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Theodore E. Warkentin, MD; Robin S. Roberts, MSc; Jack Hirsh, MD; John G. Kelton, MD

Arch Intern Med. 2003;163:2518-2524.

Background  Diagnosis of immune heparin-induced thrombocytopenia (HIT) is usually based on a fall in platelet count below 150 x 10⁹/L (standard definition of thrombocytopenia). However, this definition may be inappropriate for postoperative patients who often develop postoperative thrombocytosis. We sought to determine an improved definition of thrombocytopenia indicating HIT in postoperative orthopedic patients, including its impact on frequency and thrombotic risk of HIT.

Methods  We performed a secondary analysis of a clinical trial of 665 patients who received unfractionated or low-molecular-weight heparin following elective hip arthroplasty. Daily platelet counts and objective studies for deep vein thrombosis were performed in all patients. Laboratory detection of HIT antibodies from a 362-patient subgroup was used to define sensitivity and specificity of various definitions of thrombocytopenia to indicate HIT.

Results  The improved definition of HIT was a 50% or greater platelet count fall from the postoperative peak, as this definition had greater sensitivity (50% vs 25%) and similar high specificity (99.1% vs 99.4%) for detecting HIT-IgG compared with the standard definition. Patients with HIT who were identified using the improved definition had a higher frequency of thrombosis than patients without HIT (72.2% vs 17.3%; P<.001). The improved definition showed an even greater absolute difference in frequency of HIT between unfractionated and low-molecular-weight heparin (4.8% vs 0.6%; P<.001) compared with the standard definition (2.7% vs 0%; P = .002).

Conclusion  A 50% or greater fall in the platelet count from the postoperative peak is a sensitive definition indicating possible HIT that is associated with an increased risk of thrombosis.

From McMaster University (Drs Warkentin, Hirsh, and Kelton and Mr Roberts), the Henderson Research Centre (Dr Hirsh and Prof Roberts), and the Hamilton Regional Laboratory Medicine Program (Dr Warkentin), Hamilton, Ontario. The authors have no relevant financial interest in this article.

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