What is a QALY and how to calculate it?

The Calculation

To calculate QALYs, you multiply the years of life by the quality of life during those years. Quality of life is expressed as a utility value between 0 and 1: 1 means perfect health and 0 means death.
 

Years of life × utility value = QALYs

Three simple examples:

• One year in perfect health gives 1 QALY: 1 × 1 = 1.

• Half a year in perfect health gives 0.5 QALYs: 0.5 × 1 = 0.5.

• One year at half of perfect health, a utility of 0.5, also gives 0.5 QALYs: 1 × 0.5 = 0.5.
   

Comparing a new medicine with the current one

Cost-effectiveness studies, also called health economic evaluations, use QALYs to measure the health effect of a new medicine compared with the current one. The current standard of care is the starting point, and only the extra QALYs from the new medicine are counted.
 

Example

A patient lives 3 years with a disease. With the current standard of care, their utility is 0.7. That gives 3 × 0.7 = 2.1 QALYs.

Medicine A improves quality of life. The patient’s utility rises from 0.7 to 0.9, so they now have 3 × 0.9 = 2.7 QALYs. Medicine A adds 0.6 QALYs: 3 years × 0.2 extra utility.

Medicine B extends life. The patient lives 2 years longer, at a utility of 0.7. Medicine B adds 2 × 0.7 = 1.4 QALYs. 

Why the QALY is so useful

Quality of life is measured on the same scale for every disease. This is what makes the QALY so powerful: you can compare the health gain of a new cancer therapy with that of a new antidepressant, or of any other medical treatment.
 

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