Original ArticlesFatigue in the general norwegian population: Normative data and associations
Introduction
Fatigue is reported to be common (15–20%) in the general population [1] and may accompany physical diseases 1, 2, 3, psychiatric diseases such as depression 1, 3, 4, or pregnancy 5, 6. Fatigue is therefore a nonspecific symptom and highly prevalent (20-30%) among patients in primary health care 7, 8. Still, physicians tend to rate fatigue of little importance [9] even though chronic fatigue is associated with disability at the same level as chronic medical conditions [8].
Both the widening of the health concept, as stated by the World Health Organization (WHO) [10], to include subjectively experienced dimensions such as fatigue and chronic fatigue syndrome (CFS) have resulted in increased attention to fatigue during recent years. The existence and etiology of CFS are disputed 11, 12. This unsettled dispute relates to the relative contributions of somatic and psychological factors, because the prevalence of psychiatric disorders is high (60–70%) among those affected 4, 13. The availability of reliable data on the epidemiology of fatigue has been suggested as one strategy to resolve this dispute [14], but such data are considered scarce [3]. In addition, the prevalence estimates of fatigue in the general population have been based mostly on single-item measures [3]. In such measures, the wording may strongly influence what is actually being measured, because “feeling tired all the time” is reported ten times more often than “feeling weak” [3]. The epidemiology of fatigue, including its social and demographic associations, is therefore far from well-documented.
In regard to the increasing amount of studies on fatigue in patient populations, such as cancer patients, most studies have been cross-sectional without comparison groups or control for confounding variables such as depression [15]. Given the high prevalence of fatigue in the general population, this hinders us from ascertaining whether fatigue in patient samples differs from fatigue in healthy individuals. Furthermore, the most commonly applied measurement of fatigue by single items with different wordings 16, 17 entails high levels of random error variance due to the different meanings assigned to the wordings by the respondents. Such measures also hinder comparisons between studies.
In the absence of objective measures of fatigue, both clinicians and researchers must rely on measuring subjective fatigue, which is in fact what the patients complain of [18]. As with other subjective health status measures, the interpretation of a score may represent a challenge. This challenge implies answering questions such as: What is the clinical relevance of a score? What is a high score? Norm-based interpretation is one method to answer if an observed score is (a)typical. In regard to fatigue in patient populations, norm-based interpretations should be of special relevance, given the high prevalence of fatigue in the general population. At the group level, norms are calculated based on a sample of the general population, and norm-based interpretations imply calculating departures from the norm [19]. However, normative data related to measures of fatigue are lacking, even though norm-based interpretation has become popular with other subjective health status measures [19].
The fatigue questionnaire (FQ) has been found suitable both for clinical and epidemiological purposes [20]. The present study was performed to estimate the prevalence of fatigue and to obtain population norms for the FQ in a sample representative of the general Norwegian population. Second, the study aimed at examining the associations of sociodemographic and health variables upon the level of fatigue.
Section snippets
Material and data collection
A total of 3500 Norwegian citizens, aged 19–80 years, were randomly drawn from the National Register by the Norwegian Government Computer Centre (SDS), and were representative of the entire Norwegian population.
The subjects received a mailed questionnaire in May 1996. Subjects who had not responded within 4 weeks received one written reminder, again enclosing the questionnaire.
Measures
The questionnaire included the following measures.
Response rate
A total of 2323 filled-in questionnaires were returned after the two mailings. Among nonrespondents, 4 had died and 44 questionnaires were returned because of incorrect address, yielding 3452 eligible for the study. The response rate was thus 67%, and 74% of the respondents answered before the second mailing. The respondents were younger () than the nonrespondents () (t=−2.3, p=0.02). More females (69%) than males (66%) responded (p=0.04). The lowest
Discussion
This study confirms that fatigue is distributed continuously in the general population 6, 22. In healthy subjects, fatigue is a natural occurrence after physical or mental efforts, usually relieved by rest. In diseased subjects, fatigue refers to exaggerated “lack of energy,” which does not or only to a limited extent responds to cessation of activity. In clinical practice, the amount and duration of fatigue are therefore the relevant topics [3] — but how should the answers be interpreted for
Acknowledgements
Acknowledgments—Financial support was given by Grant No. 96028/002 from the Norwegian Cancer Society. The staff of the Palliative Medicine Unit at the University Hospital of Trondheim, especially Helene Flottorp, Anders Skjeggestad, and Gro Underland, provided invaluable assistance in data collection.
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