Beyond observed changes: how differential item functioning and response shift alter health-related quality of life...

Experiencing dialysis and receiving a kidney transplant may induce differential item functioning (DIF) and response shift (RS), reflecting changes in how patients interpret and evaluate their health-related quality of life (HRQoL). This study investigates the impact of DIF and RS on HRQoL change estimation in preemptive (PKT) and non-preemptive (nPKT) kidney transplant patients before and after transplantation.

In the prospective, multicentre, controlled PreKit-QoL study, HRQoL was assessed using the SF-36 every 6 months before transplantation, at hospital discharge, at 3 and 6 months, and annually until 5 years post-transplantation in PKT (n = 158) and nPKT (n = 226) patients. Changes in HRQoL and DIF and RS occurrence between the last visit before transplantation and 3 months post- transplantation, and between 3 and 6 months post-transplantation were investigated with an item-level DIF and RS detection algorithm, ROSALI. ROSALI estimates DIF and RS effects, and HRQoL changes accounting for these effects using Rasch models.

Four SF-36 dimensions (Bodily Pain, Vitality, Social Functioning, Mental Health) were unaffected by DIF and RS, and showed general improvement shortly after transplantation, with only changes in Bodily Pain differing between groups. Two dimensions (Role Physical, General Health) exhibited DIF and RS shortly after transplantation. Ignoring these phenomena would have masked the deterioration in Role Physical for PKT patients and slightly underestimated the improvement in General Health.

Investigating DIF and RS is essential for accurate PROM interpretation, revealing how patients' perception may evolve following transplantation. These insights inform both statistical analysis and understanding of patients' lived experience.