Cost-effectiveness and NNT

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Number Needed to Treat/Harm - The Core Formulas

  • Number Needed to Treat (NNT): Average number of patients who need to be treated to prevent one additional bad outcome.

    • Formula: $NNT = 1 / ARR$
    • A low NNT indicates a more effective intervention.
  • Number Needed to Harm (NNH): Average number of people who need to be exposed to a risk factor to cause one additional harmful event.

    • Formula: $NNH = 1 / ARI$
    • A high NNH indicates a safer intervention.
  • Core Calculations:

    • ARR (Absolute Risk Reduction): $CER - EER$
    • ARI (Absolute Risk Increase): $EER - CER$
    • CER: Control Event Rate
    • EER: Experimental Event Rate

⭐ An NNT of 1 means every single patient treated derives benefit. An NNT of 100 means that for every 100 patients treated, only one benefits.

2x2 Contingency Table for NNT Calculation

Cost-Effectiveness and NNT - The Economic Equation

  • Incremental Cost-Effectiveness Ratio (ICER): The additional cost for one unit of health outcome (e.g., per QALY gained) when comparing interventions.
  • Quality-Adjusted Life Year (QALY): A metric combining quantity and quality of life. 1 QALY = 1 year in perfect health.
  • The cost to prevent one adverse event is calculated as:
    • $Cost_{prevent} = NNT \times Cost_{treatment}$
FeatureDrug ADrug B
NNT1020
Cost/Dose$5$2
Cost to Prevent 1 Event$50$40

⭐ When comparing treatments, the one with the lower "Cost per event prevented" is more cost-effective, even if its NNT is higher. In the table, Drug B is superior economically.

Clinical Interpretation - Reading the Fine Print

  • Confidence Interval (CI) is Key: A wide CI suggests less precision. If the CI for an NNT crosses infinity or includes zero effect, the result is not statistically significant.
  • Context-Specific: NNT is not universally applicable. It is specific to the patient population and the study's timeframe.
  • Single-Outcome Limitation: NNT/NNH focuses only on one discrete outcome, ignoring potential side effects or ancillary benefits.
  • Baseline Risk Influence: In populations with very low baseline risk, the calculated NNT may be very high, making the intervention appear less impactful.

⭐ If the 95% CI for an NNT includes negative values (crossing into the NNH range), one cannot definitively conclude benefit or harm; the result is statistically insignificant.

High-Yield Points - ⚡ Biggest Takeaways

  • NNT (Number Needed to Treat) is the number of patients you must treat to prevent one additional adverse outcome.
  • A lower NNT indicates a more effective intervention.
  • NNH (Number Needed to Harm) is the number treated for one additional harmful outcome to occur.
  • A higher NNH signifies a safer treatment profile.
  • Cost-effectiveness weighs the total cost against health benefits (e.g., cost per QALY gained).
  • The cost to prevent one outcome = NNT × cost of treatment per patient.

Practice Questions: Cost-effectiveness and NNT

Test your understanding with these related questions

A 25-year-old man with a genetic disorder presents for genetic counseling because he is concerned about the risk that any children he has will have the same disease as himself. Specifically, since childhood he has had difficulty breathing requiring bronchodilators, inhaled corticosteroids, and chest physiotherapy. He has also had diarrhea and malabsorption requiring enzyme replacement therapy. If his wife comes from a population where 1 in 10,000 people are affected by this same disorder, which of the following best represents the likelihood a child would be affected as well?

1 of 5

Flashcards: Cost-effectiveness and NNT

1/9

The Mortality Rate measures the _____ due to a specific disease in a population at risk

TAP TO REVEAL ANSWER

The Mortality Rate measures the _____ due to a specific disease in a population at risk

speed of death

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