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.