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Difficulty: MediumMeasures of Association and Impact

A multicenter randomized controlled trial evaluates a novel daily topical foam aimed at preventing foot ulcers in adults with long-standing diabetes mellitus. Participants were randomly assigned to receive either the novel foam plus standard podiatric care or standard podiatric care alone. All participants were followed for 2 years. The incidence of new foot ulcers is shown in the table below:

Study GroupTotal PatientsNew Foot Ulcers Over 2 Years
Novel foam + standard care80088
Standard care alone800120

Assuming the results are statistically significant, which of the following is the most accurate interpretation of the intervention's impact?

  1. The absolute risk reduction is 4%, indicating that 25 patients must be treated with the novel foam for 2 years to prevent one additional foot ulcer.Answer
  2. B
    The absolute risk reduction is approximately 27%, indicating that 4 patients must be treated with the novel foam for 2 years to prevent one additional foot ulcer.
  3. C
    The absolute risk reduction is 4%, indicating that 4 patients must be treated with the novel foam for 2 years to prevent one additional foot ulcer.
  4. D
    The relative risk of ulceration is 1.36, indicating a 36% increase in the risk of foot ulcers with the novel foam compared to standard care.
  5. E
    The difference in total events is 32, meaning that 32 patients must be treated with the novel foam for 2 years to prevent one additional foot ulcer.

Answer

The absolute risk reduction is 4%, indicating that 25 patients must be treated with the novel foam for 2 years to prevent one additional foot ulcer.
The correct answer accurately calculates both the Absolute Risk Reduction (ARR) and the Number Needed to Treat (NNT). The Control Event Rate (CER) is 120/800=0.15120/800 = 0.15, and the Experimental Event Rate (EER) is 88/800=0.1188/800 = 0.11. The ARR is the simple difference between these rates: 0.15−0.11=0.040.15 - 0.11 = 0.04, which is 4%4\%. The NNT is calculated as the inverse of the ARR (1/0.04=251 / 0.04 = 25), meaning 25 patients need to be treated to prevent one ulcer.

Step-by-Step Solution

1
Calculate the Control Event Rate (CER).
CER=120800=0.15CER = \frac{120}{800} = 0.15 (or 15%15\%).
Establishes the baseline risk of developing a foot ulcer under standard care.
2
Calculate the Experimental Event Rate (EER).
EER=88800=0.11EER = \frac{88}{800} = 0.11 (or 11%11\%).
Establishes the risk of developing a foot ulcer when the novel topical foam is used.
3
Calculate the Absolute Risk Reduction (ARR).
ARR=CER−EER=0.15−0.11=0.04ARR = CER - EER = 0.15 - 0.11 = 0.04 (or 4%4\%).
Determines the absolute difference in risk between the standard care group and the experimental group.
4
Calculate the Number Needed to Treat (NNT).
NNT=1ARR=10.04=25NNT = \frac{1}{ARR} = \frac{1}{0.04} = 25.
Converts the ARR into a clinical metric representing how many patients must be treated to prevent one adverse event.

Key Concept

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

Measures of Association and Impact

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