A clinician measures a patient's steady-state trough concentration of an antibiotic as 6 µg/mL. The known lowest effective concentration is 3 µg/mL and the lowest toxic concentration is 9 µg/mL. Using these concentrations, calculate the TI and analyze whether the patient is currently within the therapeutic window.

• A. TI cannot be calculated using concentrations; only dose-based TIs are valid, so no conclusions about the patient's level can be drawn.
• B. TI = 3; the patient is below the effective concentration and therapy should be stopped as it is ineffective.
• C. TI = 3; the patient's trough concentration of 6 µg/mL is within the therapeutic window (between 3 and 9 µg/mL), but because TI is narrow, the patient should be monitored closely.
• D. TI = 0.33; the patient is above the toxic threshold and requires immediate cessation of therapy and emergent decontamination.

Answer: (C)