Definition:The Number Needed to treat is the number of patients that you would need to treat to prevent one primary outcome (heart attack, death, stroke, whatever)

  • This applies to patients: with the same problem studied
  • treated for the same duration as the study

    • Calculation:

    • First calculate the Absolute Risk Reduction (ARR)
    • Then take the ARR in decimal form (e.g. .05 for 5%) and divide it INTO 1. (1/ ARR = NNT)

      • Example:
      - 8% stroke rate with A. Fib decreased to 3% with Coumadin
      – Absolute risk reduction of 5%
      – NNT = 1 / ARR or 1/.05 = 20
      Therefore you need to treat 20 A. Fib patients for one year with warfarin to prevent one stroke.

      Number Needed to Harm (NNH): this is the same concept as the Number Needed to Treat except that you use:
            Incidence of Adverse Effect MINUS Incidence in the Placebo Group = Absolute Risk Increase

      The calculation is then the same using Absolute Risk Increase instead of ARR.

      Example:
      – Incidence of gynecomastia is almost zero with placebo
      – Incidence of gynecomastia is 10% with spironolactone
      – Therefore: Absolute increase in risk is 10% – 0% = 10%
      – 1 / 0.10 = 10 = NNH You need to treat 10 patients with spironolactone to cause one case of gynecomastia.

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