Abstract
BackgroundFatigue is a disabling symptom associated with reduced quality of life in various populations living with chronic illnesses. The transfer of knowledge about fatigue from one group to another is crucial in both research and healthcare. Outcomes should be validly and reliably comparable between groups and should not be unduly influenced by diagnostic variations. The present study evaluates whether the Fatigue Severity Scale 7-item version (FSS-7) demonstrates similar item hierarchy across people with multiple sclerosis, stroke or HIV/AIDS to ensure valid comparisons between groups, and provide further evidence of internal scale validity.MethodsA secondary comparative analysis was performed using data from three different studies of three different chronic illnesses: multiple sclerosis, stroke and HIV/AIDS. Each of these studies had previously concluded that the FSS-7 has better psychometric properties than the original FSS for measuring fatigue interference. Data from 224 people with multiple sclerosis, 104 people with stroke and 316 people with HIV/AIDS were examined. Item response theory and a Rasch model were chosen to analyze the similarity of the FSS-7 item hierarchy across the three diagnostic groupsResultsCross-sample differences were found for items #3, #5, #6 and #9 for two of the three samples, which raise questions about item validity across groups. However, disease-specific and disease-generic Rasch measures were similar across samples, indicating that individual fatigue interference measures in these three chronic illnesses might still be reliably comparable using the FSS-7.ConclusionsSome items performed differently between the three samples but did not bias person measures, thereby indicating that fatigue interference in these illnesses might still be reliably compared using FSS-7 scores. However, caution is warranted when comparing fatigue raw sum scores directly across diagnostic groups using the FSS-7. Further studies of the scale are needed in other types of chronic illnesses.
Highlights
Fatigue is a disabling symptom associated with reduced quality of life in various populations living with chronic illnesses
Tukey post hoc tests indicated that the multiple sclerosis (MS) group had significantly higher Fatigue Severity Scale (FSS)-7 scores than both the stroke group (p < 0.001) and the Human immunodeficiency virus (HIV) group (p < 0.001), but the stroke group and HIV group did not differ significantly from each other (p = 0.79)
Fatigue interference (FSS-7) scores were converted into equal-interval measures that can range from 0 to 100, and the means, standard deviations and ranges for the three groups were: 1) MS: 56.1 ± 16.1; 2) stroke: 48.6 ± 14.6, and 3) HIV: 47.3 ± 17.3
Summary
Fatigue is a disabling symptom associated with reduced quality of life in various populations living with chronic illnesses. In our prior work [15,16,17] we used Rasch analysis to evaluate the psychometric properties of the original 9item version of the FSS within several chronic illness groups, including multiple sclerosis (MS), stroke and HIV/AIDS These three illnesses were selected because fatigue is a well-documented and prevalent symptom in each of these patients groups [18,19,20,21,22,23,24,25]. The prior studies [15,16,17] each concluded that a 7-item version of the FSS (FSS-7) has better psychometric properties and is a reliable and valid measure of fatigue interference rather than severity as indicated by the title of the measure These studies, as well as a study by Mills et al [26], provided consistent evidence regarding the relationship between the included items and the underlying latent trait in diagnosisspecific samples. We do not know whether the FSS items function across the samples and are appropriate for use in comparative studies
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