You will often see the NNT mentioned in clinical guidelines; and when different health interventions are being prioritised or assessed for their clinical effectiveness and cost effectiveness. For example, the NNT was used to inform decisions to recommend statins for people with an elevated risk of cardiovascular disease.
The NNT is a measure used to quantify the effectiveness of an intervention or treatment. It is the average number of patients who need to be treated with a particular therapy for one additional patient to benefit.
How is NNT calculated?
In mathematical terms, the NNT = 1/[Absolute Risk Reduction]
Where Absolute Risk Reduction (ARR) = Control Event Rate (CER) – Experimental Event Rate (EER)
Control Event Rate (CER): The rate of an outcome in a control group.
Experimental Event Rate (EER): The rate of an outcome in an experimental group treated with the intervention.
For example, consider a drug that reduces the risk of heart attack from 4% to 2%. The ARR is 2% or 0.02 and the NNT is 50 (1/0.02). Hence, on average, 50 people will need to be treated to prevent one heart attack.
Importance in Clinical Medicine
The NNT is important in clinical medicine because it helps in the evaluation of the efficacy of treatments by offering a direct, patient-centred measure. It is also helpful in clinical decision making as it allows doctors and patients to make makes evidence-based decisions on treatment options. For example, when presented with data on the NNT, patients can consider how useful a medical intervention is for them.
The NNT also helps in the assessment of the balance between potential benefits and harms of treatment; and provides a uniform metric for comparing the effectiveness of different treatments.
Role of NNT in Public Health
The NNT is also important in public health because it provides a metric that can be used at a population level, offering insights into public health strategies; for example, it can help policy makers determine the most efficient use of healthcare resources. When combined with other metrics, the NNT can be a tool in assessing the cost-effectiveness of public health interventions such as preventive measures, screening and vaccination.
For example, the NNT was used by the UK JCVI to decide which population groups should be prioritised for booster Covid-19 vaccinations by considering how many people in different age groups would need to be vaccinated to prevent one hospital admission.
Limitations of NNT
The NNT does have some limitations. For example, it does not account for side effects or adverse reactions to medical interventions. It is also specific to the particular patient populations and settings from which the data to calculate the NNT was derived. For example, many adverse health outcomes are more common in older people. Hence, the NNT is not uniform over the population and will be lower in groups at higher risk such as the elderly.
Understanding NNT is crucial for both individual clinical decisions and broader public health strategies aimed at population health improvement. It provides an intuitive way to understand the practical impacts of treatment and public health interventions; and is a measure that is useful to many groups including policy makers, clinicians, public health specialists and patients.