Number needed to treat

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Conceived as a basic tool of evidence-based medicine,94,95 it has been refined for chronic pain studies and found to be suitable as a common currency of treatment effect81 for both medication and intervention96 studies. It is calculated as the inverse of the absolute risk reduction between groups and is most usefully expressed with confidence intervals.97 Figure 14.2 illustrates a simple example.

NNT is unique to each treatment and may differ according to:

• pain condition being treated;

• comparator (control group);

• conditions (environment) of the trial;

• patient population:

- baseline pain level.

• therapeutic outcome (e.g. 30 or 50 percent pain relief).

Given the variability between individual studies, it is not surprising that an analysis of NNTs for a wide variety of medications in several pain conditions has failed to confirm a pattern of efficacy between the various classes of medication that are available.98

NNTs are now widely used to define the effectiveness of treatments by health professionals99 and patients (www.besttreatments.co.uk). Although the NNT is useful for comparing treatments, care must be exercised with its use because it can only be applied in similar circumstances to those patients and circumstances from which it was derived.

• NNT is context specific. The NNT for gabapentin from a brief clinical trial in adults with trigeminal neuralgia would certainly not apply to long-term therapy of octogenarians with PHN.

Placebo group Anticonvulsant group

Response rate (effectiveness) Relative efficacy Absolute risk reduction Number needed to treat

1 (by definition) 0 (by definition)

Figure 14.2 NNT: an example of a simple clinical trial. Eight patients with identical chronic pains receive treatment with medication, of whom four receive placebo and four receive an anticonvulsant. Thirty percent pain relief is considered to be a success. One of the placebo group and two of the anticonvulsant group gets 30 percent relief. It can be seen that for every four patients treated, one (one-quarter) will respond with anticonvulsant that would not have responded with placebo. Thus the NNT is 4 - and the anticonvulsant can be considered to be a moderately effective treatment for chronic pain. Obviously larger trials are required for statistical significance!

• NNT depends on the outcome measure used. The NNT for gabapentin (2400 mg per day) as a treatment of PHN in one study was 3.88 for 30 percent relief and 5.04 for 50 percent relief (the values are different again if a lower daily dose is used). Graphs illustrating how the proportion of responders changes according to the level of analgesia achieved have recently been advocated in order to understand this further.100

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