Quality-adjusted life years (QALYs)
- Measure
They combine how long people live and how well. A year in full health is worth 1 QALY; a year in poor health is worth less.
What it is
The QALY multiplies years of life by a utility weight from 0 (death) to 1 (full health). It lets interventions that lengthen life be compared with ones that improve it. In economic evaluation, the incremental cost-effectiveness ratio divides the extra cost of one option by the extra QALYs it produces.
Formula and reading
- Total QALYs = Σ (years in each state × utility of that state).
- ICER = difference in costs / difference in QALYs; read as the cost of each extra QALY gained.
- If one option costs less and gives more QALYs, it dominates the other and no ratio is needed.
- Some very severe states can be valued below 0 (worse than death) in some methods.
Worked example
- A made-up chronic condition has two approaches. A: 10 years at utility 0.6, costing 30 million pesos per patient.
- B: the same 10 years, at utility 0.8, costing 50 million pesos.
- QALYs for A: 10 × 0.6 = 6.0. QALYs for B: 10 × 0.8 = 8.0. Gain with B: 2.0 QALYs.
- Extra cost of B: 50 − 30 = 20 million pesos.
- ICER: 20 million / 2.0 QALYs = 10 million pesos per QALY gained.
- If B also extended life to 12 years at utility 0.8: 12 × 0.8 = 9.6 QALYs; gain 3.6 and ICER = 20 / 3.6 ≈ 5.6 million per QALY.
- Reading: B gives more health at an extra cost; whether it is adopted depends on the country's threshold and budget.
QALYs versus DALYs
| Feature | QALY | DALY |
|---|---|---|
| Question | How much healthy life does an intervention gain? | How much healthy life does a population lose to a disease? |
| Weight scale | Utility: 1 = full health, 0 = death | Disability: 0 = full health, 1 = equivalent to death |
| Direction | More is better | More is worse |
| Typical use | Economic evaluation of technologies | Burden of disease and priorities |
Markers
- QALY
- Utility
- ICER
- Cost-utility
How to read it
- Health technology assessment agencies use QALYs and cost-effectiveness ratios to recommend what to fund with public money.
- A trial that improves symptoms without changing survival can show a benefit in QALYs but not in mortality.
Common pitfalls
- Dividing total cost by total QALYs of a single option: the decision rests on the incremental ratio.
- Confusing the utility scale (1 is good) with the disability scale (1 is bad).
- Forgetting that utilities depend on who values them (patients or the general public).
Test yourself
Think of the answer, then tap the question to see it.
How many QALYs are 4 years at utility 0.75?
4 × 0.75 = 3 QALYs.
How is the incremental cost-effectiveness ratio calculated?
By dividing the difference in costs by the difference in QALYs between the two options being compared.
Option B costs 12 million more and gives 3 more QALYs than A. What is the ICER?
12 / 3 = 4 million per QALY gained.
What does it mean when one option dominates another?
It costs less and produces more QALYs; it is preferred without needing to calculate the ratio.
Connections
Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources.
Sources: Chapter 20: Economic evidence (Cochrane, 2024); Making Choices in Health: WHO Guide To Cost-Effectiveness Analysis (World Health Organization, 2003).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.