Does the correspondence between EQ-5D health state description and VAS score vary by medical condition?

Background: The EQ-5D health-related quality of life instrument comprises a health state classification (health problems by severity in five domains), followed by an evaluation using a visual analogue scale (VAS). Despite the EQ-5D's use in health technology assessment and as a patient-reported outcome measure (PROM), the correspondence between the two parts of the instrument remains ill-understood. In this paper, we consider whether the association between health state classification and VAS score might vary by medical condition. Methods: EQ-5D data collected for studies of patients in four different clinical conditions or circumstances (stroke, low back pain, colposcopic investigation or cytological surveillance) were pooled to generate a sample of 3,851 patient records. VAS scores were regressed on reported problem severities, with the inclusion of intercept and slope dummy variables specific to condition. Results: The regression model achieved a goodness-of-fit of 0.54. Given its structure and the significance of the coefficients, the proportion of VAS scores which differed by condition for the same health state varied between 33.3 and 88.5 per cent of possible states. Conclusions: Many of the patients with different medical conditions or in receipt of different interventions recorded different VAS valuations, in spite of ostensibly being in the same EQ-5D-defined health states. By implication, it is probable that the same state-to-state change would by valued differently by patients experiencing different conditions.