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PHYS THER
Vol. 83, No. 5, May 2003, pp. 444-454
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Research Reports

Psychometric Properties of the Dutch Chronic Fatigue Syndrome–Activities and Participation Questionnaire (CFS-APQ)

Jo Nijs, Peter Vaes, Neil McGregor, Elke Van Hoof and Kenny De Meirleir

J Nijs, PT, MSc, MT, is Assistant at the Department of Human Physiology, Faculty of Physical Education and Physical Therapy, Vrije Universiteit Brussel, Brussels, Belgium. Address all correspondence to Mr Nijs at Vakgroep MFAB/Sportgeneeskunde, AZ-VUB KRO Gebouw 1, Laarbeeklaan 101, 1090 Brussels, Belgium (Jo.Nijs{at}vub.ac.be)
P Vaes, PT, PhD, is Professor, Department of Physical Therapy, Faculty of Physical Education and Physical Therapy, Vrije Universiteit Brussel
N McGregor, PhD, MDSc, is Faculty Member, Collaborative Pain Research Unit, Department of Biological Sciences, Faculty of Science, University of Newcastle, Callaghan, New South Wales, Australia
E Van Hoof, Clin Psychol, is Clinical Psychologist, Department of Human Physiology, Faculty of Physical Education and Physical Therapy, Vrije Universiteit Brussel
K De Meirleir, MD, PhD, is Staff Member, Chronic Fatigue Clinic, Vrije Universiteit Brussel, and is Professor and Head, Department of Human Physiology, Faculty of Physical Education and Physical Therapy, Vrije Universiteit Brussel


Submitted August 1, 2002; Accepted December 3, 2002


   Abstract
 
Background and Purpose. The Chronic Fatigue Syndrome–Activities and Participation Questionnaire (CFS-APQ) is a recently developed disease-specific assessment tool for monitoring activity limitations and participation restrictions in patients with chronic fatigue syndrome (CFS). In this study, the convergent validity, content validity, and test-retest reliability of data obtained with the Dutch-language version of the questionnaire were examined. Subjects and Methods. One hundred eleven consecutive patients with CFS were enrolled, of whom 47 fulfilled all inclusion criteria. The subjects were first asked to rate their pain, fatigue, and ability to concentrate using 3 visual analog scales, to list at least 5 activities that had become difficult to perform due to their complaints, and to complete the CFS-APQ. Furthermore, subjects were asked to complete a modified version of the CFS-APQ at home and return it to the investigators. The content of the questionnaire was reviewed using the World Health Organization's International Classification of Impairments, Disability and Health (ICIDH) beta II draft. Spearman rank correlation coefficients (R) were used for the convergent validity analysis, and intraclass correlation coefficients were computed for the assessment of the test-retest data. Results. Overall scores on the CFS-APQ correlated with the scores from the visual analog scales for pain (R=.51, P<.001) and fatigue (R=.50, P<.001). The majority of the responses (157 out of 183 answers [85.8%]) to the request to "list difficult activities" matched the content of the CFS-APQ. Using the ICIDH beta II draft, 21 out of 26 questions were found to address activities, and the remaining 5 questions measured the participation level. The Cronbach alpha coefficient was .94, and intraclass correlation coefficients for test-retest reliability of the overall scores were ≥.95 (P<.001). Discussion and Conclusion. The results substantiate the convergent validity, content validity, and reliability of the CFS-APQ scores for patients with CFS.

Key Words: Activity limitations • Chronic fatigue syndrome • Participation restrictions • Reliability • Validity


   Introduction
Top
 Abstract
 Introduction
Method
 Results
 Discussion and Conclusions
 Appendix
 References
 
Chronic fatigue syndrome (CFS) is characterized by persistent or relapsing fatigue lasting 6 or more consecutive months.1,2 To fulfill the Centers for Disease Control and Prevention (CDC) 1994 definition for CFS,1 any medical condition that may explain the presence of chronic fatigue should be excluded. In addition, self-reported chronic fatigue should result in substantial reduction in previous levels of occupational, educational, social, or personal activities. No single underlying cause has been established for all patients with CFS, possibly due to the heterogeneity of this patient group.

