# Epidemiology – Numbers Needed to Treat (NNT)

Definition

NNT = the number of patients that need to be treated in order for 1 extra patient to benefit

Alternatives to NNT include:

• Numbers Needed to Screen (NNS = No. needed to be screened for 1 to benefit)
• Numbers Needed to Harm (NNH = No. needed to be exposed to a risk factor for 1 to be harmed)

Formulae

• NNT = 1/ARR
• Absolute Risk Reduction (ARR) is calculated by the difference between the rate of event in controls and the rate of event in cases = (a/a+c) – (b/b+d)
• NNTs should always be reported with 95% Confidence Intervals for interpretation

Interpretation

• The lower the NNT the better.
• E.g. Drug FAB helps prevent strokes and has an NNT of 1.  By treating Bob with FAB this should prevent him having a stroke.  On the otherhand, drug BAD has an NNT of 50, so you would have to treat 50 Bobs in order to prevent one stroke.
• If the treatment or exposure if harmful (i.e. result is a negative number) the omit the sign and measure is renamed as NNH

• Useful to communicate benefit and harm – easy to understand (risk communication)
• NNTs can be used either for summarising the results of trials
• A clinically useful measure of the relative benefit of an active treatment over a control (better than RR or OR)
• Takes into account the frequency of the outcome – thus reflects the ublic health impact of the intervention