Théorie de l'utilité et mesure de la qualité de vie en économie de la santé.

Abstract

En économie, les individus sont supposés avoir le choix entre des actions dont les conséquences sont évaluées par leur utilité. Celle-ci est assimilable au niveau de satisfaction que procure le fait d’en disposer. Lorsque les conséquences sont incertaines, l’individu est « rationnel » s’il choisit l’action dont l’utilité espérée est la plus grande. Transposée aux choix en santé, une décision sera rationnelle si elle résulte d’un calcul analogue. Si les conséquences ne peuvent être réduites en un indicateur unique et physique, l’approche coût-utilité, qui tend à devenir la méthode de référence, consiste à les mesurer par un indicateur, le QALY (quality adjusted life years) qui pondère le temps passé dans un état de santé par un coefficient rendant compte de sa qualité. Certains économistes ont proposé d’intégrer le QALY dans la définition des priorités d’un système de santé, d’autres de l’utiliser pour accepter la diffusion d’une procédure médicale. Toutes ces propositions ont soulevé de nombreuses objections. Les économistes favorables au QALY considèrent cependant qu’il éclaire les décisions d’allocation des ressources et entraîne la réflexion sur la place de l’usager dans le système de soins.

In this paper, the issue of quality of life measurement in economic evaluation of health care is presented and discussed. In the first section of the article, methods for the economic evaluation are briefly presented within the theoretical framework of expected utility theory. In the second section, a number of situations are pointed out where health-related quality of life is an important outcome or both morbidity and mortality are affected. These types of situation give the rationale for the development of a new method, called cost-utility analysis. It enables a broad range of relevant outcomes to be combined into a single composite summary outcome like Quality Adjusted Life Years (QALYs) gained. The advantage of QALY as a measure of health outcome is that it simultaneously capture gains from reduced morbidity (quality gains) and reduced mortality (quantity gains) and integrate them in a single indicator. The combination is based on the relative desirability of the different outcomes from the individual' points of view. In the conventionnal approach to QALYs, the quality adjustment weight for each health state is multiplied by the time in the state and then summed to calculate the number of quality adjusted life-years. In a particular study, cost-utility ratios of concurrent options will be compared and an incremental analysis of costs and consequences of the alternatives will be performed. In the first years of utilisation, cost-utility analysis were conducted for specific interventions, especially innovating ones. More recently, some economists advocate that cost-utility ratios could be used to inform decisions about the allocation of health care resources between alternatives programs and to aid for determining priorities of the health care system. For such purposes, health care interventions should be compared and ranked in terms of their relative cost per QALY gained. Some economists argued that adoption and utilization of new technologies should be classified into different grades of recommendation based on their incremental cost per QALY. All these proposals have been strongly criticised. Methodological objections have been addressed. The consistency of the cost per QALY approach with welfare economic theory has also been questioned. It has been proved that QALYs can be utilities if very restrictive conditions are respected, which is very uncommon in practice. These criticisms are summarised in the last section of the paper. In conclusion, the cost-utility is an approach to be used with caution. Nevertheless, its utilisation can be necessary to inform decisions as it gives the unique opportunity to take into account individual preferences when valuing outcomes of health care interventions.