Longitudinal Psychometric Evaluation of 15 Bolt-Ons for the EQ-5D-5L in a Large-Scale Multicountry Study

OBJECTIVES: Recently, EuroQol introduced the EuroQol 5-Dimension (EQ-5D) Bolt-on Toolbox with experimental status. Psychometric evidence gaps remain for candidate dimensions, particularly regarding test-retest reliability and responsiveness. This study aimed to evaluate the longitudinal psychometric performance of 15 bolt-ons alongside the EuroQol 5-Dimension 5-Level (EQ-5D-5L). METHODS: We used data from the POPCORN project, a large, general population study. Participants from 6 countries completed an online survey in April to May 2021 (n = 8088) and 2022 (n = 4254), with a subset (n = 221) completing a third wave. The survey included bolt-ons measuring constructs relevant to the Toolbox (tiredness, sleep, cognition, and breathing) and additional bolt-ons such as social relationships, coughing, and smell/taste. Psychometric analyses assessed ceiling, validity (structural, known-group, and explanatory power), test-retest reliability, and responsiveness. RESULTS: Most bolt-ons loaded onto different factors than EQ-5D-5L dimensions. In the total general sample, the EQ-5D-5L ceiling (31.5%) was reduced most by tiredness (22.9%) or sleep (25.0%). In chronic condition subsamples, adding relevant bolt-ons-such as breathing for asthma, chronic bronchitis, and lung emphysema (relative efficiency = 1.38-1.54)-improved known-group validity. Tiredness increased explanatory power for EQ VAS by 3.5% in the total sample and contributed most explanatory power in 10 of 14 chronic conditions. Most bolt-ons showed excellent test-retest reliability when added to EQ-5D-5L (intraclass correlation coefficients = 0.897-0.924) and enhanced responsiveness in respondents reporting health deterioration, whereas only tiredness, sleep, cognition, and smell/taste improved responsiveness in those reporting improvement. CONCLUSIONS: Our findings provide new evidence on test-retest reliability and responsiveness of multiple bolt-ons, highlighting tiredness and sleep as strong bolt-ons across populations, and breathing and cognition as valuable in relevant chronic conditions.