Investigators at a regional cardiovascular center analyze data from a prospective study ev… — USMLE Step 2 CK

Investigators at a regional cardiovascular center analyze data from a prospective study evaluating a new oral anticoagulant compared to standard therapy for stroke prevention. The study followed patients with nonvalvular atrial fibrillation over a two-year period. Among the $1,000$ patients receiving the new anticoagulant, $20$ experienced an ischemic stroke. Among the $1,000$ patients receiving standard therapy, $70$ experienced an ischemic stroke. Based on these findings, what is the number needed to treat with the new anticoagulant to prevent one additional ischemic stroke compared to standard therapy?

Answer: 20 patients

Answer

The number needed to treat is 20.

The Number Needed to Treat (NNT) represents the number of patients who need to receive the experimental therapy rather than the standard therapy to prevent one additional adverse event. It is calculated as the inverse of the Absolute Risk Reduction (ARR). The ARR is the difference between the control event rate (

70/1000 = 0.07

) and the experimental event rate (

20/1000 = 0.02

). This difference is

0.05

. Therefore, the NNT is

\frac{1}{0.05}

, which equals

20

patients.

Step-by-Step Solution

Calculate the control event rate (CER)

\text{CER} = \frac{70}{1000} = 0.07

Establishing the baseline risk of the adverse event in the population receiving standard therapy.

Calculate the experimental event rate (EER)

\text{EER} = \frac{20}{1000} = 0.02

Establishing the risk of the adverse event in the population receiving the new intervention.

Calculate the Absolute Risk Reduction (ARR)

\text{ARR} = 0.07 - 0.02 = 0.05

Determining the absolute difference in event rates between the two groups.

Calculate the Number Needed to Treat (NNT)

\text{NNT} = \frac{1}{0.05} = 20

NNT is the inverse of the ARR, representing the number of patients required to receive the new treatment to prevent one additional event.

Key Concept

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

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