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Difficulty: MediumMeasures of Association and Impact

A multicenter clinical trial investigates the efficacy of a newly developed monoclonal antibody targeting proprotein convertase subtilisin/kexin type 9 (PCSK9) in patients with familial hypercholesterolemia. A total of 4,000 patients are enrolled and randomly assigned in equal numbers to receive either the monoclonal antibody or a placebo injection every two weeks. All participants remain on their maximally tolerated statin therapy. Over a 3-year follow-up period, a nonfatal myocardial infarction occurs in 100 of the 2,000 patients in the treatment group, compared to 150 of the 2,000 patients in the placebo group. Based on these findings, what is the number needed to treat (NNT) to prevent one nonfatal myocardial infarction over the 3-year period?

Answer: 40 patients

Answer

40
The Number Needed to Treat (NNT) is calculated as the inverse of the Absolute Risk Reduction (ARR). By determining the incidence of the outcome in both the placebo group (150/2000=7.5%150/2000 = 7.5\%) and the treatment group (100/2000=5.0%100/2000 = 5.0\%), we find an ARR of 2.5%2.5\% (or 0.0250.025). Dividing 11 by 0.0250.025 yields exactly 4040, meaning 4040 patients must be treated with the PCSK9 inhibitor over a 33-year period to prevent one nonfatal myocardial infarction.

Step-by-Step Solution

1
Calculate the Control Event Rate (CER) in the placebo group.
CER=1502000=0.075CER = \frac{150}{2000} = 0.075 (7.5%7.5\%)
The CER establishes the baseline risk of the outcome without the experimental intervention.
2
Calculate the Experimental Event Rate (EER) in the treatment group.
EER=1002000=0.050EER = \frac{100}{2000} = 0.050 (5.0%5.0\%)
The EER represents the risk of the outcome when the experimental intervention is applied.
3
Calculate the Absolute Risk Reduction (ARR).
ARR=CER−EER=0.075−0.050=0.025ARR = CER - EER = 0.075 - 0.050 = 0.025 (2.5%2.5\%)
The ARR quantifies the absolute decrease in the risk of the outcome attributable to the intervention.
4
Calculate the Number Needed to Treat (NNT).
NNT=1ARR=10.025=40NNT = \frac{1}{ARR} = \frac{1}{0.025} = 40
The NNT indicates how many patients must be treated to prevent one additional adverse outcome.

Key Concept

Number Needed to Treat (NNT)
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Topics

Measures of Association and Impact

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