Impact of the patient population on the risk for heparin-induced thrombocytopenia

The frequency of immune heparin-induced thrombocytopenia (HIT) varies among prospective studies. It is unknown whether this is caused by differences i n the heparin preparations, the patient populations, or the types of serolo gic assay used to confirm the diagnosis. Seven hundred forty-four patients were studied from 3 different clinical treatment settings, as follows: unfr actionated heparin (UFH) during or after cardiac surgery(n = 100), UFH afte r orthopedic surgery (n = 205), and low-molecular-weight heparin (LMWH) aft er orthopedic surgery (n = 439), Both an activation assay and an antigen as say were used to detect heparin-dependent IgG (HIT-IgG) antibodies. By acti vation assay, the frequency of HIT-IgG formation ranged from a low of 3.2% in orthopedic patients receiving LMWH to a high of 20% in cardiac patients receiving UFH; by antigen assay, the corresponding frequencies ranged from 7.5% to 50%, Both UFH use (P = .002) and cardiac surgery (P = .01)were more likely to be associated with HIT-IgG formation. However, among patients in whom HIT-IgG formed and who were administered UFH, the probability for HIT was higher among orthopedic patients than among cardiac patients (by activ ation assay: 52.6% compared with 5%; odds ratio, 21.1 [95% CI, 2.2-962.8]; P = .001; by antigen assay: 34.5% compared with 2.0%; odds ratio, 25.8 [95% CI, 3.2-1141]; P < .001), It is concluded that there is an unexpected diss ociation between the frequency of HIT-IgG formation and the risk for HIT th at is dependent on the patient population, HIT-IgG antibodies are more like ly to form in patients who undergo cardiac surgery than in orthopedic patie nts, but among patients in whom antibodies do form, orthopedic patients are more likely to develop HIT. (C) 2000 by The American Society of Hematology .