Rasch analysis of fatigue severity scale in patients with epilepsy

Objective: To utilize the Rasch model to validate and assess the psychometric properties of the Fatigue Severity Scale (FSS) in patients with epilepsy.

Methods: A total of 307 patients (age > 18 years) with a confirmed diagnosis of epilepsy were consented to participate. Exclusion criteria included patients with psychogenic nonepileptic events, cognitive disabilities, and patients who did not speak/understand English. The nine-step FSS was programmed into software administered to patients on electronic tablets, and patient responses were auto-scored. The Rasch rating scale model (RSM) was used to evaluate the unidimensionality, reliability, and targeting of the FSS. To assess unidimensionality, we examined infit and outfit mean squares. We also assessed unidimensionality of the FSS using a principal component analysis of Rasch residuals, where residuals are understood as the difference between observed and expected data values. We evaluated the internal consistency of person and item performance by examining separation reliability estimates and separation ratio. Differential Item Functioning (DIF) was calculated for gender.

Results: There was mixed evidence regarding the extent to which the FSS fit the Rasch model. Outfit values ranged from 0.52 to 2.72 and infit values were 0.60 to 2.18, strongly suggesting the presence of misfitting items: Item 1 ("My motivation is lower when I am fatigued") and Item 2 ("Exercise brings on my fatigue").

Significance: The nine-item FSS showed fair psychometric properties in this sample of patients with epilepsy. Our study provides unique, supportive information for the use of a modified version of the FSS, omitting the first two items, in patients with epilepsy. Given the prevalence of fatigue and other neuropsychiatric comorbidities of epilepsy, having a validated fatigue scale can aid healthcare providers to identify moderate-to-severe fatigue levels in patients with epilepsy and address the plausible risk factors.