The quality adjusted life year (QUALY) is a measure of the effect of a health care intervention like lung cancer screening that combines both the length in years and the quality of the resulting health state 1,2. The number of QALYs resulting from an intervention is calculated using a simple formula:
number of QALYS = number of years of survival x quality factor (utility)
Although the formula is simple, determining the values of the 2 variables is difficult, especially the quality factor. Several different approaches are available for determining the quality of a health state, but the range is always from 0 (dead) to 1 (perfect health). A year spent in perfect health is equal to one QALY, while a year spent in a health state with a utility of 0.5 is equal to 0.5 QALYs.
QALYs are often used in cost-effectiveness analysis. For example, data from the NLST trial were used to estimate the average life expectancy for patients with lung cancer screened with CT and Radiography 3. The number of life years in the CT and radiography groups was multiplied by a quality factor (0.76 for men and 0.74 for women using a health assessment survey) to determine the number of QALYs in each group. Trial subjects with cancer in the CT group had an average of 6.0521 QALYs compared to 4.8981 in the radiography group.
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