Introduction
Currently, renal replacement therapy (RRT) extends life and alleviates symptoms in patients with end-stage renal disease (ESRD) by removing metabolic waste products and excess fluid from the blood. Despite advancements in dialysis technology and management strategies, hemodialysis patients continue to experience high rates of morbidity and mortality. ESRD patients often have an average life expectancy of only about 30% compared to the general population of the same age.1 This reduced life expectancy is influenced by multiple factors, often overlooked or inadequately addressed, including frailty2 and sarcopenia.3
Frailty, a syndrome characterized by decreased physiological reserve and increased vulnerability to stressors,4 is increasingly prevalent in ESRD patients, with rates two to four times higher than in individuals with normal kidney function.5 In Thailand, frailty affects 22.1% of the general aged population, while in ESRD patients, the prevalence ranges from 14% to 73%, significantly impacting health by increasing mortality rates, hospitalization rates, confusion, falls, and the likelihood of becoming bedridden.6 Anemia, inflammation, reduced food intake, acid-base abnormalities, and hormone imbalances all contribute to frailty and protein-energy wasting in ESRD patients.7
Sarcopenia, characterized by decreased muscle mass, muscle strength, and physical performance, leads to reduced physical ability, decreased quality of life, increased falls, morbidity, and mortality in older adults.8 The prevalence of sarcopenia in ESRD patients can be as high as 28.5%.9 Sarcopenia in these patients is associated with higher rates of physical disability and nearly doubles the mortality risk compared to those without sarcopenia. The key mechanism involves both increased muscle breakdown and decreased muscle synthesis, associated with heightened inflammation in ESRD patients.10
Residual kidney function (RKF), defined as a urine output of at least 100 mL per day in patients undergoing RRT, plays a crucial role in maintaining adequate blood filtration, fluid balance, acid-base regulation, and erythropoietin production.11
The relationship between RKF and frailty in hemodialysis patients remains poorly understood. To address this gap in knowledge, this study aims to investigate the association between RKF and frailty in hemodialysis patients. Recognizing the role of RKF in frailty could prompt healthcare providers to refer ESRD patients without RKF for frailty assessment. Early detection of frailty would enable tailored prevention and treatment strategies, such as optimizing dialysis, managing anemia, reviewing, and minimizing unnecessary medications, preventing falls, recommending appropriate exercise, and addressing nutritional deficiencies, depression, and cognitive impairment.
The primary objective of this study was to investigate the association between residual kidney function (RKF) and frailty in patients with end-stage renal disease (ESRD) undergoing hemodialysis. Secondary objectives included determining the prevalence of frailty and sarcopenia in this population, as well as exploring the association between frailty, sarcopenia, and RKF with baseline demographic and clinical characteristics.
Materials and Methods
Study Design
This was a cross-sectional study conducted from October 2023 to February 2024 on ESRD patients attending the Division of Nephrology and Renal Replacement Therapy, at one urban teaching hospital in Bangkok, Thailand. Participants included adults aged over 18 years who had been undergoing regular hemodialysis for more than six months. Exclusion criteria comprised patients with neurological deficits, recent cerebrovascular accidents within the previous 6 months, severe cognitive impairment or dementia, secondary hypertension, cancer, overt cardiovascular disease, obstructive pulmonary disease, other severe illnesses, wheelchair dependency, a life expectancy of less than six months, or an inability to comply with medical treatment. The sample size was calculated using statistical methods, setting α at 0.05, a power of 80%, and incorporating risk differences in frailty occurrence between patients with and without residual kidney function (RKF). Based on these parameters, the required sample size was determined to be 89 participants, adjusted to 107 to account for a 20% loss to follow-up.
This study was conducted following the approval of the Clinical Research Ethics Committee of the Faculty of Medicine Vajira Hospital, Navamindradhiraj University, Bangkok, Thailand (Certificate of Approval No. 044/2566). The Institutional Review Board is in full compliance with international guidelines for human research protection, including the Declaration of Helsinki, The Belmont Report, CIOMS Guideline, and the International Conference on Harmonization in Good Clinical Practice (ICH-GCP).
