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Difficulty: EasyMeasures of Disease Frequency and Association

A randomized controlled trial is conducted to evaluate a new lipid-lowering medication for the prevention of myocardial infarction in patients with stable angina. Over a 5-year period, 1,000 patients receive the new medication and 1,000 patients receive a placebo. At the end of the study, 50 patients in the new medication group and 100 patients in the placebo group have experienced a myocardial infarction. Based on these results, what is the number needed to treat to prevent one myocardial infarction?

  1. A
    2
  2. B
    10
  3. 20Answer
  4. D
    50
  5. E
    100

Answer

20
The number needed to treat (NNT) is calculated as the inverse of the absolute risk reduction (ARR). First, determine the control event rate (CER): 1001000=0.10\frac{100}{1000} = 0.10. Next, determine the experimental event rate (EER): 501000=0.05\frac{50}{1000} = 0.05. The absolute risk reduction is CER−EER=0.10−0.05=0.05CER - EER = 0.10 - 0.05 = 0.05. The NNT is 1ARR=10.05=20\frac{1}{ARR} = \frac{1}{0.05} = 20. Therefore, 20 patients need to be treated with the new medication to prevent one myocardial infarction.

Step-by-Step Solution

1
Calculate the Experimental Event Rate (EER) and Control Event Rate (CER).
EER=501000=0.05EER = \frac{50}{1000} = 0.05. CER=1001000=0.10CER = \frac{100}{1000} = 0.10.
Event rates must be determined to calculate the absolute risk reduction.
2
Calculate the Absolute Risk Reduction (ARR).
ARR=CER−EER=0.10−0.05=0.05ARR = CER - EER = 0.10 - 0.05 = 0.05.
ARR represents the absolute difference in disease risk between the intervention and control groups.
3
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)
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Topics

Measures of Disease Frequency and Association

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