Introduction
Fatigue exhibits itself in symptoms like intense tiredness, lack of energy, and a feeling of profound weakness and exhaustion that can manifest mentally or physically [
1‐
3]. The prevalence of severe fatigue in the general pediatric population is high, ranging from one-fifth to about one-third [
4‐
8]. There are considerable sex and age differences in the occurrence of fatigue. Research shows that 20% of adolescent girls suffer from severe fatigue compared to only 6% of adolescent boys [
5]. Regarding age differences, 11% of children aged 11–14 years and 17% of those aged 13–16 years suffer from severe fatigue [
4].
Fatigue has a negative impact on health outcomes in children and adolescents including sleep disturbances, impaired social relationships, school absence, anxiety, depression, and poorer quality of life [
3,
5]. The negative effects of fatigue are not only seen in healthy children, children with pre-existing health problems, such as recurrent pain, also struggle with the impact of fatigue on their daily life, including sleep disturbances and reduced HRQoL [
3,
9‐
16]. Previous research also shows that fatigue negatively impacts pain characteristics such as pain intensity [
12,
16], functional disability [
16], and pain-related school absenteeism [
11,
16,
17].
Thus, the assessment and monitoring of fatigue is critical to treatment decisions and should be used in routine care of pediatric chronic pain patients [
11,
18,
19] as well as in routine or preventive application in the general pediatric population. For this, a reliable and valid measurement tool is required.
As part of the Patient-Reported Outcomes Measurement Information System (PROMIS
®;
www.nihpromis.org), funded by the National Institutes of Health (NIH), the Pediatric Short Form v2.0 - Fatigue 10a (PROMIS
® F-SF) [
20,
21] was developed. The English fixed-length short form and items from the underlying pediatric fatigue item bank have been used in several studies including children and adolescents suffering from chronic pain [
16], cancer [
22,
23], and sickle cell disease [
24]. The questionnaire proved to be a feasible and valid measure [
16,
22] and demonstrated sensitivity to change [
25]. A German version [
26] of the short form already exists but has not yet been validated for healthy children and adolescents nor for those with chronic pain.
Therefore, the aim of this study was to provide quantitative validity evidence for the use of the German version of the PROMIS® F-SF in a general population sample (school children) as well as in a pediatric chronic pain patient sample. It was hypothesized that the factor structure of the instrument yields good model fit in the conduction of confirmatory factor analyses (CFA). Good psychometric properties were expected. Convergent validity was assessed via associations with sleep quality, HRQoL, and pain characteristics; moderate to strong correlations were expected. In addition, in the context of convergent validity, sample differences were tested; pediatric chronic pain patients were expected to have higher fatigue scores than students. Additionally, age and gender distributions of the questionnaire were explored. Fatigue was expected to increase with age and girls were expected to experience more fatigue symptoms than boys.
Results
Construct validity
The overall model fits of the 1-factor model of the PROMIS
® F-SF for the school sample and the patient sample are presented in Table
2. The model fit of the patient sample was slightly worse than the one of the school sample [
39]. According to conservative recommendations [
38], all but one fit index - the SRMR - were not within the acceptable range. In both samples, all items showed high factor loadings (see Table
A1).
Table 2
Model fit indices of the 1-factor model of the PROMIS® F-SF for both samples
School sample | 492.0 | 35 | < 0.001 | 14.1 | 0.94 | 0.10 | 0.09 | 0.11 | 0.04 |
Patient sample | 110.8 | 35 | < 0.001 | 3.2 | 0.90 | 0.14 | 0.11 | 0.17 | 0.05 |
Item and scale characteristics
School sample
The PROMIS® F-SF showed excellent internal consistency with Cronbach’s α = 0.92. The average total score among students was M = 19.27 (SD = 8.18) which corresponds to a T-value of 47.9 (reference group: U.S. general pediatric population).
Patient sample
Internal consistency of the PROMIS® F-SF was excellent with Cronbach’s α = 0.93. The average total score was M = 20.96 (SD = 8.99) which converts to a T-value of 49.1 (reference group: U.S. general pediatric population).
Table
3 displays item properties of the PROMIS
® F-SF for both samples.
Table 3
Item properties of the German version of the PROMIS® F-SF for both samples
1 | 2.04 | 1.08 | 1–5 | 0.73 | 2.05 | 1.25 | 1–5 | 0.68 |
2 | 1.67 | 1.00 | 1–5 | 0.68 | 1.85 | 1.16 | 1–5 | 0.68 |
3 | 2.17 | 1.16 | 1–5 | 0.71 | 2.55 | 1.19 | 1–5 | 0.74 |
4 | 2.51 | 1.24 | 1–5 | 0.66 | 2.71 | 1.34 | 1–5 | 0.74 |
5 | 1.86 | 1.07 | 1–5 | 0.77 | 1.88 | 1.01 | 1–5 | 0.77 |
6 | 1.84 | 0.98 | 1–5 | 0.74 | 1.91 | 0.95 | 1–5 | 0.66 |
7 | 2.15 | 1.14 | 1–5 | 0.75 | 2.28 | 1.24 | 1–5 | 0.78 |
8 | 1.68 | 0.98 | 1–5 | 0.68 | 2.03 | 1.19 | 1–5 | 0.73 |
9 | 1.71 | 0.97 | 1–5 | 0.70 | 1.93 | 1.05 | 1–5 | 0.75 |
10 | 1.63 | 0.97 | 1–5 | 0.70 | 2.05 | 1.25 | 1–4 | 0.80 |
Convergent validity
Significant weak to moderate correlations were found between the PROMIS
® F-SF and the three scales of the sleep questionnaire in the school sample. In the patient sample, significant moderate correlations were found between the PROMIS
® F-SF and the scales Reinitiating Sleep and Returning to Wakefulness. Table
4 provides an overview of the correlations between the PROMIS
® F-SF and the rASWS scales.
