Drug-dependent, platelet-reactive immunoglobulin G antibodies specific for dexamethasone, irinotecan, leucovorin, and oxaliplatin were detected in both patient 1 and patient 2. Figure shows representative flow cytometric studies in which patient serum was incubated with normal group O platelets in the presence (blue peaks) and absence (red peaks) of drug 1.0 mg/mL. After washing, platelet-bound immunoglobulin G (IgG) was detected with an IgG-specific secondary antibody labeled with fluorescein isothiocyanate. Median fluorescence intensity (MFI) values are shown adjacent to each peak. A positive test result is one in which the ratio of the MFI values obtained with and without drug exceeds 1.50. MFI values obtained when normal serum was substituted for patient serum did not differ significantly from those obtained with patient serum in the absence of drug (not shown). Patient serum was also screened for DDAbs specific for ondansetron and palonosetron (antiemetics), and diphenhydramine (antihistamine), agents commonly used to reduce adverse effects in patients given combination chemotherapy for cancer. Both patients were found to have weak but reproducible DDAbs specific for ondansetron. Because patient 1 was not given ondansetron during the cycles of treatment shown in Figure 1, it is likely that the ondansetron-specific DDAb resulted from exposure to this drug during an earlier treatment cycle.