Severity of illness versus severity of disease : definitions and applications

Sammanfattning: Outcome assessment should consist of two basic steps - "selection of a meaningful outcome" and "determination of what caused the outcome". The latter step requires comparisons, and therefore information about the initial status of the patients. Severity ratings attempt to group patients with similar initial status so that outcome comparisons are meaningful. They are consequently an essential ingredient in outcome assessment.This thesis aims at expanding the knowledge about outcome measurement focusing on the severity of a case with regard to definitions and applications of severity measures. Further, theoretical knowledge is integrated with practical needs for outcome assessment and methodology is developed for modelling the use of resources and interpreting the association between different severity measures.As with illness and disease, severity of illness and severity of disease are interchangeable terms in medical literature. Based on a holistic theory of health, these terms were differentiated. Severity of a case was described by two basic components - severity of disease and severity of illness. Essentially, severity of illness concerns suffering and disability, and severity of disease concerns risk of death, risk of suffering and risk of disability. These definitions were the core of a set of criteria for classifying severity measures or outcome measures.The role of a disease severity score in explaining resources use (e.g., intensity of technology utilization and length of hospital stay) in a neonatal intensive care unit was investigated. A disease severity score based on the death risk of the initial diagnosis was found to be an indicator of intensity of therapeutic technology together with gestational age and Apgar score. However, it does not show statistical significance to explain length of stay, which was explained by intensity of diagnostic technology and birthweight.The demand for patient classification systems (standard systems to describe a patient's condition) on a national and local level was investigated. There is little experience in the use of these systems, but at the same time there is a demand from the nurse managers for standard measures of a patient's need for care.There is still little knowledge about the association between severity of illness and severity of disease. This association was studied for patients with ulcerative colitis using data analysis techniques for dimension reduction such as cluster analysis, homogeneity analysis and non linear principal components analysis. A non linear relationship between illness and disease severity measures was found. The overall clinical judgement of disease activity was more related to illness patterns than symptoms or endoscopy and laboratory findings. In addition, the two measures of illness severity (Sickness Impact Profile and Rating Form for Inflammatory Bowel Disease Patient Concerns) were found to represent complementary information about a patient's illness experience. This illustrates the importance of conceptual knowledge in the selection process of an illness measure for outcome evaluations. These findings also reinforce the need for the development of systems to support interpretation of these kinds of complex associations, as traditional statistics have been shown to be of limited value under these circumstances.