Table 4
Correlations of the PROMIS® F-SF and the rASWS scales for both samples
School sample | 1348 | − 0.18 | < 0.001 | − 0.31 | 0.010 | − 0.36 | < 0.001 |
Patient sample | 103 | − 0.06 | 0.388 | − 0.42 | < 0.001 | − 0.36 | < 0.001 |
Correlation analyses in the school sample showed a strong significant association between fatigue and HRQoL (τ = − 0.51, p < 0.001).
Table
5 displays the correlations between fatigue and pain characteristics in both samples. In the school sample, all pain characteristics were significantly correlated with fatigue showing weak to moderate associations. For the patient sample, a weak significant association was found between fatigue and pain-related disability.
Table 5
Correlations of the PROMIS® F-SF and pain characteristics
School sample | 742 | 0.17 | < 0.001 | 742 | 0.08 | 0.010 | 742 | 0.41 | < 0.001 |
Patient sample | 112 | 0.08 | 0.274 | 88 | 0.01 | 0.857 | 112 | 0.18 | 0.006 |
An independent t test indicated that the patient sample (M = 20.96, SD = 8.99) had higher fatigue values compared to the school sample (M = 19.27, SD = 8.18), (t(129.34) = − 1.94, p = 0.055, d = − 0.21). Students who suffered from pain in the past four weeks (M = 22.06, SD = 8.57) reported significantly higher fatigue scores than students without pain in the past four weeks (M = 15.85, SD = 6.15), (t(1324.56) = 15.46, p < 0.001, d = 0.82).
Sex and age distributions of the PROMIS® F-SF
Female students (M = 21.38, SD = 8.98) showed significantly higher fatigue scores than male students (M = 16.95, SD = 6.46), (t(1280.24) = − 10.45, p < 0.001, d = − 0.56). The same was found for the patient sample; an independent t test showed that girls (M = 22.89, SD = 9.33) reported significantly higher fatigue than boys (M = 17.64, SD = 7.35), (t(102.16) = − 3.32, p < 0.001, d = − 0.61). Age was significantly associated with fatigue for students (τ = 0.22, p < 0.001) as well as for patients (τ = 0.16, p = 0.022) showing increasing fatigue with rising age.
Discussion
The aim of this study was to validate the German version of the PROMIS® F-SF for both school children and pediatric chronic pain patients. CFA showed sufficient model fit for the 1-factor model. Convergent validity was supported by significant weak to strong correlations with sleep quality, HRQoL, and pain characteristics, as well as by higher fatigue scores in children with pain than in those without pain. The questionnaire had excellent internal consistency in both samples. Girls generally reported higher fatigue scores than boys and fatigue increased with age.
Construct validity
CFA revealed a slightly better model fit in the school sample than in the patient sample [
39]. When interpreting fit indices according to conservative recommendations [
38], all but one fit index - the SRMR - were not within the acceptable range. However, it should be noted that in both samples all items had high factor loadings, which indicates strong associations between the items and the factor. Furthermore, internal consistency of the questionnaire is excellent. Due to these additional psychometric characteristics and considering international comparability of questionnaire results, the model was maintained in its original form despite the limited model fit.
Item and scale characteristics
Excellent internal consistency confirms that the fatigue questionnaire is a reliable instrument and underlines the good suitability of the instrument for use in the German pediatric community as well as in the population of pediatric chronic pain patients. In previous research, the English short version of the fatigue instrument has shown sensitivity to change [
25]; this should be assessed for the German version in future research.
In general, the fatigue scores in both German samples were rather low compared to the previous research [
5,
16]. On average, both students and patients reported the occurrence of fatigue symptoms as “almost never.” T-scores were just below the mean of 50, suggesting that German children tended to have lower fatigue scores compared to the U.S. general pediatric population. This should be considered when interpreting T-scores based on the U.S. reference population.
In contrast to previous research [
4,
5], over 60% of students in the current study “almost never” experienced fatigue symptoms in the past seven days. Contrary to this, a clinical review summarized that about one-third of European children experience considerable fatigue symptoms four times a week or more often [
8]. As the mentioned studies used different instruments to assess and determine the prevalence of fatigue, a comparison with the present study is difficult.
Unexpectedly, pediatric chronic pain patients in the present study reported a rather low frequency of fatigue symptoms. Only less than 2% of patients reported fatigue symptoms “often” to “almost always.” In a previous study, using the English version of the PROMIS
® F-SF, pediatric chronic pain patients had T-scores on average 10 points higher than those of the patients in the present study [
16].
