Number Needed to Treat - NNT Calculator
- Number Needed to Treat (NNT) represents the number of patients over a given time period that one would need to treat to achieve one additional study endpoint.
- NNT will help to calculate the average number of patients who need to receive specific treatment in order to prevent one additional adverse event.
Steps to Calculate NNT
Here are 3 simple steps to calculate NNT.
- Enter Control Group Percentage
- Enter Experimental Group Percentage
- Check the NNT Calculation
NNT Calculator Formula
Here is the Formula for NNT Calculator
To calculate the NNT, you need to know the Absolute Risk Reduction (ARR); the NNT is the inverse of the ARR:
NNT = 1/ARR
Absolute Risk Reduction (ARR)
- Absolute risk reduction describes the proportion of patients that benefited from the use of experimental therapy. It's a measure of a patient's gain from a given treatment.
- In epidemiology, absolute risk reduction, risk difference, or excess risk is the change in risk of a given activity or treatment in relation to a control activity or treatment. It is the inverse of the number needed to treat (NNT).
The formulas for ARR are as follows:
ARR = Control event rate - Experimental event rate,
ARR = R₀ - R₁
ARR = 1/NNT.
ABSOLUTE RISK REDUCTION (ARR) is absolute value of the difference between the CER and the EER
ARR = |CER - EER | or = 1/NNT
Relative Risk Reduction (RRR)
- RELATIVE RISK REDUCTION (In epidemiology, the relative risk reduction is a measure calculated by dividing the absolute risk reduction by the control event rate.)
Formula: RRR (raw calculation) = |CER - EER| / CER
Alt: RRR = 1- RR where RR=reported relative risk]
- The two methods of calculation may produce different results. The reported value will adjust for other prognostic factors
NNT Calculator Calculation
- The number needed to treat (NNT) is the inverse of the absolute risk reduction (ARR) expressed as a decimal.
- The example below compares an event rate of 26% versus 16%:
- Similarly, when a study outcome is based on time of exposure (patient-years), the NNT is calculated based on cumulative event proportions.
The example below compares 12 events per 1000 patient-years versus 4 events per 1000 patient-years
If a time-based study outcome is relatively rare, the following shorter equation may be used for person-time data
How to use the NNT calculator?
Here are some data required by our NNT calculator to choose the type of your data.
- Some research results are given as percentages - e.g., group A had a 20% complete response to an old treatment, and group B had a 40% response to the new treatment.
- Some scientists prefer to use exposure data or patient years.
- They would supply you with a number of events in both groups, and the period over which the study happened. - e.g., the follow-up lasted for 5 years. During that period, 5 patients died in group A, and 10 patients died in group B.
- Enter the data for both the control and experimental groups - the control group is usually the old, well-known way of doing something. In contrast, the experimental group is the novelty that we wish to test.
- NNT helps both at an individual case basis (evaluation of the likelihood of benefit of a treatment) and in summarizing the results of therapeutic trials.
- It was also found to be a better measure of the relative benefit of an active treatment over a control than other calculations, such as the relative risk, the relative risk reduction (RRR) or the odds ratio.
- It is often used in establishing patient decision aids such as: a particular medication reduces the risk of a particular condition.
- Despite their easefulness of application, the interpretation of NNTs can only go up to a certain extent as the measure cannot be used for a meta-analysis because the baseline risk can vary considerably between the trials.
- Although clinicians can use them to discuss particular benefits of treatment, the measure itself.
- NNT cannot be used to calculate the risk/benefit ratio for an individual person and requires sample sizes.
- Similarly, this is not a measure of the degree of benefit but an expression of how many individuals are needed to undergo the treatment, for one to benefit.
Advantages of NNT
- NNTs can be used either for summarising the results of a therapeutic trial or for medical decision-making about an individual.
- The number needed to treat provides a more clinically useful measure of the relative benefit of active treatment over control than the use of the relative risk, the relative risk reduction (RRR), or the odds ratio (the ratio of the odds of an event occurring in one group to the odds of it occurring in another group).
- NNT allows a simple explanation of the likelihood of benefit, which may be used in helping patients reach a decision about whether to start a medication.
- It is used to inform patient decision aids, such as the National Institute for Health and Care Excellence (NICE) decision aid "Taking a statin to reduce the risk of coronary heart disease”.
Disadvantages of NNT
- Although NNTs are easy to interpret, they cannot be used for performing a meta-analysis.
- Pooled NNTs derived from meta-analyses can be seriously misleading because the baseline risk often varies appreciably between the trials.
- NNTs cannot be used to calculate the risk: benefit ratio for an individual person.
- It gives information to the population as a whole.
- The concept expresses the number of individuals who must be treated for one to benefit but not the degree of benefit, which will vary across conditions and circumstances.