Number needed to treat
The number needed to treat ( en. number needed to treat NNT ) is a statistical measure that indicates how many patients per unit of time (eg 1 year) must be treated with the test substance or test method to achieve the desired goal of therapy in a to achieve patient or an event to prevent (eg, heart attack). It is in addition to the absolute and relative risk reduction a means to illustrate the benefit of treatment in and corresponds to the reciprocal of the absolute risk reduction ( ARR ). A larger NNT consequently means a smaller risk reduction.
NNT is also used in preventive medicine and sometimes as the number of necessary operations screen ( en. numbers needed to screen NNS ) refers. In practical application, the NNT should be set with the NNH (s, numbers needed to harm ) in relation, that is, the number of treatments needed to achieve the desired goal of therapy in a patient with the necessary number of treatment processes, to cause damage in a patient are compared.
Calculation
You can see the relationships very well with the help of a fourfold table illustrate. This provides a way of evaluating a scientific investigation dar. Ideally, it is a prospective, controlled, randomized clinical trial.
Where:
- A is the number of patients who achieved with step 1, the objective
- B is the number of patients in which the objective was not achieved with step 1
- C for the number of patients who achieved with measure 2, the target,
- D is the number of patients in which the objective was not achieved with Measure 2.
The ARR ( absolute risk reduction) is then calculated as follows:
A positive absolute risk reduction indicates a superiority of one measure against measure 2 Has the Absolute risk reduction a negative value, it means a inferiority of measure 1 for the validity of the ARR is still their confidence intervals important.
The NNT is calculated as follows:
Or
A negative ARR would be represented not as a negative number needed to treat, but as a positive number needed to harm ( NNH ).
Rounding: If you want to specify the NNT without decimal places, so it is recommended to round up. example:
- 4.2304 patients must be treated to a patient dies less
- (at least) 5 patients must be treated to a patient dies less
The usual rounding normally would result in that the therapeutic effect would be overestimated.
Valence
It is important that the values of the NNT or ARR strictly initially apply only to the study, in which they were determined. The external validity of the study describes the transferability to the outside - that is, to a particular case or a general statement. Therefore, it is important that in addition to the NNT, the groups, the period and the destination point are described in detail. example:
See also: Relative risk reduction, absolute risk reduction