Calculate the Therapeutic Index of the given drug from the dose-response curve below:

INI-CET EXAM -2026

Pharmacology

Medium

Appeared in: INI-CET EXAM -2026

Explanation

The Therapeutic Index (TI) is a measure of a drug's relative safety, defined as the ratio of the dose that produces toxicity to the dose that produces a clinically desired or effective response.
The standard formula for calculating TI from preclinical animal studies is TI = LD50 / ED50, where LD50 is the median lethal dose and ED50 is the median effective dose.
Based on the provided graph, the LD50 is explicitly stated and shown as 400 mg/kg.
Similarly, the ED50 is stated and shown as 100 mg/kg.
Plugging these values into the formula gives: TI = 400 mg/kg / 100 mg/kg = 4.

Option A: This value (0.25) is obtained by incorrectly inverting the Therapeutic Index formula, dividing the ED50 by the LD50 (100 / 400).
Option B: This value (0.75) does not correspond to any logical calculation using the given ED50 and LD50 values. It is likely a random distractor or the result of a significant miscalculation.
Option C: A value of 2 would be calculated if the LD50 was misread from the graph as 200 mg/kg (i.e., 200/100 = 2). However, the graph clearly labels the LD50 at 400 mg/kg.

Clinical Relevance

Nursing practice connection: Knowing Calculation of Therapeutic Index from a dose-response curve helps nurses interpret findings accurately and avoid errors in routine assessment, medication administration, and patient teaching.
Understanding the Therapeutic Index is fundamental for safe nursing practice. A drug with a high TI (wide therapeutic window) is generally safer, as a large gap exists between the effective dose and the toxic dose.
Conversely, nurses must be exceptionally vigilant when administering drugs with a low TI (narrow therapeutic window), such as warfarin, digoxin, lithium, and some chemotherapy agents. This vigilance includes strict dose verification, patient education on toxicity symptoms, and monitoring of therapeutic drug levels through blood tests.
What if a different drug had an ED50 of 50 mg/kg and an LD50 of 150 mg/kg? The TI would be 150/50 = 3. Although the drug is more potent (lower ED50), its TI is lower than the drug in the question (TI=4), making it comparatively less safe and requiring more careful monitoring.

How to Approach the Question

First, identify the question's objective, which is to calculate the Therapeutic Index (TI).
Recall or look up the standard formula for TI: TI = LD50 / ED50.
Carefully examine the provided dose-response curve to extract the necessary values. Pay close attention to the axes and the legend.
From the graph's legend and curves, identify the Median Effective Dose (ED50) and the Median Lethal Dose (LD50). Here, ED50 = 100 mg/kg and LD50 = 400 mg/kg.
Substitute the identified values into the formula: TI = 400 / 100.
Perform the simple division to arrive at the final answer and select the matching option.

Concept Tested & Keywords

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E6 Pharmacology Katzung 16e pp. 64-66, 63-65