LAUSR

A 63-year-old man with a remote history of transfusion and subsequent development of anti-Fy was admitted with septic shock. Anti-Fy was confirmed in the patient's plasma (2+ reactivity); the patient's cells were typed as Fy(b–). The autocontrol was negative. The patient received one Fy(b–) packed red blood cell (pRBC). Two weeks later, after the patient received antibiotics, the autocontrol tested positive. The DAT was positive (polyspecific and anti-IgG). An eluate was prepared from the patient's cells (ELU-KIT II, Immucor Gamma, Norcross, GA) using low-ionic strength saline (LISS) for the last wash. Surprisingly, the eluate prepared from this patient's Fy(b–) cells reacted weakly with Fy(b+) cells (Figures 1 and 2, left). The eluate failed to react with Fy(b–) cells. All other common alloantibodies were ruled out. A drug-induced antibody secondary to antibiotics was deemed the most probable cause of the positive DAT. The eluate kit manufacturer warns that the last wash, LISS-based working wash solution may produce “nonspecific [antibody] uptake ... to antigen-negative red blood cells.” The mechanism is believed to be the nonspecific binding of antibodies to antigen-negative cells due to reduced zeta potential induced by LISS. Therefore, we repeated the eluate similarly, except substituted normal saline for LISS in the last wash step. The repeat eluate was nonreactive with Fy(b+) cells (Figures 1 and 2, right). Other explanations for the reactions we observed appear unlikely. Antibiotics may cause positive eluates; however, these reactions are usually seen only after drug is added in vitro. All pRBCs transfused in the preceding year were Fy(b–). Autoantibody was unlikely as no new panel reactivity was observed. No IVIG had been