Although the diagnosis of CFS requires severe activity limitations and participation restrictions (problems an individual may experience in involvement in life situations),1,2 no disease-specific measures are currently available to assess these aspects of disability in people with CFS. Exercise capacity testing appears to be valid for monitoring cardiorespiratory fitness,3,4 albeit an expensive and time-consuming option for characterizing a component of CFS. Generic questionnaires, such as the Medical Outcomes Study 36-Item Short-Form Health Status Survey (SF-36),3,5 are widely used in CFS research. The interpretation of data obtained with such generic measures is often difficult and time-consuming (eg, requiring interpretation of scores for 8 different subscales for the SF-36), which prohibits their routine application in practice. Furthermore, we believe that generic measures are unable to characterize all activity limitations and participation restrictions in patients with CFS and consequently have insufficient content validity for this population. These limitations decrease the usefulness of general questionnaires, especially when identifying activity limitations, which we believe is crucial so that patients can be taught to manage their activity levels (also known as "pacing," a strategy where patients are encouraged to achieve an appropriate balance between activity and rest to avoid exacerbations and to set realistic goals for increasing activity). Therefore, an inexpensive, quick, and valid measure to assess activity limitations and participation restrictions in patients with CFS might be of great value, especially to establish goals and to examine the effectiveness of interventions. Indeed, the primary expectation of treatment for patients with less severe disability may not to be to reduce symptoms, but to regain full participation in work and social activities.

Our group recently constructed a disease-specific and time-specific (reflecting the patients' health status during the last 7 days) questionnaire: the Chronic Fatigue Syndrome–Activities and Participation Questionnaire (CFS-APQ).6 This new assessment tool was constructed based on self-reported activity limitations and participation restrictions of 141 patients with CFS (a retrospective analysis of over 600 quality-of-life questionnaires).6 In accordance with the World Health Organization's taxonomy, the International Classification of Functioning, Disability and Health,7 our new self-assessment tool is designed to monitor activity limitations and participation restrictions in patients with CFS. In this study, the convergent validity, 2 different aspects of content validity, and reliability of the CFS-APQ scores were examined in 47 patients with CFS.


   Method
Top
 Abstract
 Introduction
 Method
Results
 Discussion and Conclusions
 Appendix
 References
 
Subjects

One hundred eleven consecutive patients with chronic fatigue who visited the Chronic Fatigue Clinic of the Vrije Universiteit Brussel for phlebotomy between the beginning of March 2001 and the end of May 2001 completed the CFS-APQ. All questionnaires were administered by our secretaries. They asked the patients to complete the questionnaire but did not provide additional explanations, not even when patients asked for clarification. This was done to minimize administrator bias. An accompanying letter explaining the exact nature and purpose of the research stipulated that patients were not obliged to participate and that anonymity was guaranteed. The introductory letter was followed by standardized sheets for the assessment of the demographic features of each patient (age, sex, and illness duration).

Patients who did not fulfill the 1994 definition for CFS1 were excluded from the sample. To fulfill the CDC criteria for CFS, unexplained, persistent, or relapsing chronic fatigue that is of new or definite onset should result in a substantial reduction in previous levels of occupational, educational, social, or personal activities.1 Furthermore, at least 4 of the following symptoms must have persisted or recurred during 6 or more consecutive months and must not have predated the fatigue: impairment in short-term memory or concentration, tender cervical or axillary lymph nodes, muscle pain, multijoint pain, headache, unrefreshing sleep, and postexertional malaise lasting longer than 24 hours.1 Any active medical condition that may explain the presence of chronic fatigue prohibits the diagnosis of CFS. All subjects underwent an extensive medical evaluation, consisting of a physical examination, medical history, exercise capacity test, and routine laboratory tests. The laboratory tests included a complete blood cell count; determination of erythrocyte sedimentation rate; a serum electrolyte panel; measures of renal, hepatic, and thyroid function; and rheumatic and viral screens. If the patients' medical history did not include information that excluded a psychiatric problem at the time of disease onset, then a structured psychiatric interview was performed. In a number of cases, further neurological, gynecological, endocrine, cardiac, or gastrointestinal evaluations were performed to fulfill the CDC criteria for CFS. The medical records also were reviewed to determine whether patients had organic or psychiatric illnesses that could explain their symptoms. Furthermore, if patients did not complete more than 50% of the items on the questionnaire appropriately or if they were younger than 18 years of age, they were excluded from the sample. Underaged patients, we believe, have different activity limitations and participation restrictions as compared with adults.