Data Collection
Written informed consent was obtained from all participants after providing detailed information about the study objectives, procedures, and potential risks. Confidentiality and privacy of the participants’ data were ensured, adhering to the principles of the Declaration of Helsinki.
Participants were recruited through the hospital’s electronic medical record system. Demographic data, including age, gender, and body mass index (BMI), and clinical information, such as underlying medical conditions, current medications, original kidney disease, dialysis duration, vascular access type, dialysis frequency, adequacy of dialysis, urea reduction ratio (URR), normalized protein catabolic rate (nPCR), history of intradialytic hypotension, oral nutrition supplementation (ONS), and dialysis ultrafiltration volumes, were extracted. Dialysis adequacy was assessed using Kt/V, calculated as the product of dialyzer urea clearance (K) and dialysis time (t), divided by the volume of distribution of urea (V).
Laboratory and Residual Kidney Function Measurement
Participants received instructions and containers for collecting a 24-hour urine sample prior to a dialysis day. The collected urine samples were brought to the hospital, where blood samples were also drawn. The following laboratory parameters were assessed:
Pre-Dialysis Blood Parameters
Hemoglobin, potassium, alkaline phosphatase (ALP), calcium, phosphorus, parathyroid hormone, albumin, vitamin D, blood urea nitrogen (BUN), iron, total iron-binding capacity (TIBC), and ferritin.
Residual Kidney Function (RKF)
Calculated using the residual renal urea clearance (KRU) formula:12
Where:
KRU: Residual renal urea clearance,
UUN: Urine urea nitrogen concentration from the 24-hour urine sample.
Urine Flow Rate: Calculated as:

TAC of SUN: Time-averaged concentration of serum urea nitrogen, derived as:13

Within two weeks of submitting the 24-hour urine sample, participants underwent assessments for frailty, nutritional status, and sarcopenia, conducted by physiotherapists at the Department of Rehabilitation Medicine, Faculty of Physical Therapy, Vajira Hospital.
Frailty Assessment
Frailty was assessed using the Thai version of the FRAIL Scale,14 a validated translation of the Simple Frailty Questionnaire15 for use in Thailand. This tool comprises five domains: Fatigue, Resistance, Ambulation, Illnesses, and Loss of Weight. Each domain is evaluated through self-reported responses, scored as either 0 or 1 point based on the presence of specific criteria. The total score ranges from 0 to 5, with higher scores reflecting a greater degree of frailty. A cutoff score of 3 or higher indicates frailty.
Sarcopenia Assessment
Sarcopenia was diagnosed following the Asian Working Group on Sarcopenia (AWGS) 2019 recommendations. Diagnosing sarcopenia involves three main components: reduced muscle mass, decreased muscle strength, and impaired physical performance. Additionally, this condition can be categorized into three levels: pre-sarcopenia, sarcopenia, and severe sarcopenia, as shown in Table 1. Muscle strength was evaluated with the Handgrip Strength Test using a digital dynamometer, with normal values defined as <28 kg for males and <18 kg for females. Physical performance was measured using the Five Time Chair Stand Test, with an abnormal score ≥12 seconds. Muscle mass was determined using the Fresenius Bioelectrical Impedance Analysis (BIA) device, and the appendicular skeletal muscle mass index (ASMI) was calculated as follows.16 Low muscle mass was defined as <7 kg/m² for males and <5.7 kg/m² for females.
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Table 1 The Diagnostic Criteria for Sarcopenia According to the Recommendations of the European Working Group on Sarcopenia in Older People (EWGSOP) in 201017 and the Asian Working Group of Sarcopenia (AWGS) in 20198
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Where: ASM: Appendicular skeletal muscle mass, calculated as:

Nutritional Status Assessment
Nutritional status was assessed using the Thai version of the Malnutrition Inflammation Score (MIS), based on the Clinical Practice Recommendation for Nutritional Management in Adult Kidney Patients 2018 by the Society of Parenteral and Enteral Nutrition of Thailand and the Nephrology Society of Thailand.18 MIS scores were categorized as follows: 1–2: Well-nourished, 3–5: Mild to moderate malnutrition, and ≥6: Severe malnutrition.
