Introduction: As the five dimensions of the EQ-5D might be insensitive to some populations or conditions, adding one or more specific 'bolt-on dimensions', to the EQ-5D may improve its validity and relevance for those groups. Various bolt-on dimensions have been identified and tested the effect on the psychometric performance and utility measurement. Therefore, this systematic review was to review all bolt-on dimensions and report the results of those two effects. Areas covered: Twelve studies were identified through PubMed, Scopus, and Web of Sciences from inception to January 2019. The bolt-on dimensions generally improved the EQ-5D's psychometric performance when compared with the standard version. However, evidence is lacking about the impact of bolt-on dimensions on responsiveness, reliability and known-groups validity. Evidence was also mixed for adding a sleep dimension, while interpersonal relationships did not promote a significant improvement on utility measurement. Expert opinion: Adding bolt-on dimensions could enhance the discriminative power and utility measurement of the EQ-5D; however, careful investigation of the sleep and interpersonal relationships is required since the evidence did not support a significant improvement. Further investigation of the identified bolt-on dimensions on all of the psychometric performance and the incremental utility scores derived from TTO is greatly encouraged.
Keywords: EQ-5D; bolt-on; health-related quality of life; psychometric performance; utility measurement.