Does Differential Item Functioning Jeopardize the Comparability of Health-Related Quality of Life Assessment Between Patients and Proxies in Patients with Moderate-to-Severe Traumatic Brain Injury?

Background Health-related quality of life (HRQoL) is clearly recognized as a patient-important outcome in patients with traumatic brain injury (TBI). Patient-reported outcomes are therefore often used and supposed to be directly reported by the patients without interpretation of their responses by a physician or anyone else. However, patients with TBI are often unable to self-report because of physical and/or cognitive impairments. Thus, proxy-reported measures, e.g., family members, are often used on the patient’s behalf. Yet, many studies have reported that proxy and patient ratings differ and are noncomparable. However, most studies usually do not account for other potential confounding factors that may be associated with HRQoL. In addition, patients and proxies can interpret some items of the patient-reported outcomes differently. As a result, item responses may not only reflect patients’ HRQoL but also the respondent’s (patient or proxy) own perception of the items. This phenomenon, called differential item functioning (DIF), can lead to substantial differences between patient-reported and proxy-reported measures and compromise their comparability, leading to highly biased HRQoL estimates. Using data from the prospective multicenter continuous hyperosmolar therapy in traumatic brain-injured patients study (240 patients with HRQoL measured with the Short Form-36 (SF-36)), we assessed the comparability of patients’ and proxies’ reports by evaluating the extent to which items perception differs (i.e., DIF) between patients and proxies after controlling for potential confounders. Methods Items at risk of DIF adjusting for confounders were examined on the items of the role physical and role emotional domains of the SF-36. Results Differential item functioning was evidenced in three out of the four items of the role physical domain measuring role limitations due to physical health problems and in one out of the three items of the role emotional domain measuring role limitations due to personal or emotional problems. Overall, despite an expected similar level of role limitations between patients who were able to respond and those for whom proxies responded, proxies tend to give more pessimistic responses than patients in the case of major role limitations and more optimistic responses than patients in the case of minor limitations. Conclusions Patients with moderate-to-severe TBI and proxies seem to have different perceptions of the items measuring role limitations due to physical or emotional problems, questioning the comparability of patient and proxy data. Therefore, aggregating proxy and patient responses may bias HRQoL estimates and alter medical decision-making based on these patient-important outcomes.