Number Needed to Screen


Number Needed to Screen, Number Needed to Treat, Absolute Risk Reduction, Relative Risk Reduction

  • Definition
  • Event Rate (Event Probability)
  1. Event Rate: (Number Events) / (Number Total Patients)
  2. Calculate for both intervention and control groups
  1. RR = (Intervention Event Rate)/(Control Event Rate)
  • Definition
  • Relative Risk Reduction (RRR)
  1. Relative Risk Reduction or RRR = 1 - (Relative Risk)
  2. RRR = (Absolute Risk)/(Control Group Event Probability)
  3. Relative Risk and Relative Risk Reduction can be misleading when applied to an individual patient
    1. Relative Risk Reduction reflects the effect of an intervention on a population
    2. Effect is not guaranteed for the individual patient
    3. Number Needed to Treat (NNT) is a better measure of the chance that a positive effect will apply to the individual patient
    4. http://www.thennt.com/thennt-explained/
  • Definition
  • Absolute Risk Reduction (ARR)
  1. ARR = (Intervention Event Rate) - (Control Event Rate)
  2. A negative result is seen with effective intervention, while a positive result is seen with a harmful intervention (worse than control)
  • Definition
  • Number Needed to Screen (NNS) or Treat (NNT)
  1. Number of patients screened to prevent one death
  2. Assumes certain period of time (e.g. one year)
  3. NNS or NNT = 1/(Absolute Risk Reduction)
  4. NNT = 100/abs(rateTreatment - rateControl)
    1. Where Intervention is intended to prevent event X (e.g. Myocardial Infarction)
    2. Where rateTreatment = Rate of event X in treatment group (e.g. 5%)
    3. Where rateControl = Rate of event X in a control group (e.g. 10%)
    4. Example: NNT = 100/(5-10)=-20
      1. Twenty persons would need to undergo intervention, to prevent one event X
      2. In this case, the result is negative, consistent with Number Needed to Treat
      3. If the result were positive, the finding would be consistent with number needed to harm
  • Resources
  • References