Figure 1. Serial platelet counts in a 55-yr-old woman who underwent elective total knee replacement. Before postoperative day 12, the only heparin exposure was intraoperative (flushing of an intraarterial catheter). Nonetheless, the acute onset of dyspnea, hypertension, tachycardia, diaphoresis, and chills occurred 10 min after a 5,000-unit (U) bolus of heparin administered on day 12 for lower-limb deep vein thrombosis. Acute heparin-induced thrombocytopenia was suggested by the abrupt and unexpected decrease in platelet count that worsened during continuing heparin use and that recovered rapidly after discontinuation of heparin. The patient died of intracranial hemorrhage on day 20. The role of the intraoperative heparin exposure in causing immune sensitization to heparin is shown by the heparin-induced thrombocytopenia antibody seroconversion (negative [Neg] testing on days 0, 2, and 4) and positive [Pos] testing on days 6, 8, 10, and 20). V/Q = ventilation perfusion.