Concepts to be considered when appraising applicability of evidence to older people - number needed to treat

Definition

Number needed to treat (NNT) - the number of patients who need to receive a treatment for one additional patient to benefit (avoid the outcome of interest), compared with a control - a clinically intuitive way to express the absolute benefit of an intervention
NNT is increasingly emphasised in evidence-based practice and shared decision-making because it communicates absolute benefit far more meaningfully to patients than relative risk reduction alone

NNT = 1 / absolute risk reduction - because it depends on absolute (not relative) risk reduction, NNT varies with baseline risk: the same relative risk reduction produces a much better (lower/more favourable) NNT in a high-baseline-risk population than in a low-risk one

Baseline risk differs systematically with age, frailty, and life expectancy - a treatment's NNT calculated in a middle-aged trial population may not transfer directly to a frail 85-year-old, whose baseline risk of the outcome (and competing risk of dying from something else first) differs substantially
NNT should always be reported/considered alongside the time frame over which it applies (e.g. "NNT of 50 over 5 years") - a favourable NNT over a longer time horizon than the patient's remaining life expectancy provides little or no real-world benefit to that individual

Present NNT to patients in absolute, concrete terms (not just relative risk reduction) to support genuinely informed shared decision-making, and explicitly consider whether the calculated NNT's time horizon is compatible with the individual patient's prognosis

Statin primary prevention, anticoagulation for AF, screening program benefit communication

A "good" NNT from a landmark trial can become a poor one in an individual frail patient once their higher competing mortality risk and shorter life expectancy are accounted for - NNT is a population-level statistic that always needs individual-level translation

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