Quality of life: Definition and assessment.

Patients, clinicians, and policy makers recognize the importance of assessing health and quality of life to inform clinical and policy decisions. Demand is thus increasing for measures available for use in clinical trials and clinical practice. Health status, functional status, and quality of life are three concepts often used interchangeably. The conceptual basis of quality of life is focused on the individual perception of his or her position in life in the context of personal goals and standards and the culture in which the person lives. Health-related quality of life can be defined as the value assigned to duration of life (how long we live) as modified by the symptoms, functional states, perceptions and opportunities influenced by disease and treatment. Clinicians focus on health-related quality of life (HRQOL) although when a person is ill or diseased, almost all aspects of life cart be health-related. Self- or interviewer-administered questionnaires can be used to measure cross-sectional differences in HRQOL between patients at a point in time (discriminative) or longitudinal changes within patients over time (evaluative instruments). Both discriminative and evaluative measures must be valid (measuring what they are supposed to measure) and have a high ratio of signal to noise (reliability and responsiveness, respectively). Reliable reproducible discriminative instruments are able to differentiate between persons. Responsive evaluative measures are able to detect important changes over time, even if those changes are small. Measures must also be interpretable in that clinicians and policy makers must be able to identify differences that are small, moderate, or large. Two basic approaches to measurement are available: generic instruments that provide summary scores and specific instruments that focus on problems associated with a single condition, patient group, or area of function. Both health profiles and instruments that generate health utilities for cost-effectiveness analyses are usual. Each approach has its strengths and weaknesses and must be selected on the basis of the purpose of assessment. Achievement of cross-cultural comparability requires attention to the content of the questionnaire and its conceptual basis, the method of translation, and the testing of the measurement properties defined above. In addition to these methodological considerations, practical considerations include communication difficulties among investigators and expense. A high level of cooperation and coordination is necessary between centers located in different countries. Investigations on health-related quality of life have led to instruments suitable for detecting minimally important effects in clinical trials, for measuring the health of populations, and for providing information for policy decisions. With an increasing number of measures available for use, criteria for selection will be based on empirical evidence, ease of use, and sociopolitical considerations, including acceptability to patients, providers, and policy makers.