Synonyms

Fatigue instrument; Fatigue measure

Definition

Krupp et al. (1988), who developed the FSS, defined fatigue as “a sense of physical tiredness and lack of energy, distinct from sadness or weakness” p. 435.

Description

The Fatigue Severity Scale (FSS) is one of the most frequently used inventories for measuring fatigue in people with chronic illnesses. The original FSS is a nine-item unidimensional questionnaire developed by Krupp, LaRocca, Muir-Nash, and Steinberg (1989) (Table 1). Each item consists of statements that are scored on a seven-point Likert type scale ranging from 1 (“strongly disagree”) to 7 (“strongly agree”). The mean score of the items is used as the FSS score. Some studies have calculated an FSS score as the sum of all nine items.

Fatigue Severity Scale, Table 1 The fatigue severity scale (FSS); English (US) version
Full size table

Cutoff Values

In order to categorize severity of fatigue in people with MS, researchers have used a variety of different criteria. Originally, the cutoff for severe fatigue was set at an FSS score ≥4 (Krupp et al., 1989). This cutoff is still used in some studies, e.g., in several studies of fatigue in people with stroke (Tang et al., 2010; van de Port, Kwakkel, Schepers, Heinemans, & Lindeman, 2007). However, more recent studies of people with multiple sclerosis mainly use a cutoff of ≥5 for categorizing severe fatigue (Johansson, Ytterberg, Hillert, Widen, & von Koch, 2008; Tellez et al., 2005). The different cutoff values have not been validated clinically. Some studies have also categorized the FSS score into three groups: low fatigue (FSS score <4), medium or borderline fatigue (FSS score ≥4 and <5), and high or severe fatigue (FSS score ≥5).

Samples

The FSS has been used in samples with, among others, people with brain injury, cancer, chronic fatigue syndrome, fibromyalgia, hepatitis C, HIV/AIDS, multiple sclerosis, neuroborreliosis, kidney diseases, Parkinson’s disease, poliomyelitis, sleep disorders/insomnia, stroke, systemic lupus erythematosus, people receiving primary care, and healthy controls.

Reliability

Internal Consistency

Studies have documented high internal consistency as analyzed with Cronbach’s alpha, i.e., alpha values ranging from 0.81 to 0.94 (Krupp et al., 1989; Kleinman et al., 2000; Mattsson, Moller, Lundberg, Gard, & Bostrom, 2008). However, when inter-item correlations were analyzed in a sample from the general population and among people with multiple sclerosis, stroke, and sleep-wake disorders, items #1 and #2 showed a relatively low correlation with the rest of the items (Lerdal, Wahl, Rustoen, Hanestad, & Moum, 2005; Valko, Bassetti, Bloch, Held, & Baumann, 2008).

Test-Retest

The FSS has shown high test-retest reliability (intra class correlation 0.82–0.94) (Kleinman et al., 2000; Gencay-Can & Can, 2012).

Validity

Factor analyses of the FSS have verified one factor (Lerdal et al., 2005; Kleinman et al., 2000).

Convergent Validity

The FSS correlates strongly with other fatigue scales (r = 0.41–0.94) (Krupp et al., 1989; Kleinman et al., 2000; Gencay-Can & Can, 2012) and in a clinical study has also been shown to be sensitive to change in levels of fatigue (Zifko, Rupp, Schwarz, Zipko, & Maida, 2002). Furthermore, the FSS has shown medium to strong relationships with other health-related quality of life domains (Mattsson et al., 2008).

Discriminant Validity

The FSS has demonstrated the ability to discriminate between healthy and chronically ill individuals (Lerdal et al., 2005; Valko et al., 2008). Furthermore, the instrument has been shown to discriminate between different sleep-wake disorders (Valko et al.), indicating satisfactory discriminant validity.

Discussion

In a recently published review of measurements of fatigue in chronic illnesses (Whitehead, 2009), the FSS was rated with the highest scores on robust psychometric properties among the 18 fatigue measurements evaluated. In the FSS, one item addresses a possible consequence of fatigue (item #1), one addresses a possible cause of fatigue (item #2), and the rest address the impact of fatigue on peoples’ ability to perform activities in daily life. Recently published studies that have assessed FSS using Rasch models in patients with multiple sclerosis (Mills, Young, Nicholas, Pallant, & Tennant, 2009; Lerdal, Johansson, Kottorp, & von Koch, 2010), in stroke (Lerdal & Kottorp, 2011) and in people with HIV/AIDS (Lerdal, Kottorp, Gay, Aouizerat, Portillo & Lee, 2011). All these studies reported inconsistent responses, with items #1 and #2 of the FSS having high positive residual statistics. In two of these studies (Lerdal, Johansson, Kottorp, & von Koch, 2009; Lerdal & Kottorp, 2011) no other items showed uniform differential item functioning in relation to sociodemographic and clinical variables. Thus, they recommended that items #1 and #2 should be removed when computing the FSS mean score (FSS-7). All items in the FSS-7 are concerned with the degree to which fatigue interferes with daily life and might be best used as a fatigue interference scale (Aouizerat, Gay, Lerdal, Portillo, & Lee, in press).

The FSS is a brief one-dimensional measurement with good psychometric properties, and it is one of the most frequently used self-report instruments for measuring fatigue. An abbreviated FSS-7 has better psychometric properties and is valid for measuring fatigue interference. Patients are instructed to choose a number from 1 to 7 that indicates their degree of agreement with each statement where 1 indicates “strongly disagree” and 7 “strongly agree” (Krupp et al., 1989).

Cross-References

Activities of Daily Living (ADL)

Burnout

Emotional Well-Being

Fatigue

Health

Health Outcomes

Health-Related Quality of Life (HRQOL)

Motivation

Perceived Quality of Life

Personal Well-Being

Quality of Life (QOL)