Heparin-independent serum-induced serotonin release. (A) SRA results using serial fourfold serum dilutions for 4 MM patients who developed HIT. All 4 patient sera showed strong serum-induced serotonin release (>80%) using 1/2 diluted serum (heparin-independent serotonin release); at higher dilutions, as expected, typical heparin-dependent serotonin release (at 0.2 IU/mL UFH) was shown, with serotonin-release inhibited at 100 IU/mL UFH. (B) SRA results using serial fourfold serum dilutions for 2 positive autoimmune HIT controls. Both sera were available from previously reported patients with delayed-onset HIT (patient 1¹⁵ ) and persisting HIT.¹⁶  (C) Relationship of platelet count recovery with decrease of heparin-independent serum-induced serotonin release. For 2 patients (1 and 4) for whom follow-up sera were available, platelet count recovery occurred in association with decrease in heparin-independent serum-induced serotonin release.