NNT and NNH Calculator

This calculator computes the Number Needed to Treat (NNT) or Number Needed to Harm (NNH) from event rates in treatment and control groups. These metrics express absolute treatment effects in clinically interpretable terms.

Enter event rates (as percentages)
Event Rate in Treatment Group (%) Percentage of patients who experienced the event in the treatment arm
Event Rate in Control Group (%) Percentage of patients who experienced the event in the control arm

Formulas

Absolute Risk Reduction (ARR)

\( \text{ARR} = \text{Event Rate}_{\text{Control}} - \text{Event Rate}_{\text{Treatment}} \)

Relative Risk Reduction (RRR)

\( \text{RRR} = \frac{\text{ARR}}{\text{Event Rate}_{\text{Control}}} \)

Number Needed to Treat (NNT)

\( \text{NNT} = \frac{1}{\text{ARR}} \) (when ARR > 0)

Number Needed to Harm (NNH)

\( \text{NNH} = \frac{1}{|\text{ARR}|} \) (when ARR < 0)

Understanding NNT and NNH

Number Needed to Treat (NNT) is the number of patients who need to receive a treatment for one additional patient to benefit. A lower NNT indicates a more effective treatment.

Number Needed to Harm (NNH) is the number of patients who need to receive a treatment for one additional patient to experience an adverse event. A higher NNH indicates a safer treatment.

These metrics translate relative effects into absolute terms that are easier to communicate to patients and use in clinical decision-making.

Clinical Interpretation

NNT and NNH are most useful when:

  • Comparing treatments with similar relative effects but different baseline risks
  • Communicating treatment benefits and risks to patients
  • Weighing benefits against harms (comparing NNT to NNH for the same treatment)
  • Making resource allocation decisions

A treatment with NNT = 10 and NNH = 100 means you would expect 10 patients to benefit for every 1 patient harmed.

Specific Examples

Example 1: A statin reduces cardiovascular events from 8% to 5% over 5 years. ARR = 3%, NNT = 34. You need to treat 34 patients for 5 years to prevent one cardiovascular event.

Example 2: An antibiotic causes C. difficile infection in 3% of patients vs 1% with placebo. ARR = -2%, NNH = 50. For every 50 patients treated, one additional patient develops C. difficile.

Example 3: A cancer screening program detects cancer in 0.5% of screened patients vs 0.4% in unscreened. ARR = 0.1%, NNT = 1000. You need to screen 1000 patients to detect one additional cancer.

Assumptions & Limitations

  • NNT/NNH are specific to the baseline risk of the population studied
  • Results may not generalize to populations with different baseline risks
  • NNT is typically rounded up to the nearest whole number
  • Confidence intervals should be considered when interpreting NNT/NNH
  • NNT/NNH do not account for the severity or duration of outcomes

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