Validity of patient-reported health-related quality of life global ratings of change using structural equation modeling

Background Patient-perceived global ratings of change are often used as anchors of health-related quality of life (HRQoL) since they are easy for clinicians to interpret and incorporate the patient's perception of change as a means to capture clinical significance. Although this approach may be preferred, the validity of the anchor-based approach is currently under scrutiny. Objective To estimate the explained variation in single-item domain-specific global ratings of change (GRCs) that is accounted for by time 1 (T1) and time 2 (T2) domain-specific summary change scores from the Short-Form 36, V2 (SF-36) Health Survey in asthma primary care patients. Methods The baseline and first follow-up enrollment data to be evaluated in this investigation were part of a larger longitudinal HRQoL study conducted from August 2000-December 2002, in which the 356 asthma patients from Midwestern primary care facilities completed telephone interviews for every two consecutive months for a year on multiple HRQoL measures, including the SF-36 and domain-specific GRCs. A structural equation modeling technique was employed to ascertain the explained variability in patient-reported GRCs for each SF-36 domain that is accounted for by the summary change scores at the two time-points for four SF-36 domains (bodily pain, general health perception, mental health, and physical functioning). The model was estimated by the maximum likelihood method with the Satorra-Bentler correction for ordinal variables using equal threshold asymptotic covariance matrices. Results Multicollinearity between T1 and T2 latent constructs clouded interpretation of the standardized structural coefficients leading to GRCs. Correlations, however, revealed that all four domain-specific GRCs were more strongly related to T2- than T1-domain summary scores, indicating that patients were not equally relying on T1 and T2 to generate the GRCs. Furthermore, T1-domain summary scores were not of equal magnitude and opposite sign as compared to T2 scores. Conclusions In this study, there is insufficient evidence to establish SF-36 domain-specific GRC validity in asthma primary care patients. Therefore, it is recommended to reassess validity before using domain-specific SF-36 GRCs to classify clinically important change over time.