

A study by Lewis et al reported that White patients had significantly less risk than nonwhite patients for thrombotic events, irrespective of the HIT presentation. [31] In this study, nonwhites were 2-3 times more likely than Whites to progress to a HIT-associated thrombotic outcome, particularly with regard to new thromboses. [31]

Men have significantly less risk than women for thrombotic manifestations in HIT. Women diagnosed with HIT and thrombosis are 1.7 times more likely than men to have a new HIT-associated thrombotic event. [31] The exact mechanism responsible for this difference remains unclear.

The higher frequency of HIT in females was found most strikingly in patients treated with UHF. There was no relationship between sex and the risk for HIT in patients treated with low molecular weight heparin (LMWH). [32] LMWH in prevention of HIT may have the greatest absolute benefit in females receiving surgical thromboprophylaxis. [32]

The mean age of patients with HIT is 62.3 ± 6.6 years. [33] In a retrospective study of 408 patients diagnosed with HIT, 66% of patients were older than 60 years. [34] Obeng and colleagues, in a retrospective cohort study of 155 patients under 21 years old with sufficient data for 4Ts scoring for HIT, found that the prevalence of HIT was significantly lower in these patients than in adults. [35]