Figure 1 shows one attempt to depict the relationship between health concepts and general quality of life, and how determinants from the internal (individual) as well as the external (social and cultural) environment influence the general quality of life.
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In this figure, health status and health-related quality of life concepts include symptoms, functional status, perceptions, and opportunity (Patrick and Erickson, 1993).
In Figure 1, these are depicted as following a linear progression from the most proximal to the individual, symptoms, to the most distal, opportunity. In reality, no such linear progression exists and symptoms may be translated directly to perceptions or opportunity without affecting function. Most important to the figure is the depiction of the determinants intrinsic to the individual and outside the individual indicating that what people report cannot be separated from the personal or sociocultural environment.
Researchers tend to approach the relationship inductively by collecting data and examining the correlations, but a priori hypotheses are important for developing systematic knowledge of how disease and treatment impacts different indicators of health outcome. The most appropriate approach to causal modeling, the use of health outcomes in meta-analyses, development and application of community-level indicators of health, and interpretation of observed health and quality-of-life measurements remain challenges for both the developers and users of these measures.
PRO terminology permits primary identification that this information comes directly from the patient and avoids confusion in using one or more concepts as an over-arching term with little specification. Some concepts can be measured from both a patient and an observer perspective, e.g., physical function. Others can only be patient-reported, e.g., pain. If a conclusion about pain is based on a non-PRO measure, it would be important to know. The term PRO makes it more likely that use of a PRO is considered in a clinical trial. Perhaps it is reasonable to confirm or add value to an objective or observed finding with a PRO. PROs in a daily diary format may capture daily variations in symptoms or function that can elucidate the mechanism of treatment effects.
Using PRO terminology requires that the concepts be identified. When using this organizing concept, investigators need to define and label the content of a specific PRO. Operational definitions of each construct or concept in use are necessary and recommended, including specification of the theoretical basis for the concept and how the concept was translated into a measurement, i.e. the mathematical definition of assigning a number to a response.