Introduction
Multimorbidity, the co-occurrence of two or more chronic diseases, is a condition that affects up to 95% of patients with chronic kidney disease (CKD) [
1]. The term multimorbidity, however, does not specify the disease types and distribution, which are more informative of the impact these diseases have on people’s health. Social determinants of health, such as health literacy [
2], may help to determine the patterns of disease distribution in multimorbid patients with CKD, thereby supporting the development of strategies to approach these patients more efficiently.
Multimorbidity is a major challenge for patients with CKD [
1], and a more comprehensive understanding of this phenomenon is needed to adapt healthcare systems to their needs [
3]. Among CKD patients, multimorbidity is the single highest independent predictor of all-cause mortality [
4], and it is an even more immediate concern than kidney function in the management of CKD, particularly during asymptomatic stages [
5]. However, current healthcare systems and medical guidelines are generally focused on a single disease, resulting in fragmented and inefficient care, which fails to meet the challenges of multimorbidity [
6]. To support the shift of healthcare systems to a more comprehensive model, we need to better understand the factors that determine the number and distribution of diseases in multimorbid patients [
7].
Low health literacy is an important determinant of health due to its negative effects on chronic diseases [
2], and it may be one of the determinants of multimorbidity among CKD patients. Health literacy is defined as the degree to which individuals have the capacity to obtain, process, and understand basic health information and services needed to make appropriate health decisions [
8]. Low health literacy is associated with CKD development and complications [
9,
10], mainly because individuals with low health literacy lack the self-management skills required for adequate CKD control [
2,
11]. This lack of skills may be especially problematic when the patient presents multimorbidity, since the co-occurrence of other diseases adds to the burden of CKD, making it more difficult to manage [
12].
Low health literacy may predict not only the presence of multimorbidity but also its patterns, i.e., the combination of co-occurring diseases. Multimorbidity is more complicated than just a sum of diseases, and the need to understand its patterns is well recognized [
13]. The impacts of multimorbidity on CKD patients may vary according to the groups of diseases that occur together [
14]. Recent research has identified some determinants of multimorbidity patterns, such as age and sex [
15], but no study has directly addressed the association between these patterns and low health literacy. A better understanding of these associations may help to optimize care for CKD patients with low health literacy.
This study aims to assess the association between health literacy and the prevalence and patterns of multimorbidity in CKD patients, overall and divided by age and sex.
Discussion
This study showed that CKD patients have a high prevalence of comorbidities. Patients with low health literacy, especially when older, were more likely to have an even higher number of comorbidities. Moreover, for both levels of health literacy, the patterns of multimorbidity were similar, in the total sample and by sex. For the subgroup of older patients, however, those with low health literacy had a higher prevalence of most comorbidities, with a relatively greater share of cardiovascular, psychiatric, and central nervous system diseases.
Low health literacy is a strong independent indicator of the number of comorbidities among CKD patients. The association between low health literacy and comorbidities showed a dose–response gradient and was independent of age, sex, lifestyle, and CKD severity. This finding reinforces the robustness of the results, and the association most likely stems from the less effective health behaviors and self-management skills of patients with low health literacy [
2]. These patients, thus, develop more comorbidities, which may negatively impact the prognosis of CKD. Our results contribute to the body of literature on the association between low health literacy and multimorbidity, which has until now shown mixed results [
23,
24]. Our study underlines the negative effects of low health literacy on multimorbidity, with a much larger sample and a more comprehensive set of comorbidities than in previous studies.
Interestingly, the patterns of multimorbidity were similar between patients with low and adequate health literacy in the total sample and when divided by sex, and only slightly different in older patients. These similar patterns indicate that, even though patients with low health literacy have more comorbidities than their adequate health literacy counterparts, the combination of comorbidities is roughly the same. This means that support of multimorbid CKD patients with low health literacy need not focus on different groups of diseases, but rather on general measures related to CKD care.
Among older adults, patients with low health literacy showed a higher prevalence of most comorbidities, which could be explained in two ways. First, patients with low health literacy have worse health behaviors and self-management skills, leading to aggravation of existing diseases and the development of new comorbidities, which accumulate over the life course [
2]. A second explanation is that patients with low health literacy have poor access to care [
25]. This could be due to a lack of resources to access care, or to a lack of knowledge to adequately use the available care. In either case, the result is a suboptimal use of medical resources for treating existing conditions and preventing new ones. The apparent differences in associations with health literacy by age suggest that the negative effects of low health literacy develop over time, being less in younger patients but more evident among older ones.
