Pharmacokinetic modeling predicts that co-administration of Drug A with a commonly used inhibitor will increase Drug A plasma levels by 70%. If Drug A has a TI of 2 under normal circumstances, analyze the impact of the interaction and recommend prescriber actions.

• A. A 70% increase in plasma levels with a baseline TI of 2 significantly increases risk of toxicity; prescribers should avoid the combination if possible, or reduce Drug A dose substantially and monitor levels and organ function closely if combination is necessary.
• B. No action is necessary because TI = 2 already accounts for common drug interactions and a 70% increase is within acceptable variability for most patients.
• C. Increase Drug A dose to overcome the interaction since higher plasma levels predict better antimicrobial killing and TI concerns are secondary.
• D. Stop monitoring plasma levels because drug interactions invalidate therapeutic drug monitoring and render it useless; rely only on clinical signs of toxicity instead.

Answer: (A)