Forty-seven of the 111 study participants fulfilled the inclusion criteria. Demographic features of the sample and descriptive statistics for the visual analog scales (VASs) for pain, fatigue, and ability to concentrate are presented in Table 1. The subjects had a mean age of 36.5 years (SD=11.1, range=18–60) and a mean illness duration of 4.8 years (SD=4.9, range=1.0–28.0), and 39 subjects (83.0%) were female.


View this table:
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[in a new window] 		Table 1. Demographic Characteristics and Mean Visual Analog Scale (VAS) Scores for Pain, Fatigue, and Ability to Concentrate

 
Fifty of the 111 study participants were excluded because their medical diagnosis was not established at the time of data analysis, mainly because they did not return for all required evaluations. Eight participants were excluded because they did not fulfill the CDC criteria for CFS1 (6 had fibromyalgia syndrome,8 1 had leukemia, and 1 was judged to be cured). Two participants were younger than 18 years of age, and another participant was excluded because the questionnaire was not completed appropriately. The medical records of 3 participants were not found, probably because they listed their maiden name on the questionnaire. All exclusion criteria are listed in Table 2.


View this table:
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[in a new window] 		Table 2. Reasons for Exclusion in the Validity Analysis (N=111) and the Test-Retest Reliability Analysis (n=42)

 
Forty-two (37.8%) of the 111 study participants returned the retest questionnaire. Thirty-four of the returned questionnaires were completed by patients who were eligible for study participation, and these questionnaires were used for the test-retest analysis. Three questionnaires were excluded from the data analysis because the medical diagnosis had not been established. One participant did not fulfill the current international definition for CFS,1 and 4 questionnaires were not analyzed because they were not completed appropriately (some data were missing for at least half of the questions). The demographic features of the retest sample (n=34) are presented in Table 3. These subjects had a mean age of 38.9 years (SD=9.2, range=20–59) and a mean illness duration of 5.6 years (SD=5.3, range=1.0–26.0), and 30 subjects (88.2 %) were female. No differences in age (t=–1.05, df=79, P=.298), illness duration (t=–0.65, df=79, P=.519), or sex characteristics (P=.753) were found between the retest sample (n=34) and the first sample (n=47).


View this table:
[in this window]
[in a new window] 		Table 3. Demographic Characteristics and Visual Analog Scale (VAS) Scores of the Sample (Reliability Analysis)

 
Measurement Instrument

The Dutch version of the CFS-APQ* was used for the assessment of activity limitations and participation restrictions in our subjects. The construction of this measure, based on self-reported activity limitations and participation restrictions of 141 patients with CFS (retrospective analysis of over 600 questionnaires such as the Karnofsky Performance Questionnaire and a modified version of the Barthel Index), is reported elsewhere.7 Total scores were obtained in 2 ways. Primarily, the scoring system of the Quality of Life Index,9–11 which was developed using the premise that people value things differently, was used to calculate an overall score (total score 1). Items were rated on a 4-point Likert-type scale, with scores ranging from 1 ("totally not agree") to 4 ("totally agree") for the satisfaction section and from 1 ("very unimportant") to 4 ("very important") for the importance verification. Item scores were calculated by multiplying the score on the satisfaction part of each question by the score on the importance verification. The reasoning behind this approach was based on the assumption that a patient with disability in activities of importance to areas of his or her life has a lower quality of life than those whose disabilities affect less important activities.9 In addition, subjects were able to mark items as not applicable for their living environment. The overall score (total score 1) was obtained by counting all item scores and then dividing that sum by the number of properly completed questions (possible score range=1–16). If the item was not applicable or when the question was not filled in properly (eg, was not completed), then that question was excluded. In this way, we dealt with missing data by assuming responses to the remaining data were the average calculated based on the available data. A second overall score (total score 2) was obtained by counting all satisfaction scores and then dividing that sum by the number of properly completed questions. In this way, the subjects' importance verification was not taken into account.

