Background
A patient with Advanced Chronic Kidney Disease (ACKD)(defined, according to the international guidelines, as chronic kidney disease at stage 4 or 5 with a glomerular filtration rate < 30 ml/min) has an increased burden of physical and psychological symptoms that are associated with a poor health-related quality of life (HRQoL) [
1‐
3]. Symptoms such as tiredness, pruritus, constipation, pain, sleep alterations, anxiety, dyspnoea, nausea, restless legs and depression have a high prevalence in these patients [
4,
5]. As uncontrolled symptoms at the end of life lead to greater suffering, their treatment in the advanced phases of the disease is a priority [
6].
Patient reported outcome (PRO) instruments are generally used to assess patients’ functional status, quality of life and symptoms [
7]. This is especially important in patients with chronic diseases such as chronic kidney disease (CKD), in which PRO measures can be used to evaluate, monitor, and facilitate the introduction of interventions [
8].
The use of validated scales may therefore be useful for standardising the evaluation of symptoms and monitoring the healthcare outcomes of these patients [
9]. Because of the variety of symptoms that might be suffered by a kidney patient, tools that can evaluate a wide range of symptoms are recommended. In addition, it is useful to have instruments that can be administered pre- and post-dialysis start, in order to better monitor patients as they progress through CKD stages. Among the tools most used for the study of symptoms in kidney patients are questionnaires that are not specific to renal patients, such as the Memorial Symptom Assessment Scale-Short Form(MSAS-SF) [
10], questionnaires modified for use with patients on dialysis, such as the Edmonton Symptom Assessment System (ESAS) [
11], and other specific questionnaires that have been developed to measure symptoms in ACKD, such as the Dialysis Symptom Index (DSI) [
12] and the Palliative care Outcome Scale-Symptoms Renal (POS-S Renal) [
13].
There are no specific symptom assessment questionnaires for CKD patients that have been adapted to the Spanish culture, Spanish being one of the most commonly spoken languages in the world and that enable us to evaluate the intensity of the various symptoms in this population [
14]. The POS-S Renal is a questionnaire that has been designed to evaluate symptoms in ACKD, and it has demonstrated its usefulness in clinical practice and in research [
13,
15]. Thus it is a tool recommended for the evaluation of symptoms in this population [
9,
16,
17]. There are at present no studies that have assessed the psychometric properties of this questionnaire.
The aims of this study were to carry out a cross-cultural adaptation of the POS-S Renal into Spanish and to perform an analysis of test-retest reliability, internal consistency, internal structure, and concurrent criterion-related validity with the MSAS-SF, a previously validated PRO measure which has been used in patients with ACKD [
18‐
20].
Discussion
The translation and cross-cultural adaptation of the POS-S Renal into Spanish using recognised international guidelines was achieved satisfactorily. To our knowledge, this study is the first cross-cultural adaptation and psychometric analysis of the POS-S Renal. The study sample included dialysis and conservatively managed patients. Although the POS-S Renal is an instrument to measure symptoms in conservatively managed ACKD patients, in our experience it has been a useful clinical assessment tool for the dialysis population. We tested responses from patients in these two categories, and the results indicated that there were no significant differences between the two groups, although a trend towards a significant difference in the responses between the groups for factor 1 was found.
Excellent content validity of the Spanish modified version was demonstrated, which indicates that all items of this instrument are relevant for the measurement of the symptomatology in ACKD [
23,
24]. This instrument showed satisfactory psychometric characteristics in terms of reliability [
27], structural validity [
31], and concurrent criterion-related validity [
32]. The sample size was adequate for all analyses [
33].
We added ‘cramps’ to the original 17-item POS-S Renal because of the frequency of cramps among patients with ACKD [
4,
5]. Although in our study the prevalence of cramps was lower than has been found in other studies on the Spanish population, this is a common symptom that is often under-evaluated and is an important cause of the early termination of dialysis sessions [
34,
35].
