Adding a fatigue item to the EQ-5D-5L improves its psychometric performance in the general population

Background Fatigue is a common and often disturbing sequela of serious chronic health conditions. In the widely applied HRQL instrument, the EQ-5D, this aspect is not included directly, for its assumed lack of additional information. We investigated the validity of this assumption by determining the gain-if any-of an additional fatigue item to the EQ-5D-5L in a general population sample. Methods A Dutch general population sample (including diseased people) completed a web-based survey including the EQ-5D-5L and the Rivermead Post-Concussion Symptoms Questionnaire (RPQ). The RPQ fatigue item was used to create the EQ-5D-5L + Fatigue. We head-to-head compared the psychometric performance contrasting the EQ-5D-5L and EQ-5D-5L + Fatigue: distribution (e.g. ceiling), informativity cf. Shannon's indices, convergent validity, domain dependency, and explanatory power. Results were compared between subgroups with and without >= 1 chronic health condition. Results The study population consisted of 3027 persons of whom 52% had a chronic health condition. The mean EQ-5D-5L utility score was 0.83 and 48% experienced some degree of fatigue. Adding the fatigue item to the EQ-5D-5L decreased the ceiling effect, increased absolute informativity (HMODIFIER LETTER PRIME = 6.44 vs. HMODIFIER LETTER PRIME = 4.90) and relative informativity (JMODIFIER LETTER PRIME = 0.46 vs. JMODIFIER LETTER PRIME = 0.42). The extra fatigue item slightly increased convergent validity (Spearman's rank correlation coefficient = - 0.61 vs. - 0.62). Domain dependency analysis showed that all EQ-5D-5L domains are dominant over the fatigue item. Explanatory power of the EQ-5D-5L + Fatigue was higher compared to the EQ-5D-5L (R-2 = 0.42 vs. 0.39). The gain is substantially larger in the subgroup with chronic health conditions. Conclusions Adding a fatigue item to the EQ-5D-5L improved all psychometric performance criteria of the enriched instrument in the general population. Effects are substantially larger in the subgroup with chronic health conditions, indicating that adding a fatigue item to the EQ-5D-5L is especially relevant in evaluating the HRQL of diseased people.