Quality-Adjusted Life Years (QALYs)

Calculation Methods

A QALY can be calculated in several ways using various methods. The quality of life can be measured using preference scales to implicitly rate the quality of health experienced by either individuals or the public in general. These tools can be either based on general attitudes or disease specific. Preference scales commonly used are the Visual Analog Scales (VASs), or feeling thermometers, the Standard Gamble (SG), and the Time Trade Off (TTO) preferences. The use of preference scales allows for the measurement of the quality of health from the perspective of the individuals toward whom the health system is directed.

The VASs use number or category rating scales, marked or unmarked line scales, or combinations of either. While the scales vary, the final measure is transformed into a scale of 0 to 1, where 0 is dead and 1 is perfect health. The individual is presented with two choices: One is treatment, which may result in a chronic health state leading to either a better state or immediate death; the other is no treatment, therefore remaining in a chronic health state leading to death. The assumption is that the life years are longer in the treatment state. This tool could also be used with temporary health states that do not lead to death. One such scale is the Health Related Quality of Life Scale (HQRL). The HQRL uses a vertical scale, analogous to a thermometer, from 0 to 1, 10 divisions between each integer, with 0 being dead and 1 being perfect health. The subjects are asked to indicate where on the scale they feel the quality of their health lies. Another published scale using preference scores is the Quality of Well-Being Scale (QWB). The preference scores can be plotted on the vertical axis of a graph against the time spent in each score or health state with time on the horizontal axis. Integrating the area under the plotted curve is a measure of the total QALY.

The SG method measures preferences for chronic states by presenting the subject with a choice between treatment, leading to either a healthy state or death, and no treatment, resulting in a continued chronic state until death, much like the VASs. However, the time in each state, if listed [Page 986]as a probability, can be altered to determine the subject's preference. SG techniques are offered by direct interview, paper, or computerized questionnaires. Any of these can be enhanced with visual aides. Variations for temporary health states can accommodate conditions not having a fatal outcome. Examples of tools incorporating SG are the Short-Form-6D and the Health Utility Index (HUI), both of which were generated from general public preferences.

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