Patient with autoimmune HIT (delayed-onset and persisting HIT) who was switched from fondaparinux to rivaroxaban during acute thrombocytopenia (group B). The gradual recovery in platelet count inversely paralleled the gradual decline in serum-induced percent serotonin release at 0 U/mL heparin (buffer control; open circles), a phenomenon that has previously been reported in patients with autoimmune HIT.²⁸  CABG, coronary artery bypass grafting; LMWH, low-molecular-weight heparin (dalteparin); POD, postoperative day; SC, subcutaneous; UFH, unfractionated heparin.