What is number needed to treat (NNT)?
BMJ 2013; 347 doi: https://doi.org/10.1136/bmj.f4605 (Published 24 July 2013) Cite this as: BMJ 2013;347:f4605All rapid responses
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The number needed to treat is the number of participants who needed to be treated with the intervention(the new drug) for one less women to experience the disease (an adverse outcome) or cure than if those same women had been treated with placebo.
These days increasingly due to ethical reasons the placebo are replaced by the existing treatment or "standard of care" (because denying the treatment is unethical) then whether the interpretation would be as follows:
The number needed to treat is the number of participants who needed to be treated with the intervention(the new drug) for one less women to experience the disease (an adverse outcome) or cure than if those same women had been treated with the existing standard of care?.
Competing interests: No competing interests
I thank Dr Sedgwick for the statistical question on Number Needed to Treat (NNT). He has nicely explained that if risk-difference (Absolute Risk Reduction or ARR) is smaller then the benefit of treatment is larger, so much so that if its value is zero then the NNT is infinite. What if the difference is negative? I agree we reject the treatment because it is more harmful. Nonetheless, this has not been mentioned in the endgame!!
Competing interests: No competing interests
The direction (positive or negative) of value of NNT depends on whether we are considering a good or bad outcome
The larger the treatment effect of drug compared with placebo in reducing the incidence of a bad outcome (disease/death) or increasing a positive or good outcome (cure), the smaller the number needed to treat. The ideal value for number needed to treat is one, in which case every subject in the intervention group would experience good outcome if treated with intervention/drug and all would not experience the outcome with placebo (ARR = 100-0 = 100 and NNT= 100/100 = 1). On the other hand, no body suffers from bad outcome in treatment group and all suffer from bad outcome in the control group will also give the NNT of 1 (ARR= 0-100= 100 and NNT = -100/100 = -1). If there was no treatment effect (that is, no difference between intervention and placebo) the absolute risk difference would be zero and the number needed to treat would therefore approach infinity.
In case we are monitoring a bad outcome the number needed to treat (NNT) is negative and if we are monitoring a good outcome the NNT is positive if the difference is derived by formula
Absolute Risk Reduction (ARR) = outcome event rate in treatment group-outcome event rate in placebo group.
Competing interests: No competing interests