Number needed to treat (NNT)

Regarding the calculation of number needed to treat (NNT), which one of the following formulae is used?

a) 1/absolute risk reduction

b) 1/the odds ratio

c) The odds ratio/absolute risk reduction

d) Relative risk reduction/absolute risk reduction

e) 1/relative risk reduction

Answer: a

Explanation

The number needed to treat (NNT) is the reciprocal of the absolute risk reduction

expressed as a decimal. For example, if a new agent reduces the risk of postoperative

vomiting from 25% to 15%, the absolute risk reduction is 10%, or 0.1. In 100 patients,

the 75 not suffering from vomiting and the 25 who were initially suffering would all

have to have the treatment to stop 10 from vomiting. You would treat 100 to stop 10

vomiting, or 10 to stop 1 vomiting; NNT is 10 or 1/0.1.

‘Number needed to treat’ is bandied around regularly when doctors discuss papers,

it is loosely understood and a simple statistical concept that is easy to define.

Reference

McQuay HJ, Moore RA. Using numerical results from systematic reviews in clinical

practice. Ann Intern Med 1997; 126(9): 712–20. Online at www.medicine.ox.ac.uk/

bandolier/booth/painpag/NNTstuff/numeric.htm (Accessed 30 October 2009)

Reference

British Thoracic Society/Scottish Intercollediate Guidlelines Network. 101 British

Guideline on the Management of Asthma: a National Clinical Guideline. NHS QIS, May

2008, revised June 2009. Online at www.sign.ac.uk/pdf/sign101.pdf (Accessed

30 October 2009)