Statistical Analysis
The general data analysis of the sample group involved analyzing and presenting the data in two parts based on the data types. These included qualitative data presented in reports through frequency distribution and percentages, categorized by the RKF into two groups: one with RKF and the other without. The comparison of the groups was performed using either the Chi-squared test or Fisher’s exact test, depending on the appropriateness of the data.
For quantitative data presented in reports, the mean, standard deviation, median, and interquartile range were used. The data were classified by the RKF into two groups: one with RKF and the other without. The comparison between groups was done using either the Student’s t-test or Mann–Whitney U-test, depending on the appropriateness of the data.
The prevalence of frailty and sarcopenia conditions was reported through frequency distribution and percentages, along with a 95% confidence interval. The comparison of the differences between the group with RKF and the group without was conducted using either the Chi-squared test or Fisher’s exact test, depending on the appropriateness of the data.
The relationship between RKF and factors associated with frailty and sarcopenia in ESRD patients undergoing hemodialysis was analyzed using a multivariable logistic regression approach. Odds Ratios (OR) and 95% confidence intervals (95% CI) were reported to evaluate the strength and direction of associations between variables. Variables with a p-value < 0.2 from the univariable logistic regression analysis were considered for inclusion in the multivariable model. The final model was developed using a stepwise selection method, with criteria for variable entry set at p < 0.10 and removal at p < 0.05.
All data analyses were performed using the computer program SPSS version 24.0, with the statistical significance level set at 0.05.
Results
Baseline Characteristics
A total of 138 patients were initially enrolled in the study after meeting the inclusion criteria. However, 28 patients were unable to provide a 24-hour urine sample prior to a dialysis day and were lost to follow-up for assessments of frailty, nutritional status, and sarcopenia, which were conducted by physiotherapists. Consequently, the final study population consisted of 110 patients. Among them, 78 patients (70.91%) were classified as having no RKF, while 32 patients (29.09%) had RKF (Figure 1).
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Figure 1 Flowchart of patient enrollment and classification based on the presence of RKF.
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Baseline demographic analysis revealed that patients without RKF were younger, with a mean age of 58.58 ± 11.76 years, compared to 64.65 ± 11.28 years in those with RKF (P = 0.014). Additionally, the dialysis duration was significantly longer in the group without RKF (56 months vs 30 months; P = 0.002). While there were no significant differences between the two groups in terms of gender, comorbidities, causes of chronic kidney disease, or laboratory parameters, beta-blocker use was markedly more prevalent among patients without RKF compared to those with RKF (75.95% vs 37.50%; P = 0.001) (Table 2).
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Table 2 Demographic, Clinical and Laboratory Data According to Residual Renal Function (RKF), Frailty, and Sarcopenia
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In the frailty analysis, frail participants were predominantly female. The Charlson Comorbidity Index was significantly higher in the frail group compared to the non-frail group [median in frail group=6 (IQR 3,7) vs median in non-frail group =5 (IQR 3,6); P = 0.041]. Additionally, the frail group had a lower mean ultrafiltration (UF) rate (2.3 L vs 2.77 L; P = 0.032), lower hemoglobin levels (9.95 g/dL vs 10.93 g/dL; P = 0.027), and lower blood urea nitrogen (BUN) levels (36.93 mg/dL vs 47.29 mg/dL; P = 0.036) (Table 2).
In the sarcopenia analysis, participants with sarcopenia were older, with a mean age of 62 years compared to 57 years in those without sarcopenia (P = 0.028). The sarcopenia group also had a significantly lower BMI (21.49 kg/m² vs 27.09 kg/m²; P = 0.001). The prevalence of diabetes mellitus (DM) and dyslipidemia (DLP) was lower among participants with sarcopenia compared to those without (DM: 41.67% vs 63.16%, P = 0.032; DLP: 33.33% vs 60.53%, P = 0.006). Measures of dialysis adequacy, including Kt/V and the urea reduction ratio (URR), were higher in the sarcopenia group (Kt/V: 1.84 vs 1.54, P = 0.002; URR: 77.23 vs 73.37, P = 0.001). In contrast, the ultrafiltration (UF) rate was significantly lower in the sarcopenia group (2.59 L vs 2.93 L; P = 0.029). The use of calcium channel blockers (CCBs) was less frequent in the sarcopenia group (59.72% vs 84.21%; P = 0.010). Laboratory analysis revealed that hemoglobin levels were significantly higher in the sarcopenia group (11.03 g/dL vs 10.35 g/dL; P = 0.029), while blood urea nitrogen (BUN) levels were lower (43.25 mg/dL vs 50.86 mg/dL; P = 0.032) (Table 2).
