Background:The population of hospitalized older patients is increasing. We investigated whether clinicians accurately detected renal insufficiency in older patients and chose correct doses of commonly prescribed antibiotics.Methods:We conducted a retrospective chart review of 1044 patients >80 years admitted to the University of California Davis Medical Center between January and December 1997 with a diagnosis of infection. Inclusion criteria included data necessary to calculate creatinine clearance using the Cockroft-Gault equation and administration of any of the study antibiotics. Patients with unstable renal function, defined as an increase in creatinine of ≥1.0 mg/dL/day were excluded. Administered dosages of each study drug were compared with the appropriate adjusted doses. We examined the variables of age, weight, serum creatinine, and sex to determine whether any were individually predictive of dosing errors.Results:Dosing errors were identified in all of the antibiotics studied, and the overall dosing error rate was 34%. The factors that were predictive of dosage errors were advanced age and low body weight. Serum creatinine and sex were not statistically significant factors.Conclusions:Widespread errors in medication dosing are made in elderly hospitalized patients. The Cockroft-Gault equation reveals significant renal insufficiency requiring dose adjustments in most elderly patients studied, especially those > 85 years of age and with low body weight. Estimation of glomerular filtration rate should be performed routinely on all admitted patients older than 80 and in any patient with low lean body mass.