BackgroundOur institution has used trimethoprim-sulfamethoxazole (TMP-SMX) as the antibacterial agent of choice for infection prophylaxis during the pre-engraftment period in the allogeneic transplant (allo-HCT) population.MethodsThis retrospective, single center study was developed to compare the safety of that antibacterial prophylaxis to fluoroquinolones in allo-HCT. The primary endpoint was time to neutrophil engraftment.ResultsA total of 366 patients were reviewed (TMP-SMX n = 332, fluoroquinolonen= 34).No difference in days to neutrophil engraftment was found (median 15 versus 16 days,p= 0.62). Hyperkalemia was more common in the TMP-SMX cohort (32.2% versus 14.7%,p= 0.035); this did not contribute to a higher rate of agent discontinuation or arrhythmia. There was no significant difference in the incidence of neutropenic fever; however, those in the TMP-SMX cohort were more likely to have microbiologically confirmed bacteremia (24.1% versus 8.8% respectively,p= 0.043). There was no significant difference in infections. No long-term implication of prophylactic antibacterial agent selection was observed in terms of graft-versus-host-disease, underlying disease relapse, or mortality.ConclusionThe use of TMP-SMX was associated with a higher likelihood of bacteremia and hyperkalemia; however, this did not result in increased hospital stay, escalation of care, or mortality. The use of TMP-SMX for prophylaxis during the pre-engraftment period for allo-HCT recipients is safe and effective.