Reliability

Test-retest reliability of data obtained with the self-report instrument was assessed by having the subjects complete a modified version of the questionnaire at home and returning it within 24 hours. The subjects were given a sealed envelope containing the modified version of the questionnaire, an instruction letter, and a stamped, addressed return envelope as they departed from the hospital and were instructed not to open the envelope until they arrived home. The modified version of the CFS-APQ (the retest questionnaire) had different fonts and a different question sequence, including VASs for pain, fatigue, ability to concentrate. The reasoning behind our using this approach was our effort to prevent subjects from replicating their answer sequence rather than responding to the questions. The instruction letter explained the nature and purpose of the research, and again we stipulated that patients were not obliged to participate and that anonymity was guaranteed. The same exclusion criteria were used as we did for the validity analysis.

Validity

To examine the unidemensionality of the questionnaire, the Cronbach alpha coefficient was calculated. To investigate the convergent validity of this new measure, 100-mm VASs for fatigue, pain, and ability to concentrate were used. The pain scores obtained with the VAS are believed to be reliable12,13 and sensitive to change.14 Additionally, before completing the questionnaire, participants were asked to list at least 5 activities or tasks that have become difficult for them to do. All answers were analyzed for their accordance with the contents of the questionnaire. Other researchers have used this method for investigating content validity.15 The CFS-APQ is designed to monitor activity limitations and participation restrictions in patients with CFS. Therefore, every item of the questionnaire was checked in the International Classification of Impairments, Disability and Health (ICIDH) beta II draft16 to examine whether the items of this measurement tool assess activity limitations and participation restrictions. At the time this study was performed, the International Classification of Functioning, Disability and Health (ICF, the latest revision of the ICIDH) was not available. Likewise, frequently reported answers to the open question that did not match the content of the questionnaire were searched in the ICIDH beta II draft.

Data Analysis

Spearman rank correlation coefficients were calculated for the analysis of the data highlighting the convergent validity of the CFS-APQ scores with the VASs for pain, fatigue, and concentration. The significance level was set at .01 to help protect against potential Type I errors. Simple descriptive statistics (frequencies and percentages) were used for the analysis of the content validity data. The test-retest data were assessed using intraclass correlation coefficients (ICCs, two-way mixed model).17 Possible differences in demographic characteristics between the first sample (n=47) and the retest sample (n=34) were assessed using the independent-samples Student t test for equality of means (age and illness duration) and the Fisher exact test (sex).


   Results
Top
 Abstract
 Introduction
 Method
 Results
Discussion and Conclusions
 Appendix
 References
 
CFS-APQ Scores

The descriptive statistics of the overall and individual item scores are listed in Table 4. Total score 1, which includes the importance verification, had a median of 9 and an interquartile range of 3.6. A score of 1 indicates no activity limitations or participation restrictions, whereas a score of 16 represents the maximum score. Total score 2 (median=2.8, interquartile range=1) does not take the importance verification into account (potential range=1–4).


View this table:
[in this window]
[in a new window] 		Table 4. Descriptive Statistics for the Chronic Fatigue Syndrome–Activities and Participation Questionnaire (CFS-APQ) Overall Scores and Individual Item Scoresa

 
Reliability

The Cronbach alpha was .94 when the total item scores were taken into account (satisfaction part multiplied by the importance verification). A similar coefficient alpha (.94) was obtained with only the satisfaction parts of the questions (part A) included, whereas the Cronbach alpha was slightly lower (.89) for the importance verifications of the items (part B).

Table 5 lists all ICCs for the total scores on the questionnaire, the individual questions as well as the VAS scores. The test-retest ICCs of the overall scores on this self-administered measure were .96 (total score 1) and .95 (total score 2). The ICCs of the total scores were >.80, except for the ICCs for questions 6 ("replacing a lightbulb," ICC=.57), 9 ("standing for 1 hour," ICC=.80), and 18 ("reading," ICC=.77). The ICCs for the VAS scores for pain, fatigue, and ability to concentrate between the test and the retest were .83, .82, and .76, respectively.


View this table:
[in this window]
[in a new window] 		Table 5. Intraclass Correlation Coefficients (ICCs, Two-Way Mixed Model) for the Chronic Fatigue Syndrome–Activities and Participation Questionnaire (CFS-APQ) Test-Retest Dataa

 
Convergent Validity

Overall scores (total scores 1 and 2) correlated (P<.01) with the scores from the VASs for fatigue and pain. Correlation coefficients for total scores 2 and 1 for the fatigue VAS were .47 (P<.002) and .50 (P<.001). Correlation coefficients for total scores 2 and 1 for the pain VAS were .44 (P<.005) and .51 (P<.001). Total score 2 correlated with the VAS for ability to concentrate (R=.41, P<.008), but there was no correlation for total score 1. No correlations between illness duration and CFS-APQ scores were observed (data not shown). All data are presented in Table 6.


