Introduction
Systemic sclerosis (SSc), or scleroderma, is a multisystem autoimmune inflammatory disorder. The disease is characterized by microvascular damage and increased deposition of collagen and other matrix molecules in skin and organ systems [
1]. SSc is more prevalent among women, with a female-to-male ratio of 4.7:1 [
2]. The clinical course can vary from limited skin thickening to severe organ damage such as pulmonary fibrosis or pulmonary arterial hypertension. The disease can be divided into two subtypes depending on the extent of fibrotic skin involvement: limited cutaneous SSc (lcSSc) and diffuse cutaneous SSc (dcSSc) [
1]. There is currently no cure for SSc; thus, treatments focus on reducing disease manifestations and improving health-related quality of life (HRQL) [
3].
Depression is more common in SSc patients than in patients with other rheumatic diseases [
4,
5]. Symptoms of depression occur in approximately one-third to two-thirds of patients with SSc [
6], depending on which questionnaire is used and whether or not the prevalence of depression is based on valid interview methods [
7]. Disease-specific symptoms such as reflux, constipation, dyspnea, digital ulcers, pain, fatigue, and changes in physical appearance are associated with negative emotions in SSc [
4]. Depressive symptoms are associated with poor HRQL [
8]. Patients with SSc who have depressive symptoms are also reported to be less physically active than those without depressive symptoms [
9]. Further, depressive symptoms are associated with lower self-efficacy and reduced likelihood of adopting health-promoting behaviors [
6,
10].
Depressive symptoms are important to detect and address in patients with SSc. One possible way to capture symptoms of depression is to use patient-reported outcome measures (PROMs) [
4]. The Patient Health Questionnaire-9 (PHQ-9) is one such PROM that is reliable and valid in SSc in for example English [
11,
12]. The basis of the items in the PHQ-9 is also equivalent to the criteria for depression [
13]. A slightly shorter version, the PHQ-8, also exists, wherein the ninth item (thoughts of self-harm and death) is omitted [
13]. A high correlation between the PHQ-9 and the PHQ-8 has been found in patients with SSc, and the PHQ-8 is preferred in SSc [
14]. Thus, a Swedish version of the PHQ-8 for patients with SSc was of interest for the present study. Health professionals may use the PHQ-8 to detect and facilitate communication about symptoms of depression [
15‐
17], support self-management of these symptoms, and refer patients to the appropriate healthcare provider.
The PHQ-8 in Swedish has not been psychometrically evaluated in SSc. However, a linguistic validated version of the PHQ-9 in Swedish can be found on the Pfizer website [
18]. A Swedish version of the PHQ-9 has support for internal consistency and concurrent validity among patients with affective disorder diagnoses; meanwhile, high internal consistency and structural validity among patients with self-reported depression have been reported [
19,
20]. To determine the quality of PROMs, their measurement properties in the target population need to be studied and the ability of patients to properly understand questions about symptoms of depression is of importance [
21]. The present study aimed to investigate different aspects of the validity and reliability of the PHQ-8 in Swedish for individuals with SSc.
Results
Of the total of 101 patients, most had lcSSc and, at the median, mild disease severity in their peripheral vascular system, as well as normal heart and kidney systems (Table
1). The patients (
n = 90) participating in the testing of aspects of construct validity and reliability had, at the median, mild disease severity of the lung system. The patients (
n = 11) in the assessment of content validity had, at the median, moderate disease severity of the lung system and, at the median, greater skin involvement than those in the assessment of construct validity and reliability (Table
1).
The PHQ-8 Swe total score was at median 6 (interquartile range (IQR): 2–12; n = 11), the PHQ-8 Swe total score for n = 90 patients see aspects of reliability. Of the patients (n = 90) who completed the PHQ-8 Swe, 53% had no significant depressive symptoms, 30% had mild symptoms, 15% had moderate symptoms, 1% had moderately severe symptoms, and 1% had severe symptoms. The final item in the PHQ-8 (not included in the total score), assessing the difficulties of symptoms of depression in different daily life situations, was, at the median, in the first measurement occasion 1 (i.e., “somewhat difficult”; min–max: 0–3; n = 81) and at retest 1 (i.e., “somewhat difficult”; min–max: 0–2; n = 72). There were no statistical changes over time (p = 0.84).
Content validity and linguistic adjustments
The results of the evaluation of the content validity of the PHQ-8 Swe are presented in the domains of comprehensibility, relevance, and comprehensiveness [
21], with illustrative quotations in Table
6. Overall, the PHQ-8 Swe was experienced as being easy to understand, relevant in item content, and covering important aspects of depression in SSc. However, the following main changes to the PHQ-8 Swe were carried out to boost understanding: The tenses in items 1, 3, 5, 6, and 7 were altered to maintain the same tense throughout all items. Further, item 1 was changed from “little interest” to “felt less interest” and in item 8 the words “could have” were added to prevent misunderstandings in the Swedish language. This change in item 8 is in line with the English original [
18] and information from Kurt Kroenke (personal communication, 2017). Finally, the last item (not included in the total score) was clarified. Table
4 contains the PHQ-8 Swe for individuals with SSc.
