Protective index

The protective index is a comparison of the amount of a therapeutic agent that causes the therapeutic effect to the amount that causes toxicity. Quantitatively, it is the ratio given by the toxic dose divided by the therapeutic dose. A protective index is the toxic dose of a drug for 50% of the population (TD₅₀) divided by the minimum effective dose for 50% of the population (ED₅₀). A high protective index is preferable to a low one: this corresponds to a situation in which one would have to take a much higher dose of a drug to reach the toxic threshold than the dose taken to elicit the therapeutic effect. A drug should ordinarily only be administered if the protective index is greater than one, indicating that the benefit outweighs the risk.

<math>\mbox{Protective index} = \frac{\mathrm{TD}_{50}}{\mathrm{ED}_{50}}</math>

The protective index is similar to the therapeutic index, but concerns toxicity (TD₅₀) rather than lethality (LD₅₀); thus, the protective index is a smaller ratio. Toxicity can take many forms, as drugs typically have multiple side effects of varying severity, so a specific criterion of toxicity must be specified for the protective index to be meaningful. Ideally a choice is made such that the harm caused by the toxicity just outweighs the benefit of the drug's effect. Thus, the protective index is a more accurate measure of the benefit-to-risk ratio than the therapeutic index, but is less objectively defined. Nevertheless, the therapeutic index can be viewed as an upper bound to the protective index for a given substance.