Number needed to treat

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The number needed to treat (NNT) is a way of summarizing the benefit of an intervention to improve health care.[1][2] The NNT has been proposed to improve quantitative literacy.[1] The calculations are derived from the results of a randomized controlled trial of an intervention.

Two-by-two table for a screening program
Outcome
Present Absent
Experimental (intervention) group Cell A Cell BTotal in experimental group
Control group Cell C Cell DTotal in control group
Total with outcome Total without outcome

Contents

Calculations

Event rates

\mbox{Experimental event rate}=\left (\frac{\mbox{Cell A}}{\mbox{Cell A + Cell B}}\right )
\mbox{Control event rate}=\left (\frac{\mbox{Cell C}}{\mbox{Cell C + Cell D}}\right )

Measures of efficacy

\mbox{Absolute risk reduction} =\left (\mbox{Experimental event rate} - \mbox{Control event rate}\right )
\mbox{Number need to treat}=\left (\frac{100}{\mbox{Absolute risk reduction}}\right )

Deriving the NNT from the odds or risk ratios

The odds ratio may be used to derive the number needed to treat:[3][4]

For odds ratios less than 1:[4]

NNT = \frac{1 - CER * (1 - OR)}{CER * (1 - OR)* (1 - CER)} \mbox{, where CER is control event rate and OR is odds ratio}

For odds ratios greater than 1:[4]

NNT = \frac{1 + CER * (OR - 1)}{CER * (OR - 1)* (1 - CER)} \mbox{, where CER is control event rate and OR is odds ratio}

The relative risk ratio may be used to derive the number needed to treat:[3][5]

NNT = \frac{1}{CER * (1-RRR)} \mbox{, where CER is control event rate and RRR is relative risk ratio}

The relative risk reduction may be used to derive the number needed to treat:[3][5]

NNT = \frac{1}{CER * (RRR)} \mbox{, where CER is control event rate and RRR is relative risk reduction}

Example

Variations

Years-needed-to-treat to add 1 year of life is proposed to estimate treatment effects.[6]

References

  1. ↑ 1.0 1.1 Laupacis A, Sackett DL, Roberts RS (1988). "An assessment of clinically useful measures of the consequences of treatment". N. Engl. J. Med. 318 (26): 1728–33. PMID 3374545.
  2. ↑ Wen L, Badgett R, Cornell J (2005). Number needed to treat: a descriptor for weighing therapeutic options. Am J Health Syst Pharm 62 (19): 2031–6. DOI:10.2146/ajhp040558. PMID 16174840.
  3. ↑ 3.0 3.1 3.2 Furukawa TA, Guyatt GH, Griffith LE (February 2002). Can we individualize the 'number needed to treat'? An empirical study of summary effect measures in meta-analyses. Int J Epidemiol 31 (1): 72–6. PMID 11914297.
  4. ↑ 4.0 4.1 4.2 McQuay HJ, Moore RA (May 1997). Using numerical results from systematic reviews in clinical practice. Ann. Intern. Med. 126 (9): 712–20. PMID 9139558.
  5. ↑ 5.0 5.1 Chatellier G, Zapletal E, Lemaitre D, Menard J, Degoulet P (February 1996). The number needed to treat: a clinically useful nomogram in its proper context. BMJ 312 (7028): 426–9. PMID 8601116. PMC 2350093.
  6. ↑ Levy WC, Mozaffarian D, Linker DT, et al. (March 2009). Years-needed-to-treat to add 1 year of life: a new metric to estimate treatment effects in randomized trials. Eur. J. Heart Fail. 11 (3): 256–63. DOI:10.1093/eurjhf/hfn048. PMID 19164422.

See also

External links

Tables of NNTs for various treatments:

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