Background: When studying health-related quality of life (QOL), disease-specific instruments have the advantage of measuring the unique effects of particular medical conditions. Almost every disease-specific QOL instrument uses its own metric, and measures QOL in its own content areas. The unfortunate result is that scores from different disease-specific QOL instruments cannot be compared. In contrast, the seven-item Quality of Life Disease Impact Scale (QDIS-7) has response choices on only one scale (one metric) and its content is standardized. Thus, the QDIS-7 should allow disease-specific QOL to be compared across different diseases. We therefore tested whether, unlike scores from the traditional mutually-incompatible metrics, those from the single-metric QDIS-7 are comparable across diseases. Methods: Responses to the QDIS-7 questions (regarding global QOL, physical functioning, role functioning, social functioning, vitality, mental health, and health outlook) were used to compute a single score, based on an item-response model. When the QDIS-7 was completed by respondents with different diseases, the content of the question-items was the same, and the only difference was the name of the disease to which the respondents explicitly attributed any impact on their QOL. In an online survey, 2,627 adults who had sought care for headache, low-back pain, asthma, or diabetes, each responded to the QDIS-7 and to a previously-validated disease-specific QOL instrument ("legacy scale") that was developed to measure QOL in their specific disease. We examined the slopes from four regressions of legacy-scale scores on QDIS-7 scores. Similarity of those slopes would support the hypothesis that the QDIS-7 enables quantitative comparisons of disease-specific QOL across those four different medical conditions. Results: For all four groups, the regression-line slopes were nearly the same: 0.12 to 0.14 legacy-scale standard deviations per 1-point difference in QDIS-7 score. Thus, each 10-point difference in QDIS-7 scores is equal to slightly more than one standard-deviation difference in legacy-scale scores, for all four groups. Conclusions: The relationships of score differences on the legacy measures to score differences on the QDIS-7 (i.e., the slopes) were similar across the four groups, which is consistent with the idea that the QDIS-7 enables comparisons of disease-specific QOL across different medical conditions.

The authors have declared no competing interest.

Funding for parts of this study came from the Ministry of Education, Culture, Sports, Science and Technology of Japan, through the Japan Society for the Promotion of Science.

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On August 21, 2020, the plan for this study (201611-3) was approved by the Institutional Review Board of the the Institute for Health Outcomes and Process Evaluation Research (iHope: <http://www.i-hope.jp/>).

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