Knowing NNT

Summary

In this episode of Sketchy EBM, host Anthony Croco explains the concept of Number Needed to Treat (NNT), which quantifies how many patients must be treated to achieve one positive outcome. He illustrates the calculation of NNT through an example comparing control and treatment groups, demonstrating the significance of Absolute Risk Reduction. Through fictitious studies with varying NNT values, he highlights the complexity of evaluating treatment benefits versus risks and costs. Ultimately, Croco emphasizes that a 'good' NNT depends on context, making it essential for healthcare providers to critically assess treatment options.

Takeaways

• 😀 NNT (Number Needed to Treat) represents the number of patients that must receive a specific treatment for one patient to benefit.
• 📊 The calculation of NNT is based on the absolute risk reduction, which is the difference in event rates between the control and treatment groups.
• 🔍 A simple example shows that if the control group has a 67% bad outcome and the treatment group has a 33% bad outcome, the ARR is 33%.
• 🧮 The formula for NNT is 1 divided by the absolute risk reduction (NNT = 1/ARR).
• 💡 An NNT of 3 means that for every three patients treated, one will experience a positive outcome.
• ⚠️ A lower NNT is generally preferred, indicating a more effective treatment with fewer patients needing to be treated for one to benefit.
• 🤔 However, the interpretation of NNT depends on the context, including the severity of benefits and potential risks.
• 💸 The cost of treatment is a crucial factor in evaluating NNT; expensive treatments may have a high NNT but offer limited benefit.
• 😷 Even a high NNT can be acceptable if the treatment is free and has no adverse effects, as demonstrated in the example of preventing mortality.
• 🔗 The ultimate assessment of what constitutes a 'good' NNT depends on evaluating the benefits, harms, and costs of the treatment.

Q & A

What is the Number Needed to Treat (NNT)?

The Number Needed to Treat (NNT) is the number of patients that need to be exposed to a specific intervention for one patient to experience a positive effect.

How is NNT calculated?

NNT is calculated using the formula: NNT = 1 / Absolute Risk Reduction (ARR), where ARR is the difference between the event rate in the control group and the event rate in the treatment group.

What does Absolute Risk Reduction (ARR) represent?

Absolute Risk Reduction (ARR) represents the difference in the event rate of a specific outcome between the control group and the treatment group.

Can you provide an example of how to calculate NNT?

In a study with a control group event rate of 2/3 (bad outcome) and a treatment group event rate of 1/3 (bad outcome), the ARR would be 1/3 (or 33%). Thus, NNT would be calculated as 1 / 0.33, resulting in an NNT of 3.

What is the significance of a lower NNT?

A lower NNT is generally considered better because it indicates that fewer patients need to be treated for one to benefit from the intervention. However, context and outcomes also matter.

What are some examples of different NNT values discussed in the transcript?

Three fictitious studies are provided: NNT of 5 with a benefit of one less flatulence per day (but a 50% risk of fatal MI), NNT of 50 with no adverse effects but costing $1 million per dose to prevent one hospital admission, and NNT of 50,000 with a free therapy that reduces mortality.

What should be considered when evaluating an NNT value?

When evaluating an NNT value, one should consider the treatment's benefits, potential harms, and associated costs to determine if it is justifiable.

What does an NNT of 50 indicate in terms of hospital admissions?

An NNT of 50 indicates that for every 50 patients treated, one patient benefits by not having to be admitted to the hospital, but the treatment is very expensive at $1 million per dose.

Why is the NNT of 50,000 considered justifiable despite being high?

The NNT of 50,000 is considered justifiable because it provides a significant benefit (one less mortality) without adverse effects and is free of cost, suggesting a strong public health value.

What overall message does the host convey about NNT?

The host emphasizes that the value of NNT is not just in the number itself but depends on the context, including treatment benefits, risks, and costs.