Girolami and colleagues (page 2955) report their study of the incidence and clinical impact of heparin-induced thrombocytopenia (HIT) in 598 medical patients who received subcutaneous unfractionated heparin for various prophylactic and therapeutic indications. They found that 5 of 598 patients (0.8%) developed HIT (or about 1% at 2 weeks by time-to-event analysis). Remarkably, 3 of the 5 patients (60%) developed one or more HIT-associated thrombotic events (2 fatal outcomes). The odds ratio for thrombosis observed was very high at 40.8 (95% CI, 5.2-162.8; P < .001) and parallels the known high risk of thrombosis previously observed in surgical patients who develop HIT (Warkentin et al, N Engl J Med. 1995;332:1330-1335). Thus the main conclusion is stark: although HIT occurs somewhat less often in medical than in surgical patients (1% vs 3%-5%), it remains a very serious, life-threatening adverse drug effect. Thus their findings support the College of American Pathologists's recent recommendation (Warkentin, Arch Pathol Lab Med. 2002;126:1415-1423) that routine monitoring of platelet counts be performed in medical patients receiving unfractionated heparin in prophylactic or therapeutic doses.
The study provides other interesting insights. For example, the authors found that previous exposure to heparin (occurring more than 3 months earlier) was not associated with an increased risk of developing HIT. This is consistent with the emerging view that typical immunologic memory is not a feature of HIT (Warkentin and Kelton, N Engl J Med. 2001;344:1286-1292). Also, Girolami and coworkers presented the serial platelet counts in their 5 HIT patients: this illustrates that medical patients who develop HIT do not display the “inverted V” platelet count profile characteristic of HIT after surgery (which happens because the rising platelet count following surgery is then complicated by the falling platelet count of HIT; see Warkentin et al, cited above). The authors should be congratulated for illuminating how, and how often, HIT impacts medical patients receiving unfractionated heparin.