Table 6
Content validity of the Patient Health Questionnaire-8 in Swedish for individuals with systemic sclerosis
Comprehensibility | The instruction, items, and response options were generally experienced as easy to understand. However, the fact that several items addressed multiple aspects was experienced to be challenging, as was estimating the number of days that each response option referred to. Some linguistic difficulties were expressed; for example concerning tense, “little interest” (Item 1) could possibly be understood as “having some interest,” while “hopeless” (Item 2) could possibly be interpreted to mean “hopeless as a person.” HPs expressed that some items could be perceived as emotionally demanding for patients, especially those with recent disease onset, and that the PHQ-8 was problem-based, not possibility-based, and could thus upset patients. Moreover, the title of the questionnaire does not clearly express what the PHQ-8 aims to assess, which may make the purpose of the questionnaire unclear. Overall, however, the PHQ-8 was experienced to be appropriate, with a suitable amount of items and response options. HPs expressed that any patient could complete PHQ-8 as long as the questionnaire is carefully introduced and responses concerning frequent symptoms of depression are followed up by HPs. Below are some experiences in quotations: “They [the items] are so concrete. I know exactly what to think about: my mental health in the last 14 days” (P7) “They [the response options] can be confusing… I don’t know what the difference is between ‘several days’ and ‘more than half of the days’” (P10) “ [difficult to understand]… this item, item 6, several questions are included in that item” (P4) “Poor appetite or eating too much [in Item 5]—what is ‘too much’?” (HP1) “Extra-demanding items, which I think can arouse patients’ emotions, like Item 2; there’s a sense of hopelessness. And Item 6: feel bad about yourself or that you’re a failure or have let yourself or your family down. I think that those [items] can be a little more emotionally demanding” (HP2) |
Relevance | The time frame, the previous two weeks, was described as being appropriate, and the items were experienced as relevant and not redundant. Items 3, 4, 5, 7, 8 and the final item (not included in the total score) were described as possible referring to somatic symptoms or consequences of SSc other than depression. Here are some quotations: “I think that it’s good. It deals with daily things, and how they are, both eating and sleeping and how to feel” (P1) “No [no need to remove items]. It [PHQ-8] is also very descriptive of how you could feel” (P4) “In Item 3, to be tired is associated with the disease. You do feel [tired] almost every day; for example, I used to rest every day” (P4) “Moving slowly [in Item 8] is what we often experience at the clinic. It’s quite obvious... speaking slowly is maybe something that I do not associate… I can’t say that I noticed that patients were too slow in that way… I don’t know whether I think that it’s relevant” (HP1) |
Comprehensiveness | Key symptoms of depression in SSc were described to be covered by the questionnaire. Some HPs expressed uncertainty in assessing depression, and some patients expressed limitations in personal experiences with depression. Still, items were suggested to be added, such items could cover tearfulness, meaning of life, thoughts about death or the future, demanding situations (e.g., loneliness, physical limitation, and limitations in activities), self-management strategies, and treatment adherence. Below are some quotations: “I think that they [the items] sum it up very well, everything, yes” (P9) [Suggesting adding an item] “Being diagnosed with this disease isn’t fun. If someone reads about it [the diagnosis] online, it could make them really depressed….But otherwise I think that they [the items] cover it” (HP6) |
Construct validity
Structural validity: The CFA for the single factor had a near “reasonable” fit with fit indicators: AIC 97.3, CFI 0.891, RMSEA 0.128, and CMIN/DF 2.47. The two-factor model provided a better fit for the data, revealing an “acceptable” fit and AIC 81.5, CFI 0.953, RMSEA 0.086, and CMIN/DF 1.66.
Hypotheses testing for construct validity: Convergent validity was supported by strong correlations between the PHQ-8 Swe and the assessment of pain (HAQ-DI VAS); fatigue (MAF); and physical role function, bodily pain, vitality, social function, and mental health (RAND-36). Moderate correlations were found between the PHQ-8 Swe and disability (HAQ-DI); gastrointestinal symptoms, lung symptoms, Raynaud’s phenomenon, digital ulcers, and overall disease severity interference with daily activities (SHAQ VAS); physical function, general health, and emotional role function (RAND-36); and disease severity of the lung system (MSS) (Table
7). Divergent validity was obtained with weak correlations between the PHQ-8 Swe and skin involvement (mRSS); disease severity of peripheral vascular, heart, and kidney systems (MSS); and disease duration (Table
7).
