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The QALYs concept is simplicity itself, and can readily be grasped by taking a look at Figure 2. On this figure, the horizontal axis indicates the life years, while the vertical axis is QOL. QOL is represented as a number between 0 (death) to 1 (perfect health), known as a utility. Let us consider the change in the longevity and utility values for two different patients.
Figure 2 Quality-Adjusted Life Years (QALYs)
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Patient A was initially in perfect health (utility = 1), but was afflicted by a disease in the tenth year, whereupon their utility value fell to 0.8. After spending a ten years in this state, Patient A was further afflicted with another disease in the twentieth year (from the start of assessment), causing the utility value to fall to 0.5. The utility value slipped to 0.2 ten years later, and the patient died ten years after that.
Patient B, by contrast, was not in very good health from the start, and had a utility value of 0.5. However, the utility value remained at this level for the succeeding 40 years, at the end of which Patient B died.
At the outset, the two patients both had 40 years left to live, but clearly differed from each other in respect of the quality experienced during these 40 life years. Because it does not incorporate a mechanism for taking into account any change in QOL, a survival analysis for the long-term prognosis would unfortunately not draw any distinction between these two patients. QALYs was devised to remedy this shortcoming. The thinking behind it is very simple; it rests merely on weighting the life years by the utility (QOL).
In the case of Patient A, for example, QALYs would be calculated as follows.
| * Patient A's QALYs = [10 years · 1.0] + [10 years · 0.8] + [10 years · 0.5] + [10 years · 0.2] = 25 QALYs |
The same calculation for Patient B would be as follows.
| * Patient B's QALYs = [40 years · 0.5] = 20 QALYs |
QALYs has enabled a clear distinction to be drawn between two patients who are not distinguished by mere survival analysis.
Although almost all diseases have an influence on both life expectancy and QOL, clinical trials have conventionally considered only one of these factors. Even where they took both into account, they generally evaluated each separately instead of integrally. While this approach may have been sufficient for clinical evaluations, objective assessment of pharmaceutical value requires assessment based on QALYs. |