View this table:
[in this window]
[in a new window] 		Table 6. Convergent Validity (R) of the Chronic Fatigue Syndrome–Activities and Participation Questionnaire (CFS-APQ) Data From Visual Analog Scales (VASs) for Pain, Fatigue, and Ability to Concentrate (n=47)a

 
Content Validity

In response to the request to list at least 5 activities or tasks that have become difficult to do, 47 participants with CFS reported 183 answers. Table 7 presents all different answers and corresponding frequencies. One hundred fifty-seven answers (85.8%) matched the content of the CFS-APQ. "Work" was mentioned most frequently (21 times [11.5%]), even more than "housekeeping" (17 times [9.3%]), "walking" and "reading" (both 13 times [7.1%]), and "sports" and "driving a car" (both 11 times [6.0%]). Seven subjects (3.8%) mentioned "sleeping," indicating it was the ninth most prevalent item in this inquiry. In the ICIDH beta II taxonomy, however, "sleeping" is classified as a mental function and is therefore not suitable for a measure that assesses activity limitations and participation restrictions.


View this table:
[in this window]
[in a new window] 		Table 7. Content Validity of the Chronic Fatigue Syndrome–Activities and Participation Questionnaire (CFS-APQ) Data Based on Activities and Tasks That Became Difficult to Do as Reported by 47 Patients With Chronic Fatigue Syndrome

 
Using the ICIDH beta II draft, 5 out of 26 questions of the CFS-APQ were found to measure different aspects of the participation level. These items were: "caring for a child," "sports and active recreation," "functioning at school/at work," "working full-time/attending school full-time," and "social activities." All other items were related to the activity-domain (Tab. 8.)


View this table:
[in this window]
[in a new window] 		Table 8. Content Validity of the Chronic Fatigue Syndrome–Activities and Participation Questionnaire (CFS-APQ) Data Using the International Classification Impairments, Disability and Health (ICIDH) Beta II Drafta

 

   Discussion and Conclusions
Top
 Abstract
 Introduction
 Method
 Results
 Discussion and Conclusions
Appendix
 References
 
Reliability

The results of the test-retest reliability analysis, we contend, indicate what we would consider sufficient reliability, especially for overall scores (ICC=.96 for total score 1, ICC=.95 for total score 2) and the total scores on each question (apart from questions 6, 9, and 18, the ICCs were >.80). We conclude, therefore, that nearly all individual items of this new questionnaire can be presented with sufficient reliability, particularly because the correlation coefficients were similar to those obtained for the VASs for pain, fatigue, and ability to concentrate. The imperfect (ie, not suggesting a linear correlation: R=1) reliability coefficients for these symptoms suggest the difference in test-retest scores might be caused by a change in health status (reflected by the change in severity in pain, fatigue, and ability to concentrate). Cognitive disturbances (eg, processing speed, impaired working memory, poor learning of information, improper divided attention) appear to be characteristic for patients with CFS18,19 and, therefore, may prevent a higher degree of reliability for self-assessment tools in this population.

Validity

The Cronbach alpha coefficient was .94 for both total scores, suggesting a high degree of consistency among the different items. Consequently, the calculated Cronbach alpha coefficients suggest that the different items of the questionnaire measure the same construct. These data suggest sufficient convergent validity of the CFS-APQ scores with VASs for pain and fatigue. The way the CFS-APQ was constructed, based on self-reported activity limitations and participation restrictions of 141 patients with CFS,6 supports disease specificity and content validity of this new measure. In our study, 47 participants with CFS reported 183 activities that became difficult to do, and the majority (157 responses [85.8%]) complied with the content of the questionnaire (ie, the answers were found in the content of the questionnaire). The most prevalent of the nonfitting items ("sleeping") is a mental function, indicating it is unsuitable for a questionnaire aiming at monitoring activity limitations and participation restrictions. These results, we believe, substantiate the comprehensiveness of item selection. In addition, reviewing the content of the CFS-APQ in the ICIDH beta II draft indicated that this measure focuses on the activity and participation domains.

