What does a Number Needed to Treat (NNT) of 6 mean?

Understanding Number Needed to Treat (NNT) of 6

A Number Needed to Treat (NNT) of 6 means that you need to treat 6 patients with the intervention to prevent one additional adverse outcome (or achieve one additional beneficial outcome) compared to the control or standard treatment.

What NNT Measures

• NNT is calculated as the inverse of the absolute risk reduction (ARR): NNT = 1/ARR × 100 1
• When ARR = 16.67%, the NNT equals 6 (1/0.1667 = 6) 1
• NNT represents the average number of patients who need to receive the experimental therapy to prevent one additional adverse outcome (or achieve one beneficial outcome) compared to the control therapy 2

Clinical Interpretation of NNT = 6

• An NNT of 6 is generally considered a clinically meaningful effect size in medical practice 3
• This means that for every 6 patients treated with the intervention:
  • 1 patient will benefit (achieve the desired outcome or avoid the adverse outcome)
  • The other 5 patients would have the same outcome whether they received the intervention or control 1

Contextualizing NNT Values

• Lower NNT values indicate more effective treatments (fewer patients needed to treat for one to benefit) 3
• For comparison:
  • Evolocumab (Repatha) has an NNT of 67 over 2.2 years to prevent one major cardiovascular event in patients with atherosclerotic cardiovascular disease 4
  • Ezetimibe plus simvastatin shows an NNT of 50 over 6 years for secondary prevention 4
  • An NNT of 6 is substantially better than these examples, indicating a more effective intervention 1, 4

Clinical Decision-Making with NNT

• NNT should be considered alongside:

  • The severity of the outcome being prevented 5
  • The risks and adverse effects of the treatment 5
  • The costs of treatment 5
  • The time period over which the NNT applies 1

• The threshold NNT (the point at which benefits equal negative consequences) can be calculated as: NNT(T) = 1/(AER₁·RV₁ +...+ AERₙ·RVₙ), where AER is the adverse event rate and RV is the relative value 5

Important Considerations When Interpreting NNT

• NNT values are specific to:

  • The population being studied 6
  • The specific intervention being used 6
  • The time period of observation 1
  • The baseline risk in the control group 5

• Measurement errors in continuous outcome measures can lead to bias in NNT calculations 7

• Always check if the NNT is statistically significant - non-significant NNTs should be reported as "NS" rather than a numerical value 1

Common Pitfalls in NNT Interpretation

• Failing to consider the time period over which the NNT applies (a 5-year NNT of 6 is different from a 1-year NNT of 6) 1
• Not accounting for the severity of the outcome being prevented (an NNT of 6 to prevent death is more meaningful than an NNT of 6 to prevent mild symptoms) 5
• Overlooking the baseline risk in the population (NNT is more favorable in higher-risk populations) 5, 6
• Not considering adverse effects of treatment when evaluating the clinical utility of an NNT 5