The test–retest reliability was high (r = 0.887 to total score), with values beyond those found in the modified ESAS in dialysis patients (r = 0.7 to total score) [
11].
Internal consistency analysis indicated a satisfactory degree of interrelatedness among the items of the instrument [
27]. Factor 1(α = 0.78) and factor 2 (α = 0.56) showed the highest and lowest results, respectively, with values below those found in the Arabic translation and modification of the DSI (Cronbach’s α = 0.91 overall) [
36].
The two-factor solution obtained in the EFA accounted for a significant proportion of variance and showed support for the presence of construct validity, which provides support for evidence of the instrument validity. The factor analysis performed to evaluate the internal structure of the POS-S Renal confirmed the two-factor model, in line with other symptom assessment tools [
25]. However, we performed a CFA, and the fit indices of the CFA model were satisfactory [
29]. Nonetheless, two POS-S Renal items, namely, restless legs and changes in skin, did not load on either of these two factors. Consequently, it appears that these symptoms function as individual items. A possible explanation for this is that these two symptoms are best regarded as ‘causal indicators’ not ‘indicator variables’, and this suggests inherent problems with the application of the factor analysis [
37]. Causal indicators are variables external to the patient that could affect symptoms, and indicator variables are unobservable variables which can be inferred from scores on multiple self-report items. Thus, the inclusion of indicator variables in factor analysis could lead to uninterpretable results [
37,
38].
The concurrent criterion-related validity analysis with the MSAS-SF was supported by a strong correlation with factor 1 and total score, and a fair correlation with factor 2, which provides support for evidence that the scores of the POS-S Renal are an adequate reflection of a previously validated instrument that has been widely used in patients with ACKD. These results are in line with those found by the authors of the Arabic translation of the DSI and the modified ESAS with the Kidney Dialysis Quality of Life-Short Form (KDQOL-SF) [
11,
36].
The extent and severity of the symptom burden in patients with ACKD demonstrates the importance of using an appropriate clinical tool [
4,
5,
13,
15]. Likewise, the multidimensional nature of the symptoms should be considered in these patients [
39]. The POS-S Renal is a validated instrument that provides information about a large number of physical and psychological symptoms. This also demonstrates the value of POS-S Renal as a useful and clinically appropriate symptom assessment tool to facilitate a multidimensional symptom evaluation in ACKD.
A patient with many mild symptoms can have a score identical to a patient with fewer, but more distressing symptoms, and a high score on even a single symptom could indicate a high level of patient distress for one symptom even if the patient’s total score is low. Thus, clinicians must look at the patient’s score on each of the 18 Spanish modified version of the POS-S Renal symptoms on its own, even though a total score can be obtained.
This study provides access to a PRO instrument to assess symptoms in CKD stage 4–5 for Spanish speaking populations. This single-page 18-item PRO is a self-administered questionnaire that is comprehensible and easy to complete. Hence, this study shows that this instrument will be of value in the symptom assessment of patients with ACKD in clinical practice and in research. Although this study contributes by filling the knowledge gap on the validation and usefulness of POS-S Renal, as well as, on symptoms assessment in ACKD, subsequent studies should be carried out to develop more refined measuring tools in this area [
40].
The limitations to consider in this study include the lack of longitudinal data concerning other psychometric characteristics. Although our study shows a preliminary analysis of the psychometric properties of a modified POS-S Renal, sensitivity to change and minimally important differences were not evaluated. Further work using longitudinal data to determine responsiveness is needed to define the use of these measures in cohort studies and clinical trials that evaluate interventions. Finally, given the lower internal consistency for factor 2, a confirmatory factor analysis in a wide sample should be carried out to explore and interpret the behaviour of this factor.
Acknowledgements
We would like to thank the Cudeca Foundation and the Nephrology Department at the General University Hospital of Carlos Haya of Málaga for their contribution to the development of this project.