Primary Outcomes
The comparison of Frailty according to RKF status between the two groups is summarized in Table 3. Frailty rates did not differ significantly between groups with and without RKF (21.88% vs 10.26%; P > 0.05). Multivariate logistic regression found no association between RKF and frailty (Table 4).
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Table 3 Frailty According to Residual Renal Function (RKF) Status
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Table 4 Univariate and Multivariate Analysis of Factors Associated with Frailty
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Secondary Outcomes
Frailty classifications included robust (37 patients, 33.64%), pre-frailty (58 patients, 52.73%), and frailty (15 patients, 13.64%) (Figure 2).
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Figure 2 Rate of frailty.
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Sarcopenia was classified into three categories: no sarcopenia (38 patients, 34.55%), sarcopenia (35 patients, 31.82%), and severe sarcopenia (37 patients, 33.64%). The overall prevalence of sarcopenia, including both sarcopenia and severe sarcopenia, was 72 patients (65.45%) (Figure 3).
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Figure 3 Rate of sarcopenia.
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Factors associated with frailty included a history of cerebrovascular accident (CVA) (adjusted OR 10.303, 95% CI 1.168–90.887; P=0.036) and TIBC levels (adjusted OR 0.972, 95% CI 0.946–0.999; P=0.047) (Table 4). Factors associated with the absence of RKF included age (adjusted OR 0.93, 95% CI 0.888–0.974; P=0.002), longer dialysis duration (adjusted OR 1.020, 95% CI 1.004–1.036; P=0.009), and higher Malnutrition Inflammation Score (MIS) (adjusted OR 1.203, 95% CI 1.007–1.437; P=0.041) (Table 5). Sarcopenia was associated with age (adjusted OR 1.054, 95% CI 1.008–1.102; P=0.02) and Kt/V values (adjusted OR 0.417, 95% CI 0.238–0.728; P=0.002) (Table 6).
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Table 5 Univariate and Multivariate Analysis of Factors Associated with Absence Residual Renal Function
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Table 6 Univariate and Multivariate Analysis of Factors Associated with Sarcopenia
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Discussion
Previous studies have demonstrated an association between kidney function and frailty in patients with chronic kidney disease (CKD). Lower estimated glomerular filtration rate (eGFR) levels have been linked to a higher prevalence of frailty, while an increase of 5 eGFR units has been inversely associated with frailty.19 As CKD progresses to end-stage renal disease (ESRD) requiring hemodialysis, most patients retain some residual kidney function (RKF), which is essential for maintaining fluid and metabolic balance and positively contributing to overall health.
Frailty and sarcopenia prevalence were examined in this study. The prevalence of frailty was 13.64% (15 patients), consistent with global data showing frailty rates among ESRD patients on dialysis range from 14% to 73%.6 Conversely, sarcopenia was observed in 31.82% (35 patients), higher than the reported prevalence of 28.5% in ESRD patients.9
Our study provides insights into the relationship between RKF and frailty in ESRD patients undergoing hemodialysis. However, we did not find a significant correlation between RKF and frailty in this population. This contrasts with CKD patients, where kidney function appears to play a central role in the development of frailty. One possible explanation is the multifactorial nature of frailty in ESRD, which involves factors beyond kidney function alone. For instance, a history of cerebrovascular accident (CVA) emerged as a significant predictor of frailty, increasing the risk by up to 10-fold. This aligns with existing data linking CVA and frailty, often referred to as “cognitive frailty.” Features of cognitive frailty, such as leukoaraiosis, atrophy, and old vascular lesions or infarcts, are associated with poorer functional and cognitive outcomes post-stroke.20 These findings underscore the need for further research to elucidate the mechanisms linking CVA and frailty in ESRD patients.
