Some physicians use another measure of risk and benefit, the number needed to treat (NNT), in considering the consequences of treating or not treating. The NNT is the number of patients to whom a clinician would need to administer a particular treatment to prevent 1 patient from having an adverse outcome over a predefined period of time. (It also reflects the likelihood that a particular patient to whom treatment is administered will benefit from it.) If, for example, the NNT for a treatment is 10, the practitioner would have to give the treatment to 10 patients to prevent 1 patient from having the adverse outcome over the defined period, and each patient who received the treatment would have a 1 in 10 chance of being a beneficiary.
If the absolute risk reduction is large, you need to treat only a small number of patients to observe a benefit in at least some of them. Conversely, if the absolute risk reduction is small, you must treat many people to observe a benefit in just a few.
An analogous calculation to the one used to determine the NNT can be used to determine the number of patients who would have to be treated for 1 patient to experience an adverse event. This is the number needed to harm (NNH), which is the inverse of the absolute risk increase.
How comfortable are you with estimating the NNT for a given treatment? For example, consider the following questions: How many 60-year-old patients with hypertension would you have to treat with diuretics for a period of 5 years to prevent 1 death? How many people with myocardial infarction would you have to treat with β-blockers for 2 years to prevent 1 death? How many people with acute myocardial infarction would you have to treat with streptokinase to prevent 1 person from dying in the next 5 weeks? Compare your answers with estimates derived from published studies (). How accurate were your estimates? Are you surprised by the size of the NNT values?
Physicians often experience problems in this type of exercise, usually because they are unfamiliar with the calculation of NNT. Here is one way to think about it. If a disease has a mortality rate of 100% without treatment and therapy reduces that mortality rate to 50%, how many people would you need to treat to prevent 1 death? From the numbers given, you can probably figure out that treating 100 patients with the otherwise fatal disease results in 50 survivors. This is equivalent to 1 out of every 2 treated. Since all were destined to die, the NNT to prevent 1 death is 2. The formula reflected in this calculation is as follows: the NNT to prevent 1 adverse outcome equals the inverse of the absolute risk reduction. illustrates this concept further. Note that, if the absolute risk reduction is presented as a percentage, the NNT is 100/absolute risk reduction; if the absolute risk reduction is expressed as a proportion, the NNT is 1/absolute risk reduction. Both methods give the same answer, so use whichever you find easier.
It can be challenging for clinicians to estimate the baseline risks for specific populations. For example, some physicians may have little idea of the risk of stroke over 5 years among patients with hypertension. Physicians may also overestimate the effect of treatment, which leads them to ascribe larger absolute risk reductions and smaller NNT values than are actually the case.14
Now that you know how to determine the NNT from the absolute risk reduction, you must also consider whether the NNT is reasonable. In other words, what is the maximum NNT that you and your patients will accept as justifying the benefits and harms of therapy? This is referred to as the threshold NNT.15
If the calculated NNT is above the threshold, the benefits are not large enough (or the risk of harm is too great) to warrant initiating the therapy.
Determinants of the threshold NNT include the patient's own values and preferences, the severity of the outcome that would be prevented, and the costs and side effects of the intervention. Thus, the threshold NNT will almost certainly be different for different patients, and there is no simple answer to the question of when an NNT is sufficiently low to justify initiating treatment.
The bottom line
NNT is a concise, clinically useful presentation of the effect of an intervention. You can easily calculate it from the absolute risk reduction (just remember to check whether the absolute risk reduction is presented as a percentage or a proportion and use a numerator of 100 or 1 accordingly). Be careful not to overestimate the effect of treatments (i.e., use a value of absolute risk reduction that is too high) and thus underestimate the NNT.