Table 7
Construct validity (correlations) of the Patient Health Questionnaire-8 in Swedish for individuals with systemic sclerosis
Patient-reported outcome measuresA |
Disability, pain, and disease interference with daily activities, SHAQ |
HAQ-DI | 0.63 | <0.001 |
HAQ-DI VAS |
Pain | 0.70 | <0.001 |
SHAQ VAS |
Gastrointestinal symptoms | 0.50 | <0.001 |
Lung symptoms | 0.48 | <0.001 |
Raynaud’s phenomenon | 0.41 | <0.001 |
Digital ulcers | 0.51 | <0.001 |
Overall disease severity | 0.64 | |
Fatigue, MAF | 0.74 | <0.001 |
Health related quality of life, RAND-36 |
Physical function | − 0.50 | <0.001 |
Physical role function | − 0.67 | <0.001 |
Bodily pain | − 0.72 | <0.001 |
General health | − 0.58 | <0.001 |
Vitality | − 0.80 | <0.001 |
Social function | − 0.76 | <0.001 |
Emotional role function | − 0.62 | <0.001 |
Mental health | − 0.67 | <0.001 |
Disease variablesB |
Skin involvement, mRSS | 0.19 | 0.072 |
Disease severity, MSS |
Peripheral vascular system | 0.20 | 0.061 |
Lung system | 0.39 | <0.001 |
Heart system | − 0.07 | 0.534 |
Kidney system | − 0.19 | 0.072 |
Disease durationa | 0.07 | 0.547 |
Aspects of reliability
In terms of internal consistency, the Cronbach’s alpha was 0.85 and corrected item-to-total correlation had a median of 0.61 (min–max: 0.41–0.76;
n = 87). Of the 90 patients who completed the PHQ-8 Swe on the first test occasion, 81 of them responded to it on the retest occasion. The median of the total score of the PHQ-8 Swe was 4 on the test occasion (IQR: 2–9;
n = 89) and also 4 on the retest (IQR: 1–7;
n = 81). The ICC was 0.83 (
n = 81) for the total score, and the weighted kappa coefficient had a median of 0.72 for the items (min–max: 0.60–0.79). There were no significant differences between test occasions in the total score (
p = 0.15) or in seven of the eight items (Table
4).
The total score had no floor or ceiling effects (n = 89).
Discussion
This study evaluates aspects of validity and reliability of the PHQ-8 Swe for individuals with SSc. The results indicate that content validity was satisfactory overall; however, some items could be interpreted as not only related to a depressive symptom but also covering somatic symptoms related to SSc. Further, based on the interviews, some linguistic adjustments were performed. The CFA revealed a better fit for the two-factor model than the one-factor model. The PHQ-8 Swe for individuals with SSc correlates more to self-reported disability, pain, disease interference with daily activities, fatigue, and HRQL than to disease severity assessments except for a moderate association with lung disease severity. Internal consistency and the test–retest reliability of the PHQ-8 Swe total score were sufficient and there were no floor or ceiling effects.
In terms of content validity, items were expressed as generally relevant and easy to understand, though some linguistic adjustments were made to the PHQ-8 Swe to increase the understanding for individuals with SSc. Further, some HPs experienced a fear of upsetting patients due to the potentially emotionally demanding items. In general, this consideration among HPs is probably unnecessary because individuals with SSc are likely to exhibit depressive symptoms during the disease course [
42], which is of important to capture.
Some items in the PHQ-8 Swe were found to cover symptoms or problems possibly attributed to the somatic symptoms of SSc. When patients (
n = 90) completed the PHQ-8 Swe, items 1 (interest/pleasure), 3 (sleep), and 4 (tired/little energy) held the highest median scores, while items related to sleeping problems and tiredness cover symptoms that can be attributed to somatic symptoms of SSc [
43]. Strong correlations were found between the PHQ-8 Swe and fatigue and vitality; others have found similar results [
12]. Fatigue could be related to somatic symptoms but on the other hand, fatigue is also a part of the core criteria for depression.
Our interviews indicated that it was difficult to estimate the verbal response options in the PHQ-8. In some previous studies, the verbal response options were changed to the exact number of days, but the original verbal setting has stronger validation data [
13]. Items that were suggested for inclusion in the PHQ-8 Swe involved meaning of life, demanding situations for mental health, and self-management strategies. Psychosocial support [
42] and support for self-management strategies such as physical exercise are important to these patients [
44]. However, the PHQ-8 consists of criteria for depression, and it would be problematic to include items beyond these criteria.
Construct validity by structural validity of the PHQ-8 Swe showed a nearly “reasonable” fit with a one-factor solution, while the two-factor model was considered to have an “acceptable” fitting model and provide a better fit to the data. The authors have not found any results in terms of structural validity regarding the PHQ-8 in patients with SSc, though the PHQ-9 previously confirmed both a single- and two-factor model without substantive differences between them [
11].
