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Comparative Study
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Problems and Solutions in Calculating Quality-Adjusted Life Years (QALYs)

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Comparative Study

Problems and Solutions in Calculating Quality-Adjusted Life Years (QALYs)

Luis Prieto et al. Health Qual Life Outcomes.

Abstract

The quality-adjusted life-year (QALY) is a measure of the value of health outcomes. Since health is a function of length of life and quality of life, the QALY was developed as an attempt to combine the value of these attributes into a single index number. The QALY calculation is simple: the change in utility value induced by the treatment is multiplied by the duration of the treatment effect to provide the number of QALYs gained. QALYs can then be incorporated with medical costs to arrive at a final common denominator of cost/QALY. This parameter can be used to compare the cost-effectiveness of any treatment. Nevertheless, QALYs have been criticised on technical and ethical grounds. A salient problem relies on the numerical nature of its constituent parts. The appropriateness of the QALY arithmetical operation is compromised by the essence of the utility scale: while life-years are expressed in a ratio scale with a true zero, the utility is an interval scale where 0 is an arbitrary value for death. In order to be able to obtain coherent results, both scales would have to be expressed in the same units of measurement. The different nature of these two factors jeopardises the meaning and interpretation of QALYs. A simple general linear transformation of the utility scale suffices to demonstrate that the results of the multiplication are not invariant. Mathematically, the solution to these limitations happens through an alternative calculation of QALYs by means of operations with complex numbers rooted in the well known Pythagorean theorem. Through a series of examples, the new calculation arithmetic is introduced and discussed.

Figures

Figure 1
Figure 1
QALYs pictured as rectangular areas.
Figure 2
Figure 2
QALYs calculated following the multiplicative model.
Figure 3
Figure 3
QALYs calculated following the multiplicative model after linearly transforming the Utility scale (Y') as Y' = 2Y+1.
Figure 4
Figure 4
The Cartesian Plane and the graphical representation of a Complex Number (P).
Figure 5
Figure 5
QALYs calculated following the Complex Number model.
Figure 6
Figure 6
QALYs calculated following the Complex Number model after linearly transforming the Utility scale (Y') as Y' = 2Y+1.
Figure 7
Figure 7
Functions relating Utility and QALY values in the Complex Number and Multiplicative Models.

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