Examkin

Question

Difficulty: MediumMeasures of Disease Frequency and Association

Investigators are evaluating the efficacy of a newly developed long-acting monoclonal antibody for preventing severe respiratory syncytial virus (RSV) infections in high-risk premature infants. A total of 5,0005,000 infants are enrolled and randomized in a 1:1 ratio to receive either the monoclonal antibody or a placebo injection at the start of the RSV season. All participants are followed for 150150 days. At the end of the follow-up period, data analysis reveals that 5050 infants in the monoclonal antibody group and 150150 infants in the placebo group required hospitalization for severe RSV infection.

Based on these study results, what is the number needed to treat to prevent one RSV-related hospitalization?

Answer: 25 patients

Answer

25
The Number Needed to Treat (NNT) quantifies the number of patients who must receive a specific intervention to prevent one additional adverse event. It is calculated by first determining the event rates in both the control and intervention groups. Here, the event rate in the placebo (control) group is 150/2,500=0.06150 / 2,500 = 0.06. The event rate in the monoclonal antibody (intervention) group is 50/2,500=0.0250 / 2,500 = 0.02. The Absolute Risk Reduction (ARR) is the difference between these two rates (0.06−0.02=0.040.06 - 0.02 = 0.04). Finally, the NNT is the reciprocal of the ARR: 1/0.04=251 / 0.04 = 25. Thus, 25 infants need to receive the monoclonal antibody to prevent one hospitalization for severe RSV.

Step-by-Step Solution

1
Determine the number of patients in each study group.
Because 5,0005,000 infants were randomized in a 1:1 ratio, there are 2,5002,500 infants in the monoclonal antibody group and 2,5002,500 infants in the placebo group.
Accurate denominators are required to calculate the correct event rates for each study arm.
2
Calculate the event rate in the control group (ERcontrolER_{control}).
ERcontrol=1502500=0.06ER_{control} = \frac{150}{2500} = 0.06 (or 6%6\%).
The baseline risk of the outcome without the intervention must be established.
3
Calculate the event rate in the intervention group (ERinterventionER_{intervention}).
ERintervention=502500=0.02ER_{intervention} = \frac{50}{2500} = 0.02 (or 2%2\%).
The risk of the outcome with the intervention must be established to compare against the baseline.
4
Calculate the Absolute Risk Reduction (ARR).
ARR=ERcontrol−ERintervention=0.06−0.02=0.04ARR = ER_{control} - ER_{intervention} = 0.06 - 0.02 = 0.04.
ARR represents the absolute difference in risk between the two groups, which is required to calculate the NNT.
5
Calculate the Number Needed to Treat (NNT).
NNT=1ARR=10.04=25NNT = \frac{1}{ARR} = \frac{1}{0.04} = 25.
NNT is the inverse of the ARR, representing how many patients must be treated to prevent one additional adverse outcome.

Key Concept

The Number Needed to Treat (NNT) is calculated as the reciprocal of the Absolute Risk Reduction (ARR), where ARR is the difference in event rates between the control and intervention groups (NNT=1ARRNNT = \frac{1}{ARR}).
Rate this question

Topics

Measures of Disease Frequency and Association

More questions from this topic