Heparin induced thrombocytopenia (HIT) is a transient hypercoagulability state and pro-thrombotic disorder caused by heparin use.1 Several diagnostic algorithms have been developed to diagnose HIT including the “4Ts” prediction model score which provides an estimate of pre-test probability for HIT by assigning scores based on the degree of thrombocytopenia.2 Previous studies showed that HIT is more common with the use of unfractionated heparin (UFH) rather than low molecular weight heparin (LMWH), and that surgery adds more risk.1,3
However, a recent Cochrane review4 comparing the incidence of HIT and HIT complicated by thrombosis in patients exposed to UFH versus LMWH in randomized controlled trials (RCTs) of postoperative heparin therapy was done to confirm or refute what is anecdotally known about the incidence of HIT with the two heparins. Two RCTs with 923 participants, of which 442 received LMWH (enoxaparin or certoparin) and 481 received UFH, were analyzed. LMWH showed superiority over UFH in reduction of both HIT and HIT complicated with venous thromboembolism (VTE) (risk ratio (RR) 0.24, 95% confidence interval (CI) 0.07 to 0.82; P = 0.02, and (RR 0.20, 95% CI 0.04 to 0.90; P = 0.04, respectively). The results suggest that patients treated with LMWH would have a relative risk reduction (RRR) of 76% in the likelihood of developing HIT and a RRR of 80% for developing HIT complicated by VTE compared with patients treated with UFH. The Cochrane reviewers pointed out the limitations of their review: most importantly, the small sample size, the moderate quality of the included trials, and the unequal distribution of patients on UFH compared to LMWH. Nonetheless, their conclusion was consistent with what clinicians know about the incidence of HIT and HIT with thrombosis with UFH and LMWH. A Cochrane review will definitely add to the body of evidence that is supporting the use of LMWH as a front-line agent, especially in surgical patients who are at risk for developing thrombosis.
1. Warkentin TE. Management of heparin-induced thrombocytopenia: a critical comparison of lepirudin and argatroban. Thrombosis Research 2003;110:73–82.
2. Ahmed I, Majeed A, Powell R. Heparin induced thrombocytopenia: diagnosis and management update. Postgrad Med J 2007;83:575–582.
3. Kam PC et al. Direct thrombin inhibitors: pharmacology and clinical relevance. Anaesthesia 2005,60:565–574.
4. Junqueira DR, Perini E, Penholati RR, et al. Unfractionated heparin versus low molecular weight heparin for avoiding heparin-induced thrombocytopenia in postoperative patients. Cochrane Database Syst Rev. 2012 Sep 12;9:CD007557. doi: 10.1002/14651858.CD007557.pub2.