Measuring Physical and Cognitive Fatigue in People With Post-Polio Syndrome: Development of the Neurological Fatigue Index for Post-Polio Syndrome (NFI-PP)

Abstract

Background: Fatigue in post-polio syndrome (PPS) has been shown to affect quality of life adversely. There is currently no disease-specific measure of fatigue for PPS.

Objective: To develop a scale to measure fatigue in PPS that meets rigorous psychometric standards.

Design: Qualitative followed by validation and test-retest studies.

Setting: Polio clinic followed by national questionnaire studies.

Participants: A total of 45 participants from polio clinic for qualitative; 319 participants from clinic or self-referral for validation study, of whom 87 completed the retest questionnaire.

Methods: Draft questionnaire items on PPS fatigue were derived from transcripts of qualitative interviews. After cognitive debriefing, the draft measure was administered by mail along with comparator questionnaires to a new sample.

Main outcome measurements: Draft PPS fatigue measure, Fatigue Severity Scale, and visual analog scale for fatigue.

Results: Analysis of 271 of 319 (85%) questionnaires identified a 2-factor solution (RMSEA 0.058). For the physical subscale, a 20-item scale showed good fit (χ²P = .189), strict unidimensionality (t-test 5.17%), and reliability 0.91. For the cognitive subscale, a 7-item scale showed excellent fit (χ²P = .917), strict unidimensionality (t-test 5.2%), and reliability 0.89. Evidence of a "difficulty factor" emerged also supporting a total score that showed good fit (χ²P = .151), strict unidimensionality (t-test 0.4%), and reliability consistent with group use at 0.73. Test-retest correlations for all scales were greater than 0.85. Standard error of measurement on metric ranges was 5.4 for total, 2.9 for physical, and 1.69 for cognitive domains. With the latent estimate of the total score transformed to a 0-100 scale, the mean score was 49.5 (SD 6.9). Spearman correlations with the Fatigue Severity Scale and visual analog scale were 0.60 and 0.55, respectively.

Conclusions: A patient-derived Neurological Fatigue Index for PPS, with physical and cognitive subscales and a total score, has demonstrated good reliability, appropriate concurrent validity, and satisfies the Rasch measurement model. A raw-score to interval scale transformation is available for parametric applications and the calculation of change scores.

Level of evidence: III.