OBJECTIVES: XLRP patients experience progressive vision loss, resulting in significant disability and vision-related quality of life (VRQoL) impacts. This study aimed to psychometrically evaluate the Low Luminance Questionnaire (LLQ) in XLRP to support its suitability as a secondary endpoint to evaluate task difficulty under low luminance in clinical trials of novel therapies for XLRP patients.

METHODS: Floor and ceiling effects, item fit, response categories performance, unidimensionality, internal consistency reliability, convergent, divergent, and known‑groups validity were assessed using baseline data from Phase I/II and natural history studies in XLRP (data on file).

RESULTS: The LLQ domains have a good capacity to measure improvement and deterioration as indicated by item spread on the latent continuum on the Wright-Andrich map, except for the Driving domain (item floor effects: 35.3%-41.2%). Average inter-item correlations within the specific domains ranged from 0.202-0.933. All domains showed very high internal consistency (α=0.777‑0.971), and all item-total correlations were moderate to very high (0.451-0.971), with the exception of item 12 from the Mobility domain (Concerned that You Might Fall at Night), which had an item-total correlation of 0.248. All domains had well‑performing response categories and showed robust evidence of unidimensionality based on Velicer’s minimum average partial test (except for the Peripheral Vision domain).

CONCLUSIONS: In this study, the LLQ was psychometrically validated in an XLRP population. Most domains demonstrated a good capacity to capture improvement and deterioration of the condition, with all domains showing high internal consistency and well-performing response categories. Velicer’s minimum average partial test showed strong evidence for unidimensionality for most domains. These findings support the LLQ as a valid, self-reported measure of task difficulty under low luminance/at night in XLRP patients.