Our results should be interpreted with some caution. The reliability and validity of scores obtained with the Dutch version of the CFS-APQ were assessed. No data are currently available to validate the English version of this questionnaire (Appendix). The patients included in this study and those used for constructing the questionnaire6 all fulfilled the current American definition for CFS.1 Because of the assumed socioeconomic similarities between American and Belgian citizens, the content of the Dutch CFS-APQ is likely to represent the most frequent activity limitations and participation restrictions in American patients with CFS. In Belgium, where predominantly Dutch- and French-speaking patients are seen in practice, validation of an English version is nearly impossible. We hope our report will convince English-speaking physical therapists that it would be worthwhile to investigate psychometric properties of the English version of the CFS-APQ.

Our questionnaire was designed to establish goals and outcomes of physical therapists' interventions. Our reliability data were obtained using mail for response. Therefore, we cannot be sure that each participant completed the questionnaire without assistance or within 24 hours following the first scoring. This is particularly relevant because a number of questionnaires were returned up to 5 weeks after the participants' initial visit to the Chronic Fatigue Clinic. The use of a mailed questionnaire may have biased the reliability analysis because we may have collected data from only highly motivated respondents. Our sample was derived from consecutive patients and was not randomly selected, and this also is a limitation. The questionnaire was given to all patients with CFS visiting the Chronic Fatigue Clinic of the Vrije Universiteit Brussels; therefore, we relied on a sample of convenience. Selection bias can only be due to appointment scheduling, but our secretaries were blinded to patients' medical records when they scheduled the appointments. Moreover, a sample consisting of consecutive patients visiting a clinic is more likely to represent routine clinical practice.

Our combining 2 different, but related, constructs (the satisfaction and importance verification of each question) might be inappropriate. Because the total score 1 as well as total score 2 (using only the satisfaction part) appeared to be reliable and valid, we believe this is unlikely to be the case. Likewise, the assumption that all item scores of the CFS-APQ can be summed together implies to us that activity limitations and participation restrictions represent a one-dimensional construct. The revised World Health Organization's classification system (the ICF) combines activities and participation into one health-related domain. This indicates to us that the data from the CFS-APQ represent a one-dimensional construct. However, dimensionality studies of the CFS-APQ that can support the interpretation of a one-dimensional construct are currently lacking.

In general, self-report questionnaires offer an easy and inexpensive way to monitor the patient's perceived quality of life, which cannot be offered by laboratory data.20 In our opinion, the use of this self-administered assessment tool, which acknowledges that people value things differently, should enable physical therapists to treat the patient rather than treating the disease. Indeed, we believe that because of the incorporation of importance verification to each item in the questionnaire, this health status instrument can be used to monitor at least a part of patients' expectations for care.

Our data, we argue, provide evidence for sufficient test-retest reliability, convergent validity, and 2 different aspects of content validity of the scores obtained with the Dutch CFS-APQ. Before the use of this new measure in clinical practice and research settings can be advised, however, responsiveness and other forms of validity should be examined.


   Appendix
Top
 Abstract
 Introduction
 Method
 Results
 Discussion and Conclusions
 Appendix
References
 


Figure 1
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[in a new window] 		Appendix. Content of the Chronic Fatigue Syndrome—Activities and Participation Questionnaire (CFS-APQ) In English

 


   Footnotes
 
Mr Nijs, Dr Vaes, and Ms Van Hoof provided concept/research design. Mr Nijs, Dr Vaes, Dr McGregor, and Ms Van Hoof provided writing. Mr Nijs and Dr De Meirleir provided data collection, and Mr Nijs, Dr Vaes, and Dr McGregor provided data analysis. Mr Nijs provided project management, and Dr De Meirleir provided fund procurement, subjects, facilities/equipment, and institutional liaisons. Dr Vaes, Dr McGregor, and Ms Van Hoof provided consultation (including review of manuscript before submission).

* The English version of the CFS-APQ is shown in the Appendix. The Dutch version of the questionnaire (as well as a French version) is available upon request from the corresponding author. Back


   References
Top
 Abstract
 Introduction
 Method
 Results
 Discussion and Conclusions
 Appendix
 References
 

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