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Difficulty: MediumMeasures of Association and Impact

A randomized controlled trial is conducted to evaluate the efficacy of a newly developed calcitonin gene-related peptide (CGRP) receptor antagonist compared to a placebo for the prevention of chronic migraines. Over a 66-month follow-up period, the incidence of patients progressing to daily migraines in the placebo group was 5%5\%. The investigators report that the new medication provides a relative risk reduction of 40%40\% for this progression compared to the placebo. Based on these findings, what is the number needed to treat with the new medication for 66 months to prevent one additional patient from progressing to daily migraines?

  1. A
    2
  2. B
    3
  3. C
    20
  4. D
    34
  5. 50Answer

Answer

The correct number needed to treat is 50.
The number needed to treat (NNT) is the number of patients who must receive a particular treatment to prevent one additional adverse event. It is calculated as the inverse of the absolute risk reduction (ARR). When given the control event rate (CER) and the relative risk reduction (RRR), the ARR must first be determined by multiplying the two values (ARR=CER×RRRARR = CER \times RRR). In this scenario, the CER is 5%5\% (0.050.05) and the RRR is 40%40\% (0.400.40). Thus, the ARR is 0.05×0.40=0.020.05 \times 0.40 = 0.02. The NNT is calculated as 1/0.021 / 0.02, which equals 5050.

Step-by-Step Solution

1
Identify the given variables from the scenario.
Control Event Rate (CER) = 5%5\% (0.050.05) and Relative Risk Reduction (RRR) = 40%40\% (0.400.40).
These are the necessary components to calculate the absolute risk reduction when the event rate in the intervention group is not explicitly provided.
2
Calculate the Absolute Risk Reduction (ARR).
ARR=CER×RRR=0.05×0.40=0.02ARR = CER \times RRR = 0.05 \times 0.40 = 0.02.
ARR represents the actual difference in risk between the two groups, which is required to find the NNT.
3
Calculate the Number Needed to Treat (NNT).
NNT=1ARR=10.02=50NNT = \frac{1}{ARR} = \frac{1}{0.02} = 50.
NNT is defined mathematically as the inverse of the absolute risk reduction.

Key Concept

Calculating Number Needed to Treat (NNT) from baseline risk and relative risk reduction.
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Topics

Measures of Association and Impact

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