Protective Index

What is this formula?

The Protective Index (PI) is a pharmacological safety measure that compares the median toxic dose to the median effective dose. Unlike the Therapeutic Index, which often uses a lethal endpoint, the Protective Index focuses on toxicity and is generally more relevant for evaluating drug safety in clinical settings.

When to use it

Use this formula when assessing the relationship between therapeutic efficacy and toxic side effects of a drug.

Example

A medication has a median toxic dose of 150 mg/kg and a median effective dose of 30 mg/kg.

Data:

TD50 = 150 mg/kg

ED50 = 30 mg/kg

Formula:

PI = TD50 / ED50

Substitution:

PI = 150 / 30

Result:

PI = 5

Interpretation:

The median toxic dose is five times greater than the median effective dose, indicating a moderate safety margin.

Applications

- Drug development

- Clinical pharmacology

- Comparative drug safety analysis

- Toxicology studies

- Therapeutic risk evaluation

Note

The Protective Index is often considered more clinically relevant than the classical Therapeutic Index because toxicity usually occurs before lethality. Different studies may define toxic endpoints differently, which can affect TD50 values and the resulting Protective Index. Therefore, comparisons should ideally be made using consistent toxicity criteria.