We found that patients with CKD had a high prevalence of specific comorbidities, which could overburden patients and negatively impact health outcomes. In the multimorbidity patterns studied, the most prevalent disease domains regarded gastrointestinal, endocrine, and cardiovascular diseases. These diseases are already known for co-occurring with CKD, given that they share various risk factors [
26,
27]. The presence of multiple comorbidities increases the overall disease burden for patients [
28]. Associated with this, it may lead to more polypharmacy with consequent poorer medication adherence, to increased psychological distress, and to a worse quality of life [
1,
29]. Furthermore, the presence of these comorbidities could accelerate eGFR decrease, either because combining different treatments becomes more intricate for patients, or because gastrointestinal, endocrine, and cardiovascular disease might be in the causal pathway toward CKD [
1,
19,
30].
Strengths of this study include its large population-based sample, with a wide selection of physical and mental chronic diseases, diagnosed by a combination of subjective and objective methods, using an internationally accepted coding system, and reported by age and sex. Furthermore, we employed latent class analysis, a sophisticated analysis technique to capture the heterogeneity in disease distribution. Latent class analysis is a person- rather than disease-oriented technique, providing results that meet current guidelines for patient-centered research and care.
Some limitations of our study should also be taken into account. First, we performed a cross-sectional analysis, which did not allow us to study whether multimorbidity patterns and their association with low health literacy change over time. Second, the health literacy questionnaire which we used is a self-report instrument focusing mainly on functional health literacy. This may have led to underestimations of low health literacy due to self-report, thereby reducing the power of our analysis. Nonetheless, our instrument is a validated tool that has been used in other studies [
31]. Third, data on albuminuria were missing for 54% of our sample. However, this is unlikely to lead to important selection bias because albuminuria was assessed for a random subgroup of the Lifelines sample and its assessment stopped because of logistical reasons not related to CKD. Fourth, 65% of the participants answered the health literacy questionnaire. This reduced the power of our study, as the number of comorbidities was lower than among the participants that did not answer the questionnaire.
Public health practitioners and healthcare professionals should be aware of the importance of addressing low health literacy already at younger ages to prevent the development of multimorbidity during the life course. This aligns with current guidelines that focus more on prevention in healthcare services [
32]. Moreover, the similar patterns of multimorbidity presented in both health literacy groups suggest that it is not necessary to change the focus of the care of low-health-literate patients regarding which groups of comorbidities should be prioritized. Care for them should be directed at the same comorbidities as those of their adequate-health-literate counterparts, but with extra support to overcome the challenges intrinsic to low health literacy. In clinical practice, this support could be achieved by using health-literacy-friendly strategies to improve self-management, health behaviors, and the quality and accessibility of CKD care. Patients could benefit from informational materials designed as narratives, such as photo stories, which are more recognizable, relevant, and engaging to patients [
33,
34]. These materials could be used to promote the acquisition of health-related skills and CKD knowledge, and to increase patients’ self-efficacy and motivation. Moreover, strengthening social support by engaging family members or friends of the patient could enhance disease management at home. Clinicians could also be supported by strategies to facilitate decision-making with patients with low health literacy [
35]. This could be achieved through training to help clinicians tailor their communication strategies to the specific needs of each patient.
Future research should study the factors that mediate the association between low health literacy and multimorbidity to support the development of interventions to assist CKD patients. Ideally, research should be performed longitudinally, noting the trajectory and evolution of comorbidities, as well as health literacy, along the life course.
Conclusion
This study shows that, in a context of high prevalence of comorbidity, CKD patients with low health literacy are more likely to have a higher number of comorbidities than patients with adequate health literacy. Moreover, the multimorbidity patterns are similar for both groups of health literacy, differing slightly at older ages. This age difference suggests that the negative effects of low health literacy are more evident in aging groups. Therefore, improving low health literacy could be an intervention targeted to decrease multimorbidity along the life course of CKD patients.
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