A patient is receiving intravenous gentamicin. A serum drug test reveals toxic levels. The dosing is correct, and this medication has been tolerated by this patient in the past. Which could be a probable cause of the test result?

• A. The patient is taking another medication that binds to serum albumin
• B. A loading dose was given to reach a faster plateau
• C. A drug not completely dissolved could embolize
• D. A drug given at a frequency longer than its half-life would be subtherapeutic

Answer: (A)

Drug interactions that change how drugs bind in the blood can alter the amount of active drug available. If another medication binds strongly to serum albumin, it can compete for those binding sites and push more gentamicin into the unbound, active form. That increase in free gentamicin raises the risk of toxicity, even when the dosing is appropriate and the patient has tolerated the drug in the past. The scenario fits a mechanism where a concomitant albumin-binding drug elevates the active drug fraction, leading to a toxic serum level. The other options don’t align with how toxicity would be produced in this setting: a loading dose would cause a higher initial concentration but isn’t about current dosing; issues with dissolution wouldn’t drive a systemic toxic level; and dosing too infrequently would more likely lead to subtherapeutic rather than toxic levels.