Formula for Therapeutic Index
The Therapeutic Index of a drug is calculated as:
- AUC / dose
- LD50 / ED50 — Correct Answer
- ED50 / LD50
- EC50 / IC50
Explanation:
Correct Answer: LD50 / ED50
Therapeutic Index (TI) = LD50 / ED50, where:
- LD50 = Dose that produces death in 50% of the population.
- ED50 = Dose that produces desired effect in 50% of the population.
Significance
- A higher TI = safer drug (large margin between effective and lethal doses).
- A lower TI = narrow safety margin (e.g. digoxin, aminoglycosides, warfarin — require careful monitoring).
Quantal Dose Response Curve
The TI is derived from the quantal dose-response curve, which plots the cumulative frequency distribution of animals showing a response against log dose.
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