The therapeutic index (TI) of a drug is defined as the ratio of TD50 to ED50. A drug with a narrow therapeutic index requires:
- A Therapeutic drug monitoring because small changes in dose or pharmacokinetics can shift the drug from the therapeutic to the toxic range ✓
- B Less frequent monitoring as small dose changes have little clinical impact
- C Higher doses to ensure efficacy before approaching toxic levels
- D Extended interval dosing to avoid accumulation
Correct answer: A. Therapeutic drug monitoring because small changes in dose or pharmacokinetics can shift the drug from the therapeutic to the toxic range
Explanation
Therapeutic index (TI = TD50/ED50) quantifies the margin between effective and toxic doses. A narrow TI (e.g., digoxin, lithium, warfarin, phenytoin, aminoglycosides) means that relatively small changes in dose, absorption, distribution, or elimination can cause concentrations to cross from effective into toxic territory. Therapeutic drug monitoring (TDM) — measuring plasma drug concentrations — is essential for these drugs. A large TI (e.g., penicillins) allows a wide safety margin without requiring concentration monitoring.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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