Our study also identified an intriguing association between frailty and lower total iron-binding capacity (TIBC). Frail patients in this study had a median TIBC of 186 μg/dL, which is significantly lower than the normal range of 250–450 μg/dL. This finding is consistent with previous research showing that low TIBC levels are linked to slower walking speeds, a key component of frailty syndrome.21 The role of TIBC in frailty development may be related to its influence on nutritional status and iron metabolism. These results underscore potential intervention points, such as optimizing nutritional and hematologic parameters, to reduce frailty risk in patients with end-stage renal disease (ESRD).
Factors contributing to RKF loss include younger age, longer dialysis duration, and higher Malnutrition Inflammation Score (MIS). Younger patients often experience acute, severe kidney failure necessitating dialysis initiation, which may lead to RKF loss over time. Prolonged dialysis is associated with a decline in RKF, with an estimated decrease of 0.18–0.33 mL/min/month during the first year, followed by a gradual decline.22 Additionally, higher MIS scores, indicative of malnutrition, are linked to the accumulation of waste products and metabolic abnormalities, further contributing to RKF loss.23 Risk factors for sarcopenia include advanced age and low Kt/V, a measure of dialysis adequacy in hemodialysis patients. Insufficient Kt/V values are associated with the development of uremic sarcopenia, a specific type of sarcopenia caused by inadequate dialysis. Inadequate dialysis leads to metabolic acidosis, insulin resistance, systemic inflammation, and increased expression of angiotensin II and myostatin.24 These factors activate the ATP-dependent ubiquitin-proteasome system, the primary pathway for skeletal muscle protein degradation. Additionally, the retention of indoxyl sulfate, a uremic toxin, exacerbates mitochondrial dysfunction and promotes the overexpression of muscle atrophy-related genes, including atrogin-1 and myostatin, further driving muscle loss in uremic sarcopenia.25
This study has several strengths worth acknowledging, although these should be interpreted within the study’s context and limitations. To our knowledge, it is among the first studies to explore the relationship between RKF and frailty in ESRD patients undergoing hemodialysis, with an adequate sample size to allow for meaningful analysis. Additionally, 24-hour urine samples were collected prior to dialysis sessions to minimize reliance on self-reported urine volumes, and frailty screening was conducted within two weeks of urine collection, alongside laboratory testing, to reduce confounding from acute illness. The study also assessed malnutrition using the MIS and sarcopenia using hand grip tests, the five-time chair stand test, and bioelectrical impedance analysis (BIA), with trained physical therapists ensuring consistency in evaluations.
Nevertheless, this study has its limitations. The cross-sectional design restricts our ability to establish causality or determine temporal relationships between RKF and frailty. Prospective longitudinal studies are needed to better understand these associations. Additionally, the lack of a standardized frailty assessment tool for ESRD patients introduces variability, as different tools might yield different results. Potential unmeasured confounders, such as socioeconomic status, dietary habits, and physical activity, could also influence the outcomes. Lastly, self-reporting bias or misclassification in variables like the Thai version of the FRAIL Scale and MIS score may have affected the findings.
Conclusion
While there was no significant association between RKF and frailty in hemodialysis patients, our study did reveal that a history of CVA and low TIBC levels were significant contributing factors for frailty. These findings underscore the importance of comprehensive frailty screening and interventions targeting nutritional and hematologic parameters. Future longitudinal studies are needed to explore causal relationships and inform strategies for improving patient outcomes.
Acknowledgments
This study was supported by the Navamindradhiraj University Research Fund, Navamindradhiraj University (Contract No. 032/2566).
The authors would like to thank the Division of Nephrology and Renal Replacement Therapy and the Division of Geriatrics, Department of Medicine, Faculty of Medicine Vajira Hospital, Navamindradhiraj University, for their support and collaboration throughout the study.
The authors also appreciate Associate Professor Varalak Srinonprasert (Division of Geriatric Medicine, Department of Medicine, Faculty of Medicine Siriraj Hospital, Mahidol University) for granting permission to use the Thai version of the FRAIL scale.
Disclosure
The authors reported no conflicts of interest in this work.
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