The Valuation of the International Prostate Symptom Score (IPSS) for Use in Economic Evaluations
Introduction
In clinical sciences, it is common to begin a clinical report with a description of the magnitude of the health problem to emphasise the relevance of the study objectives. Often, such remarks inadequately describe the magnitude of the problem, however, as in most cases the impact of heath problems on the quality of life of the patients is not based on validated measurement. For instance, in Lower Urinary Tract Symptoms (LUTS) suggestive of Benign Prostatic Obstruction (BPO), authors often refer to the high prevalence of the condition in older males, but the relative impact of LUTS suggestive of BPO on quality of life compared to other health problems is unknown.
Quality of life instruments used in economic evaluation are especially suitable to estimate the impact of a condition on quality of life, because their outcome is the value or “utility” people attach to health states. These utilities have a standardised range of 1.0 for perfect health and 0.0 for death, which allows for relative statements about the impact of quality of life. A simple multiplication of the utilities with the life years gives the number of “Quality-Adjusted Life Years” (QALYs). Because QALYs allow for comparisons and relative statements across very different conditions, interventions and diseases, they have become the preferred outcome in health economics [1].
Health state utilities are elicited in interviews with members of the general public using techniques such as “time trade-off” (TTO). Questionnaires such as the EQ-5D have been validated using TTO, so that outcome can be used directly to estimate QALYs. Nevertheless, the use of these instruments in urology is rare, probably because urologists do not expect these instruments to be very sensitive for the very specific urinary problems associated with LUTS [2]. If a disease-specific questionnaire like the International Prostate Symptom Score (IPSS) could be validated using TTO, urologists would have a sensitive tool for utility measurement and QALY-analysis in LUTS suggestive of BPO. Such measurement would allow us to describe the impact of LUTS suggestive of BPO on quality of life and facilitate a state-of-the-art economic evaluation of various treatment options. The purpose of this article is to describe how the IPSS, in a context of LUTS suggestive of BPO, was valued, and how these values should be interpreted.
Section snippets
Materials and methods
In this study, we valued the impact on quality of life of patients with LUTS suggestive of BPO, by valuing health states defined by the IPSS using TTO. In order to facilitate the administration of TTO, we reduced the number of health states defined by the IPSS using factor analysis.
Basically the study involved two steps: the first step was the reduction of the number of health states of the IPSS using factor analysis and merging of answer levels, and the second step contained the determination
Factor analysis
The factor analysis separated the seven symptom scores of the IPSS into two factors which could be interpreted as obstructive symptoms (incomplete emptying, intermittency, weak stream and hesitancy) and irritative symptoms (frequency, urgency and nocturia). Similar results were found in the studies of Welch et al. [10] and Yu et al. [11]. As Table 1 shows, the three types of factor analysis reflected the same correlation structure.
Following the factor analysis, the six response categories (from
Discussion
We determined values for the health states of the IPSS in a context of LUTS suggestive of BPO. The values for the IPSS health states in this context, as described by the questionnaire, were relatively high; they were located at the top of the utility scale, where 1.0 stands for the value of perfect health. Other conditions affect quality of life to a larger extent. For example, patients with moderate rheumatoid arthritis have a quality of life between 0.60 and 0.70, and patients with
Acknowledgements
This research effort was financed by an unrestricted grant from Yamanouchi Europe.
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