Hypotheses testing for construct validity revealed weak correlations between the PHQ-8 Swe and skin involvement as well as the objectively assessed disease severity of peripheral vascular, heart, and kidney systems. This suggests divergent validity and that the PHQ-8 Swe does not capture these somatic aspects of the disease in our sample. Similar results, including physician-rated disease severity, have been described in previous studies on the PHQ-9 in SSc [
12,
34]. One reason for the low correlations between the PHQ-8 Swe and disease severity could be that the disease severity of the assessed organ systems in the included sample was, at the median, mild or normal. However, the moderate association between the PHQ-8 Swe and the lung system indicates that lungs were more affected than the other assessed organ systems in our sample. Different associations between disease manifestation of the lung system and depressive symptoms have been described in SSc [
4,
8], but to our knowledge, no strong associations have been presented [
8]. Findings supporting convergent validity revealed moderate-to-strong correlations between the PHQ-8 Swe and disability, pain, disease interference with daily activities, fatigue, and HRQL; these results were comparable to those of previous studies on the PHQ-9 in SSc [
12,
34]. There were strong correlations between the PHQ-8 Swe and pain/bodily pain, fatigue/vitality, physical role function, social function, and mental health, indicating that the PHQ-8 Swe for individuals with SSc reflects both physical and mental aspects. The medical treatment indicates that pain was a problem in our sample, which the strong correlation between pain and PHQ-8 Swe also implies, an association in agreement with previous reports [
45]. Our results align with those of other studies indicating that symptoms of depression are associated with decreased HRQL more than with organ manifestations that may be life-threatening [
8]. However, the relationships are complex; living with depressive symptoms can influence the person’s experienced life situation and, thus, may influence the completion of PROMs.
Although the results (
n = 90) of the assessments of the MSS and the medical treatment may indicate severe disease for a number of patients in our sample and that patients also could have other rheumatic diseases and comorbidity, such as cardiovascular diseases, only 17% of the patients had at least moderate symptoms of depression on the PHQ-8 Swe. An earlier study of PHQ-8 in SSc has shown that 26% had at least moderate symptoms of depression which is somewhat higher than in our sample [
14]. However, there are more patients in percent, in our study, with at least moderate symptoms of depression compared to the general population in Sweden and the USA [
24,
46]. Nevertheless, due to the risk for overestimation when using PROMs, a diagnosis of depression must be confirmed by validated diagnostic interviews [
7].
A sufficient internal consistency was found, and these results are comparable with those from the PHQ-9 in patients with SSc [
11,
12]. The ICC confirmed sufficient test–retest reliability. To the best of our knowledge, the test–retest reliability of the PHQ-8 has not been assessed previously in SSc. Kroenke et al. [
13] assessed the PHQ-9 for test–retest reliability in a primary care setting and found a strong association between the test occasions. The agreement between the items in our study in the test–retest was moderate to substantial [
41], though a significant difference was obtained in item 3 in the test–retest procedure. The latter might be the result of fluctuations in trouble falling or staying asleep. However, the difficulties in the symptoms of depression manifesting in daily life did not differ between test occasions, implying stability in the consequences during the testing period. Thus, the test–retest reliability of the PHQ-8 Swe for individuals with SSc is satisfactory for the total score.
One limitation of our study is that convergent validity was not tested with another instrument assessing depression, such as the Center for Epidemiologic Studies Depression Scale [
12]. This was not feasible because no questionnaires assessing depression have been psychometrically tested in Swedish among individuals with SSc. However, we found a strong association between the PHQ-8 Swe and mental health in RAND-36. Another limitation is that we did not evaluate the associations between the PHQ-8 Swe and all organ systems in the MSS as well as to comorbidities such as cardiovascular diseases. Nevertheless, among patients with comorbidities, there were almost equally amount that reported
no significant depressive symptoms (PHQ-8 scores 0–4) as depressive symptoms of different severity (PHQ-8 scores 5–24) (data not shown). A further limitation is that approximately half of the patients scored no significant symptoms of depression during the latest two weeks, though they could have experienced depressive symptoms earlier [
42]. On the other hand, one-third of the patients scored mild symptoms of depression, while one-sixth had moderate-to-severe symptoms of depression.
In conclusion, in this psychometric study with in majority individuals with lcSSc, the content validity was satisfactory the reliability was sufficient and there were no floor or ceiling effects. The PHQ-8 Swe was more strongly associated with self-reported disability, pain, disease interference with daily activities, fatigue, and HRQL than to disease severity assessments, except for a moderate association with lung disease severity. As health professionals struggle to support patients with SSc in self-management, identifying symptoms of depression by the PHQ-8 Swe could be one of several means. Future studies in SSc on other aspects of validity, such as investigating the PHQ-8 Swe’s ability to discriminate between patients with a confirmed diagnosis of depression by validated interviews and those without a diagnosis, are needed.
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