Regardless, low fatigue scores were evident in both students and patients with chronic pain. This may indicate a general cultural difference in fatigue between German samples and other European or American samples. To our knowledge, there have been no comparative international studies on the severity of fatigue in children and adolescents with and without chronic pain. The validation of the German version of the PROMIS® F-SF and the large variety of translations of the PROMIS® instrument could enable such studies and create comparability through the standardization of measurement instruments.
Convergent validity
The associations between sleep quality and fatigue were all in expected directions, indicating that the more fatigue symptoms were experienced, the worse the quality of sleep. This was evident in both samples, with the strength of the association lower than expected, ranging from weak to moderate. The two scales Reinitiating Sleep and Returning to Wakefulness were moderately associated with the fatigue assessment. Surprisingly, the Going to Bed scale was only weakly associated with the fatigue questionnaire. However, these results support the convergent validity of the fatigue measure given the moderate association with two out of three subscales.
Further evidence of convergent validity is the large negative association between fatigue and HRQoL in the school sample, indicating that the more fatigue symptoms were experienced, the worse the HRQoL. This is in line with the previous research showing that fatigue worsens HRQoL especially in children with long-term conditions (for example chronic pain) [
3,
11].
Convergent validity was also inspected using pain characteristics. These findings were not in line with expectations as the strength of associations was lower than anticipated. In both samples pain-related disability had the strongest association, followed by pain intensity and pain-related school absence. The latter two were not significant in the patient sample and only weak in the school sample. A similar weak correlation (
r = 0.27,
p < 0.001) between pain intensity and fatigue was found in a study of pediatric chronic pain patients using the English version of the PROMIS
® F-SF [
16]. In another study of patients with juvenile idiopathic arthritis and juvenile dermatomyositis, parents' ratings of worst pain were moderately correlated with fatigue (
r = 0.51,
p < 0.001) [
12]. Additionally, there is some research that attributes fatigue as having an important role in school functioning [
17]. For instance, fatigue is considered to be a predictor of school functioning in pediatric chronic pain patients [
16] and a mediator between pain and school functioning [
11]. To our knowledge, the direct association between school absence and fatigue has not yet been investigated and needs further research attention, especially in pediatric chronic pain patients. Overall, the associations between fatigue and pain characteristics in the school sample, albeit weaker than expected, provide preliminary support for convergent validity of the questionnaire (especially the association with pain-related disability). However, the extremely weak associations of fatigue with pain intensity and pain-related school absence in the patient sample were surprising. Further research is needed to better understand these associations and whether results will be replicated with other pain measures.
As a final aspect of convergent validity, it was hypothesized that pediatric chronic pain patients would have higher fatigue scores than students. While this was true, there was no significant difference between the two samples. Regardless, there was a significant difference between students with pain and students without pain; students who reported pain in the past four weeks had significantly higher fatigue scores than students who did not report pain. Even more surprising was that students with pain showed higher fatigue scores than pediatric chronic pain patients.
Overall, results of this study provide preliminary support for convergent validity of the German version of the fatigue questionnaire.
Sex and age distributions of the PROMIS® F-SF
There was a sex difference for fatigue in both study samples. In line with the previous research [
5,
24], girls in tertiary care as well as in the general population reported significantly higher fatigue scores than boys. However, there is one study using the English version of the PROMIS
® F-SF that has not found a significant sex difference in fatigue [
16].
In accordance with the previous research [
4,
16], age had a significant positive association with fatigue in both study samples, indicating that the older the participants were, the more they experienced fatigue symptoms. In contrast, another study found no significant association between fatigue and age, however, the participants in that study were young adults between 15 and 30 years [
24]. The increase in fatigue symptoms with age may be specific to younger children.
Limitations
The presented study has several limitations that need to be considered when interpreting the results. It should be noted that pain characteristic instruments used to assess convergent validity had different reference periods than the fatigue instrument (fatigue questionnaire: last seven days, pain-related instruments: last four weeks). Further research is needed to better understand whether results will be replicated with other pain measures. In addition, the patient sample self-reports of certain pain characteristics had to be partially replaced by parent proxy-reports. Despite the proven high agreement between parent and child reports for pain characteristics [
34,
35], this may have weakened the associations between measures. Last, patient sample is relatively small. Psychometric studies that use larger or other clinical samples are needed to further support the findings presented here.
Conclusion
This study shows that the German version of the PROMIS® F-SF is a reliable instrument with good psychometric properties. It provides preliminary support that the measure is valid for assessing fatigue in children and adolescents with and without chronic pain. The multiple translations of the PROMIS® instrument allow international comparisons and further research regarding fatigue. Therefore, this study contributes to the improvement of international research in the field of fatigue.
Acknowledgements
We would like to thank Alexandra van der Valk for editing the manuscript for English language. The authors would like to thank the Berlin Institute of Health at Charité – Universitätsmedizin Berlin, Clinical Study Center, German PROMIS® National Center, Germany, for providing the questionnaire and for their support.
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