title: Heparin-induced thrombocytopenia in patients treated with low-molecular-weight heparin or unfractionated heparin.
abstract: BACKGROUND
  Heparin-induced thrombocytopenia, defined by the presence of heparin-dependent IgG antibodies, typically occurs five or more days after the start of heparin therapy and can be complicated by thrombotic events. The frequency of heparin-induced thrombocytopenia and of heparin-dependent IgG antibodies, as well as the relative risk of each in patients given low-molecular-weight heparin, is unknown.
  
  
  METHODS
  We obtained daily platelet counts in 665 patients in a randomized, double-blind clinical trial comparing unfractionated heparin with low-molecular-weight heparin as prophylaxis after hip surgery. Heparin-induced thrombocytopenia was defined as a decrease in the platelet count below 150,000 per cubic millimeter that began five or more days after the start of heparin therapy, and a positive test for heparin-dependent IgG antibodies. We also tested a representative subgroup of 387 patients for heparin-dependent IgG antibodies regardless of their platelet counts.
  
  
  RESULTS
  Heparin-induced thrombocytopenia occurred in 9 of 332 patients who received unfractionated heparin and in none of 333 patients who received low-molecular-weight heparin (2.7 percent vs. 0 percent; P = 0.0018). Eight of the 9 patients with heparin-induced thrombocytopenia also had one or more thrombotic events (venous in 7 and arterial in 1), as compared with 117 of 656 patients without heparin-induced thrombocytopenia (88.9 percent vs. 17.8 percent; odds ratio, 36.9; 95 percent confidence interval, 4.8 to 1638; P < 0.001). In the subgroup of 387 patients, the frequency of heparin-dependent IgG antibodies was higher among patients who received unfractionated heparin (7.8 percent, vs. 2.2 percent among patients who received low-molecular-weight heparin; P = 0.02).
  
  
  CONCLUSIONS
  Heparin-induced thrombocytopenia, associated thrombotic events, and heparin-dependent IgG antibodies are more common in patients treated with unfractionated heparin than in those treated with low-molecular-weight heparin.
authors.affiliation: Department of Pathology, McMaster University, Hamilton, Ontario, Canada.
authors.forename: T E
authors.initials: TE
authors.lastname: Warkentin
authors.forename: M N
authors.initials: MN
authors.lastname: Levine
authors.forename: J
authors.initials: J
authors.lastname: Hirsh
authors.forename: P
authors.initials: P
authors.lastname: Horsewood
authors.forename: R S
authors.initials: RS
authors.lastname: Roberts
authors.forename: M
authors.initials: M
authors.lastname: Gent
authors.forename: J G
authors.initials: JG
authors.lastname: Kelton
mesh_terms: D016001:Confidence Intervals; D004311:Double-Blind Method; D017984:Enoxaparin / Q000627:therapeutic use*; D005260:Female; D006493:Heparin / Q000009:adverse effects* / Q000276:immunology / Q000627:therapeutic use; D006801:Humans; D007074:Immunoglobulin G / Q000032:analysis; D016017:Odds Ratio; D010976:Platelet Count; D011183:Postoperative Complications / Q000517:prevention & control; D012307:Risk Factors; D013921:Thrombocytopenia / Q000139:chemically induced* / Q000150:complications; D013924:Thrombophlebitis / Q000517:prevention & control; D016896:Treatment Outcome
chemical_list: D017984:Enoxaparin; D007074:Immunoglobulin G; C006871:calcium heparin; D006493:Heparin