A multicenter longitudinal study on development and validation of the quality of life scale QLICD-DM (V2.0) for diabetes mellitus based on classical test theory and generalizability theory

PURPOSE: Quality of life (QOL) research in diabetes has long been a concern with many specific instruments being developed, but no scale for diabetes has been developed based on the modular approach. This study aimed to develop and validate the Diabetes Mellitus Scale of the System of Quality of Life Instruments for Chronic Diseases QLICD-DM (V2.0) by a modular approach and mixed methods. METHODS: The Scale was developed based on procedural decision-making methods and by combining the general module and the specific module. The instrument was used to assess the quality of life of 242 diabetic patients both before and after treatments. Under Classical Test Theory (CTT), the psychometric properties of the scale were assessed with regard to validity, reliability and responsiveness by correlation analyses, structural equation modeling, as well as t-tests. In addition, G-study and D-study in Generalizability Theory (GT) were used to validate the scale further. RESULTS: The QLICD-DM (V2.0) was developed with a 14-item specific module and a 28-item general module in the final scale. For all domains, Cronbach's α values were greater than 0.70 with the exception of physical function (0.68), the test-retest reliability correlations r and ICCs were greater than 0.80 with the exception of physical and social function (0.65,0.64). The theoretical construct was supported by correlation analyses and confirmatory factor analysis using structural equation modeling, which demonstrated good construct validity. There were significant differences (P < 0.05) in the domains of physical function, specific module and the total scale before and after treatments. The standardized response means (SRMs) of the physical, psychological, social, and the specific module were 0.36, 0.14, 0.11 and 0.28 respectively. All G-coefficients were all greater than 0.70 with the exception of the physical domain (0.683), further confirming the reliability of the scale further. The overall error was found to be small in the G-study and in the D-study, indicating a high level of accuracy. CONCLUSION: The QLICD-DM (V2.0) not only integrated characteristics of generic and disease-specific instruments but also demonstrated good reliability, validity, and moderate responsiveness, and it could serve as a quality of life assessment tool for diabetes patients.