Calculating Number Needed to Treat (NNT)
The Formula
NNT is calculated as the inverse of the absolute risk reduction (ARR): NNT = 1/ARR, or when expressed as a percentage, NNT = 100/ARR. 1, 2
Step-by-Step Calculation Process
1. Calculate Absolute Risk Reduction (ARR)
- ARR = Event rate in control group - Event rate in treatment group 2
- Express event rates as proportions (e.g., 0.102 for 10.2%) 2
- For example: If control group has 10.2% events and treatment group has 7.9% events, then ARR = 10.2% - 7.9% = 2.3% 2
2. Apply the NNT Formula
- NNT = 1/ARR (when ARR is expressed as a decimal) 1, 2
- NNT = 100/ARR (when ARR is expressed as a percentage) 2
- Using the example above: NNT = 100/2.3 = 43.5 2
3. Round the Result
Essential Requirements for Valid NNT Calculation
Study Design Prerequisites
- The calculation must be based on a statistically significant difference between treatment groups 2, 3
- Requires a dichotomous endpoint (event occurs or does not occur, such as death, stroke, or myocardial infarction) 2, 3
- Data should come from well-designed clinical studies, preferably randomized controlled trials or meta-analyses 4, 3
Required Reporting Elements
- Baseline risk of the population studied 5
- Time horizon (the duration over which the NNT applies) 1, 2, 5
- 95% confidence intervals for the NNT 5, 4
Interpreting the Result
What NNT Means Clinically
- NNT represents the number of patients who must receive the treatment for one additional patient to benefit (compared to control) over the specified time period 1, 3
- For every NNT patients treated, (NNT-1) patients would have the same outcome regardless of which treatment they received 1
- NNT values range from 1 (perfect treatment effect) to infinity (no treatment effect) 2
Clinical Significance of Different NNT Values
- Lower NNT values indicate more effective interventions 3, 6
- NNT values between 10-20 generally indicate strong clinical benefit 1
- NNT values of 20-50 represent acceptable therapeutic benefit depending on condition severity 1
- When NNT exceeds 100, the intervention requires treating large numbers for minimal population-level benefit 1
Critical Pitfalls to Avoid
Common Calculation Errors
- 29.4% of published studies use methods not aligned with basic methodological recommendations, with meta-analyses being particularly problematic (56.5% using non-recommended methods) 5
- Failing to account for study design type when selecting calculation method 5
- Using inappropriate methods for time-to-event outcomes versus binary outcomes 5
Interpretation Errors
- Comparing NNTs across studies with different follow-up periods without adjustment 2
- For chronic conditions, use annualized NNT (ANNT) to account for varying study durations 2
- NNT values are specific to the intervention, population, and outcome studied—do not directly compare across different populations 2
- Ignoring that NNT must be balanced against Number Needed to Harm (NNH) 1, 3
Incomplete Reporting
- 72.5% of studies fail to report baseline risk 5
- 74.5% fail to report time horizon 5
- 62.7% fail to report confidence intervals 5
- Without these elements, NNTs are uninterpretable and potentially misleading 5
Balancing Benefit and Harm
- When NNH approaches or falls below NNT, the intervention may cause more harm than benefit 1
- The therapeutic index (NNH/NNT ratio) should exceed 2-3 for favorable risk-benefit profiles 1
- Example: Tamoxifen has NNT of 16 for breast cancer prevention versus NNH of 73 for endometrial cancer and NNH of 115 for venous thromboembolism 1