## Learning Outcomes

- To understand the concept of numbers needed to treat
- To have an increased awareness of the efficacy of specific analgesics

**The principles of numbers needed to treat and numbers needed to harm**

With the introduction of Clinical Governance in the UK has come a drive for evidence based practice. To provide the strongest possible evidence systematic reviews and meta-analysis are used to inform our practice. Systematic reviews summarise large amounts of information and may be more likely than individual trials to describe the true clinical effect of an intervention [1]. Traditional statistical outputs from these systematic reviews are not immediately transferable to clinical practice and this is where numbers needed to treat (NNTs) prove valuable. This number can be calculated easily from raw data from statistical outputs and the principle in its calculation can be applied to treatment efficacy, adverse events (harm) or other end points [1].

NNT is the number of persons who must be treated for a given period to achieve an outcome (e.g. 50% pain relief) or to prevent an event (prophylaxis) who would otherwise have not done so if they received placebo. The NNT defines the treatment-specific effect of an intervention and knowing or estimating the number needed to harm is also an important part of the equation.

A very small NNT (that is one that approaches 1) means that a favourable outcome occurs in nearly every patient who received the treatment and in few patients in a comparison group. Although NNTs close to 1 are theoretically possible, they are almost never found in practice as this means that a treatment is 100% effective and there is no placebo effect. An NNT of 2 or 3 indicates that a treatment is very effective.

Although NNTs are powerful instruments for interpreting clinical effects, they also have important limitations:

- An NNT is generally expressed as a single number, which is known as its point estimate and the true value of the NNT can be higher or lower than the point estimate determined through clinical studies.
- It is inappropriate to compare NNTs across disease conditions particularly when the outcomes of interest differ. McQuay and Moore [1] provide an example of this in that an NNT of 30 for preventing deep venous thrombosis may be valued differently from an NNT of 30 for preventing a disabling stroke or for preventing death. Therefore, the concept expressed by the NNT is thus one of frequency, not of utility; its numerical value is a function of the disease, the intervention and the outcome.
- NNTs are not fixed quantities. The NNT for a specified intervention in an individual patient depends not only on the nature of the treatment but also on the risk at baseline (that is the probability at baseline that the patient being considered will experience the outcome of interest) Because that risk may not be the same for all patients, an NNT that is provided by the literature may have to be adjusted to compensate for the patient’s risk at baseline.
- An NNT is always based on an outcome for a specified period. Only when the outcome is the same and is measured during the same period is a comparison valid.

Please read the ‘Oxford league table of analgesics in acute pain’, located at: http://www.medicine.ox.ac.uk/bandolier/booth/painpag/Acutrev/Analgesics/Leagtab.html

Consider the following:

- Do you think that the NNT and NNH are useful concepts?
- Do they or would they steer you in your prescribing, administering or advising role?
- What do you find less useful about these concepts?

### References

McQuay, H.J., Moore, R.A., 1997. Using numerical results from systematic reviews in clinical practice.. Ann Intern Med, Ann Intern Med 126, 712-20.

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