The annual meeting of the American College of Chest Physicians recently took place in Montreal. One of the presentations (reported in eMedicine news) concerned the prevalence of heparin-platelet factor 4 (HPF4) antibodies in patients about to undergo open heart surgery. HPF4 antibodies are found in patients with heparin induced thrombocytopenia (HIT) and are instrumental in its pathogenesis. The Milwaukee investigators screened patients before surgery and found antibodies in 5.4%. Antibody positivity was associated with longer ICU stays and a higher incidence of prolonged mechanical ventilation, limb ischemia, renal dialysis and gastrointestinal complications. These findings raise the question of whether all cardiac surgery candidates should be screened.
I’m anxious for some details not available in this sound bite version. What was the mechanism of the bad outcomes associated with antibody positivity? How many of those patients actually met criteria for HIT?
We previously knew that many cardiac surgery patients (up to 50 %!) treated with unfractionated heparin (UFH) test positive for antibodies during their perioperative course but of those only 2% develop HIT. In contrast, fewer orthopedic surgery patients treated with UFH develop antibodies but substantially more antibody positive patients in the orthopedic group develop HIT. Thus the presence of antibodies is more predictive in orthopedic patients than in cardiac surgery patients. 
An iceberg model has been proposed for HIT.  At the base of the iceberg is a relatively large number of patients who have antibodies. Toward the surface is a smaller number who develop a drop in the platelet count, and at the tip of the iceberg are those who develop thrombosis. The interacting risk factors are complex and the actual number of antibody positive patients who develop HIT seems to vary with the patient population, as the above data suggest.