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Difficulty: HardMeasures of Association and Impact

An international research consortium recently published the findings of a large-scale, phase III, double-blind trial. The study assessed a novel oral small-molecule immunosuppressant against a placebo for maintaining clinical remission in adults with severe ulcerative colitis.

A total of 6,0006,000 participants were enrolled and randomly assigned in a 1:11:1 ratio to receive either the novel drug or the placebo. All participants were followed for a 5252-week maintenance phase. By the end of the study, clinical disease relapse occurred in 435435 patients in the placebo group and 312312 patients in the intervention group.

Based on these trial results, what is the number needed to treat (NNT) to prevent one disease relapse over the 5252-week period? (Calculate the exact value and apply the standard biostatistical rounding rule for NNT).

Answer: 25 patients

Answer

25
The correct answer is derived by calculating the absolute risk reduction (4.1%4.1\%) and finding its reciprocal to get the number needed to treat (24.3924.39). Following standard epidemiological guidelines, NNT values are always rounded up to the next highest integer, yielding 2525.

Step-by-Step Solution

1
Determine the number of patients in the control and experimental groups.
Control group = 3,0003,000 patients; Experimental group = 3,0003,000 patients.
To calculate event rates, we need the denominator for each group. The total is 6,0006,000 randomized in a 1:11:1 ratio.
2
Calculate the Control Event Rate (CER).
CER=4353000=0.145\text{CER} = \frac{435}{3000} = 0.145
CER is the incidence of the outcome in the placebo group.
3
Calculate the Experimental Event Rate (EER).
EER=3123000=0.104\text{EER} = \frac{312}{3000} = 0.104
EER is the incidence of the outcome in the intervention group.
4
Calculate the Absolute Risk Reduction (ARR).
ARR=CER−EER=0.145−0.104=0.041\text{ARR} = \text{CER} - \text{EER} = 0.145 - 0.104 = 0.041
ARR represents the absolute difference in risk between the two groups.
5
Calculate the Number Needed to Treat (NNT) and apply the correct clinical rounding rule.
NNT=10.041≈24.39\text{NNT} = \frac{1}{0.041} \approx 24.39
Rounded up to the next whole number: 2525
NNT is calculated as the inverse of ARR. Because one cannot treat a fraction of a patient, NNT is always rounded UP to the next whole number to guarantee the prevention of at least one outcome.

Key Concept

Calculation of Absolute Risk Reduction (ARR) and Number Needed to Treat (NNT) from raw event data, followed by the application of the conservative NNT rounding rule (always rounding up).
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Topics

Measures of Association and Impact

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