Critical evaluation: Usefulness and NNT

Last updated: Tuesday, July 14, 2015

The number needed to treat (NNT) is an expression that literally describes the number of patients that we would need to treat with anotheraban for 2 years to prevent one stroke. It is calculated as shown below, remembering that if you’ve been using percentages throughout the calculation then use 100 as the numerator to make the maths work. NNTs are normally rounded up to whole numbers:

This means that 50 patients need to be treated with anotheraban for 2 years to prevent one stroke.

In the same trial, 2 patients in the placebo group and 82 in the anotheraban suffer from life-threatening bleeding. We can describe how many patients we would need to treat for one to suffer from major bleeding (harm) using the number needed to harm (NNH). We first need to work out the absolute risk increase (ARI) of major bleeding:

Then the calculation is similar to that used for NNTs:

NNHs are normally rounded down.

Commissioners, or the people buying a service in which anotheraban was to be used, would need to consider these calculations carefully. Is it worth treating 50 patients for 2 years to prevent one stroke when for every 25 patients treated, one of them will have a major bleed that might kill them?