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Putting prevention into practice (Green Book)

Appendix 3

Assessing the benefit of treatment/intervention:
number needed to treat (NNT)

While expressing the benefits of treatment as a relative risk reduction is useful as a measure of the clinical impact of treatment, it can be deceptive, especially when the outcome of interest is very uncommon. A more productive way of expressing the benefit of an intervention is to calculate the number needed to treat (NNT).1,2

The NNT is a measure of the number of people who need to be treated (often for a specified time period) in order to prevent one event or achieve the treatment target. For example, brief advice (3–5 minutes) by a GP that incorporates assessment of interest in quitting, provision of pharmacotherapy and arranging follow up has an NNT of 14. If the GP provided this advice to 14 smokers then one would quit for at least 12 months, as shown in the table below.

Target area

GP time

intervention

NNT

outcome

Smoking3

3–5 minutes (up to 1 minute)

Brief behavioural counselling using the 5As*

1 in 14 (1 in 20)

Quit for at least 12 months

Hazardous drinking4,5

3–5 minutes

Brief behavioural counselling using the 5As*

1 in 10

25–30% reduction in alcohol consumption

Exercise6-9

3–5 minutes

Brief behavioural counselling using the 5As*

1 in 10

Engage in at least 30 minutes exercise for 30 minutes three times a week

* 5As: Ask, Assess, Advise, Assist, Arrange

The return on effort for providing effective interventions to assist patients with stopping smoking, cutting down on their drinking, or taking up exercise is good. NNT is also helpful to the patient. It provides an estimate of the benefit they may gain by adhering to a screening program, changing their health related behaviour or following a recommended treatment. The second table highlights the absolute reduction in clinical illness and disease associated with various prevention or clinical activities. Taking smoking cessation again as the example, the relative risk reduction in cancer risk is 30–50% after 10 years of abstinence.10 In absolute terms, there is one less smoking related death per year for every 100 smokers who quit. From the table above, GPs spending 3–5 minutes per smoker will have one smoker quit per 17 men counselled. Hence the NNT to reduce smoking related deaths per year is one in 1700.

Estimated NNT for a range of common screening and clinical activities


Target area

GP time

intervention

NNT

outcome

Falls prevention in the elderly11

10–15 minutes

Medication review, correct sensory deficits, balance and strengthening exercises, attention to home environment

1 in 8

Prevention of one significant fall

Smoking12

3–5 minutes

Brief behavioural counselling using the 5As

1 in 1000

Prevention of one death per year from smoking related causes

Screening for colorectal cancer13

3–6 minutes

Haem occult, appropriate Rx and follow up

1 in 1374

Prevention of one colorectal cancer over 5 years of the intervention

Mammography in women aged 50–59 years14

3–5 minutes

Mammogram and appropriate treatment and follow up

1 in 2451

Prevention of one breast cancer over 5 years of the intervention

Middle aged men with hyperlipidaemia and multiple CVD risk factors14

6–10 minutes

Lipid lowering agent for 5 years

1 in 53

1 in 190

Prevention of one nonfatal myocardial infarction

Prevention of one all cause death

Mild hypertension in the elderly15

6–10 minutes

Prescription of an antihypertensive for 5 years

1 in 83

Prevention of one cardiovascular event


As it can be difficult to estimate accurately the baseline level of risk, the following table provides the GP with a range of estimates of the NNT depending upon the reported relative risk reduction and the level of baseline risk. From a population perspective, policy makers would like to compare each of the various interventions to determine the value of each and to assist them in decision making about the provision of appropriate resources.

NNT example

NNT Calculation table

If you know the (approximate) probability of an event (1% in the above example) and the risk reduction achieved by an intervention, then you can calculate the NNT using the table below.

NNT Example

Clinical benefit doesn’t take into account the costs of providing the interventions, programs and follow up. The most valid comparison, putting the various interventions on a reasonably equivalent footing would be to include an additional column that reports the cost per quality adjusted life year saved (QALY).16 This comparison is beyond the scope of this monograph. For more information about NNT, see reference 2 and 16.

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