Once-daily dosing intravenous tobramycin is commonly used to treat cystic fibrosis pulmonary exacerbations. Clinicians often utilize historical therapeutic drug monitoring data to individualize the dose among patients who have been treated with tobramycin previously. This case series involves three patients with cystic fibrosis who had supra-therapeutic tobramycin levels despite use of a once-daily dosing that produced therapeutic drug levels during a previous hospital admission, raising questions about the validity of these levels. Investigation into several potential sources of error led to the discovery of an analyzer error in the laboratory. Once the laboratory's tobramycin analyzer was recalibrated, the reported levels were comparable to historical levels. This case series emphasizes the clinical importance of critically analyzing reported levels, and specifically, the importance of utilizing past therapeutic drug monitoring data, if available, for all patients treated with intravenous tobramycin. If a patient was therapeutic on a similar dose of tobramycin during a previous admission, a dose adjustment may not be necessary, and clinicians should consider repeating levels while pursuing alternative explanations for the discrepant serum levels.