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Difficulty: MediumMeasures of Disease Frequency and Association

A randomized controlled trial is conducted to evaluate the efficacy of a novel antiplatelet agent in reducing the incidence of recurrent ischemic stroke. A total of 1,000 patients with a history of recent ischemic stroke are enrolled and randomly assigned to receive either the novel agent or standard aspirin therapy. The patients are followed for three years. In the group receiving the novel agent (n=500n=500), 15 patients experience a recurrent ischemic stroke. In the group receiving standard aspirin therapy (n=500n=500), 40 patients experience a recurrent ischemic stroke. Based on these trial results, how many patients need to be treated with the novel antiplatelet agent for three years to prevent one additional recurrent ischemic stroke?

  1. A
    2
  2. B
    18
  3. 20Answer
  4. D
    25
  5. E
    33

Answer

20 patients need to be treated with the novel antiplatelet agent to prevent one additional recurrent ischemic stroke.
The Number Needed to Treat (NNT) is an epidemiological measure that indicates how many patients must receive a specific intervention instead of the control therapy to prevent one additional adverse outcome. It is calculated by taking the inverse of the Absolute Risk Reduction (ARR). The formula is NNT=1ARRNNT = \frac{1}{ARR} First, determine the risk of the outcome in both study arms. The risk in the standard therapy (control) group is 40500=0.08\frac{40}{500} = 0.08. The risk in the novel agent (intervention) group is 15500=0.03\frac{15}{500} = 0.03. Next, calculate the ARR by finding the difference: 0.08−0.03=0.050.08 - 0.03 = 0.05. Finally, calculate the NNT: 10.05=20\frac{1}{0.05} = 20. Therefore, 20 patients need to be treated with the novel agent for three years to prevent one stroke.

Step-by-Step Solution

1
Calculate the absolute risk (incidence) of the event in the control group (standard aspirin therapy).
Control Event Rate (CER) = 40500=0.08\frac{40}{500} = 0.08 (or 8%8\%).
This establishes the baseline risk of recurrent stroke without the new intervention.
2
Calculate the absolute risk (incidence) of the event in the intervention group (novel agent).
Experimental Event Rate (EER) = 15500=0.03\frac{15}{500} = 0.03 (or 3%3\%).
This establishes the risk of recurrent stroke while on the novel treatment.
3
Calculate the Absolute Risk Reduction (ARR).
ARR=CER−EER=0.08−0.03=0.05ARR = CER - EER = 0.08 - 0.03 = 0.05 (or 5%5\%).
ARR represents the absolute difference in risk between the control and intervention groups.
4
Calculate the Number Needed to Treat (NNT).
NNT=1ARR=10.05=20NNT = \frac{1}{ARR} = \frac{1}{0.05} = 20.
NNT is defined as the inverse of the Absolute Risk Reduction.

Key Concept

Number Needed to Treat (NNT) and Absolute Risk Reduction (ARR)
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Topics

Measures of Disease Frequency and Association

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