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Difficulty: MediumMeasures of Association and Impact

An investigative team conducts a multicenter prospective trial assessing a newly developed targeted therapy, Navituzumab, for the prevention of relapses in patients with relapsing-remitting multiple sclerosis. A total of 2,500 patients are enrolled and equally divided into two groups: one receiving standard interferon-beta therapy and the other receiving Navituzumab.

The relapse data over a 3-year follow-up period are presented in the table below:

Treatment GroupTotal PatientsPatients with Relapse
Standard Therapy1,250250
Navituzumab1,250150

Based on these study findings, how many patients need to be treated with Navituzumab instead of standard therapy for 3 years to prevent one additional clinical relapse?

  1. A
    8
  2. B
    12
  3. 13Answer
  4. D
    40
  5. E
    125

Answer

13
The correct answer is 13. The Number Needed to Treat (NNT) is calculated as the reciprocal of the Absolute Risk Reduction (ARR). The Control Event Rate (CER) is the incidence of relapse in the standard therapy group, which is 250/1250=0.20250 / 1250 = 0.20. The Intervention Event Rate (IER) is the incidence in the Navituzumab group, which is 150/1250=0.12150 / 1250 = 0.12. The ARR is the difference between these rates: 0.20−0.12=0.080.20 - 0.12 = 0.08. The NNT is 1/0.08=12.51 / 0.08 = 12.5. By epidemiological convention, NNT is always rounded up to the next whole number to ensure that at least one outcome is prevented. Therefore, 12.512.5 rounds up to 13.

Step-by-Step Solution

1
Calculate the incidence of relapse in both the control and intervention groups.
Control Event Rate (CER) = 250/1250=0.20250 / 1250 = 0.20 (20%20\%). Intervention Event Rate (IER) = 150/1250=0.12150 / 1250 = 0.12 (12%12\%).
These rates are required to determine the absolute risk difference between the two treatments.
2
Calculate the Absolute Risk Reduction (ARR).
ARR = CER−IER=0.20−0.12=0.08\text{CER} - \text{IER} = 0.20 - 0.12 = 0.08 (8%8\%).
ARR represents the absolute difference in risk of the outcome between the standard therapy and Navituzumab groups.
3
Calculate the raw Number Needed to Treat (NNT).
NNT = 1/ARR=1/0.08=12.51 / \text{ARR} = 1 / 0.08 = 12.5.
NNT indicates the number of patients that must be treated over a specific period to prevent one adverse outcome.
4
Apply epidemiological rounding rules to the NNT.
12.512.5 is rounded up to 1313.
NNT must always be rounded up to the next highest whole number. Treating 12 patients would theoretically prevent only 0.960.96 relapses, which is less than one complete event.

Key Concept

Number Needed to Treat (NNT) calculation and appropriate rounding rules.
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Topics

Measures of Association and Impact

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