Psychometric Analysis of the Patient-Reported Plexiform Neurofibromas Quality of Life Measure Using KOMET Study Data

OBJECTIVES: The plexiform neurofibromas quality of life (PlexiQoL) tool is a patient-reported outcome measure developed through rigorous qualitative methods that specifically assesses the basic needs of adults with neurofibromatosis type 1-associated plexiform neurofibromas. This study evaluated the psychometric properties of PlexiQoL to provide an early estimate of clinically meaningful within-patient change in the context of the KOMET (NCT04924608) study data.

METHODS: PlexiQoL comprises 18 items linked to appearance, relationships, independence, role fulfillment, and pleasure; lower scores indicate better QoL. In KOMET, PlexiQoL was assessed at baseline; Day 28 of cycles (C)2, 4, 8, 12, 16, 20, 24; and Day 28 of every six cycles thereafter. PlexiQoL psychometric analysis used the Rasch measurement theory (RMT) to assess targeting of individual items, suitability of the response scale, item fit, local independence, reliability, and measurement invariance. Classical test theory analyses assessed internal consistency (Cronbach’s alpha coefficient), test–retest reliability (intraclass correlation coefficient; ICC), and construct validity using both EuroQol-5-dimension-5-level (EQ-5D-5L) and EQ-5D-5L visual analog scale (VAS). The distribution-based method was used to determine meaningful score difference (MSD) due to lack of a suitable anchor.

RESULTS: With 145 participants from baseline, RMT analysis of pooled PlexiQoL scores up to C12 (n=647 available assessments) indicated acceptable targeting by distribution match, acceptable item fit according to statistical indicators, no local dependence between items (one residual correlation was >0.3), and modest reliability (Person Separation Index: 0.79). Cronbach’s alpha coefficient (baseline: 0.84; N=142) and the range in ICC (0.75−0.94) indicated modest–good PlexiQoL reliability. PlexiQoL score showed a clear, positive association with the EQ-5D-5L items and EQ-5D-5L-VAS score. Distribution-based estimates for meaningful change were 2.23 (half standard deviation) and 1.76 (standard error of measurement).

CONCLUSIONS: Consistent with previous data, PlexiQoL score demonstrated satisfactory psychometric performance. A two-point decrease is a tentative estimate of MSD for PlexiQoL total score.