Abstract
Background: Little evidence is available on oxalate balance in peritoneal dialysis (PD) patients. Patients and Methods: We performed a cross-sectional observational pilot study with 62 adult PD patients to document oxalate balance and explore its association with PD-related peritonitis. Plasma oxalate concentration, levels of oxalate excretion in 24-h urine, and peritoneal dialysis effluent were evaluated. The peritoneal oxalate transport status and renal and peritoneal oxalate clearances were calculated according to the PD-related peritonitis history. Results: PD patients with a history of peritonitis had a statistically significantly lower peritoneal oxalate clearance, daily peritoneal oxalate excretion, and overall oxalate removal rate compared with the peritonitis-free PD patients. They had a 4-fold risk of plasma oxalic acid increase, and even a single episode of dialysis-related peritonitis resulted in plasma oxalate elevation. Conclusion: Peritoneal oxalate clearance plays an important role in oxalate balance in PD patients and, therefore, dialysis-related peritonitis is a significant predictor for hyperoxalemia. Further well-designed clinical trials need to be undertaken before the association between peritonitis and oxalate balance in PD patients is more clearly understood.
Oxalate is an ionized form of a potentially toxic oxalic acid formed from endogenously synthesized and exogenously ingested oxalates (1, 2). In physiological conditions, the bulk of circulating oxalate (90-95%) is excreted through the kidneys, whereas the remainder (5-10%) through the terminal parts of the small intestine and colon (1-4). A decline in kidney function leads to decreased oxalate clearance and, ultimately, hyperoxalemia in end-stage kidney disease (ESKD) (1, 5-7). The accumulation of oxalate is associated with oxidative stress and systemic inflammation (1, 8, 9), high cardiovascular risk (9-11), and increased mortality rate (1, 11) in patients with kidney stones and ESKD. Nevertheless, little evidence is available on oxalate balance in ESKD patients in general and peritoneal dialysis (PD) patients in particular; almost all scientific data on this issue were published during the 1980s and 1990s (6, 12-14).
Worldwide, PD is a key element of kidney replacement therapy (15-17). Nonetheless, peritonitis remains one of the major challenges associated with severe clinical complications of PD despite technological advances (17, 18). Detrimental effects of dialysis-related peritonitis on the characteristics of peritoneal transport and the alterations of the peritoneal membrane have been documented (19-21). However, the potential effect of dialysis-related peritonitis on oxalate balance in PD patients has never been evaluated before. We hypothesized that the alteration of the peritoneal membrane due to dialysis-related peritonitis could decrease the peritoneal clearance of oxalate and, consequently, its removal levels. To test this hypothesis, the present study aimed to define oxalate balance and explore its association with dialysis-related peritonitis in PD patients.
Patients and Methods
Study design. This cross-sectional observational pilot study was carried out in accordance with the Declaration of Helsinki and conducted between January 2019 and May 2020 at the State Institution ‘Institute of Nephrology of the National Academy of Medical Sciences’ (Kyiv, Ukraine). The study was part of the ongoing Institute project “Effect of oxalate and urate metabolism on the evolution of kidney disease” (ClinicalTrials.gov Identifier: NCT04399915; Domestic Trial Registration Number: 0119U000002). The study protocol was approved by the Ethics Committee of the Institute (Protocol Number: 8/2018; September 22, 2018). Written informed consent was obtained from all participants before enrollment in the study.
Sample size. The sample size was estimated using a priori power analysis (G*Power Software, version 3.1.9.4) (22) based on the study by Marangella et al. (6). A minimum of 42 participants (21 participants in each group) was found to be required to achieve 95% power for the detection of differences in plasma and urine oxalate concentrations between groups, with a significance level of 0.05, using the Student’s t-test or the non-parametric Mann-Whitney test. Based on the fact that the reported incidence of peritonitis was significantly higher in a large unselected population of PD patients than that reported in single-center short-term studies (23, 24), the number of PD patients with a history of peritonitis was doubled. Therefore, we enrolled 62 adult ESKD patients from two dialysis centers who completed 3 months of continuous ambulatory PD.
Inclusion and exclusion criteria. Inclusion criteria for the enrollment in the study were: age >18 years, dialysis treatment for at least 3 months, and a stable clinical condition. The enrolled patients fulfilled the target weekly urea clearance level (Kt/V≥1.7) and did not take antibiotics for at least 3 months before the beginning of the study. Exclusion criteria were: hospitalization during the 3 months preceding the initiation of the study, history of systemic disease, malignancy, acute inflammation, and immunosuppressive treatment.
Study participants. Among the 62 enrolled patients, 33 (53%) were male and 29 (47%) female with an average age of 50.5±13.5 years and a PD vintage of 37±24 months. All PD patients were treated with a normal dwell time (4-5 h during daytime and 8-10 h at night) and received commercially available glucose-based Dianeal PD solution (Baxter Healthcare SA, Castlebar, Ireland) of different concentrations (1.36% and 2.27%) and Icodextrin (Baxter Healthcare SA, Castlebar, Ireland). Dialysis was targeted to achieve a value of Kt/V≥1.7 according to the Kidney Disease Outcomes Quality Initiative Clinical Practice Guidelines for Peritoneal Dialysis Adequacy of the National Kidney Foundation (25).
The recruited patients were divided into two groups according to PD-related peritonitis history: the peritonitis group included 41 patients with at least one past episode of PD-related peritonitis and the peritonitis-free group included 21 patients without any episode of peritonitis.
Clinical and laboratory measurements. The demographic data including age, gender, body mass index (BMI), body surface area (BSA), nosology of ESKD, PD vintage, all past peritonitis episodes, and diabetes status were collected from the medical records of the enrolled participants. BMI was calculated as weight (kg) divided by the square of the height (m2). BSA was calculated using the DuBois & DuBois formula (26).
Dialysis-related peritonitis was defined as peritoneal dialysis effluent (PDE) containing >100 white cells/μl with >50% polynuclear leukocytes (27). Peritonitis rate was calculated as the number of infections in all patients divided by the number of patient-months on PD. Anuria was defined as a 24-h urine volume <100 ml.
Whole blood samples were collected from the patients after an overnight fasting period and processed immediately. Routine biochemical parameters including blood and daily dialysate concentrations of urea and creatinine, serum albumin, C-reactive protein, glucose, electrolytes, and lipid profile parameters were carried out using the automatic analyzer Flexor junior (Vital Scientific, Spankeren, the Netherlands). Hematological parameters of blood were determined using the ABX Micros-60 (Horiba Medical, Montpellier, France).
Dialysis adequacy in PD patients was determined by measuring the weekly peritoneal creatinine clearance (CrCl) (normalized to 1.73 m2 of BSA) and Kt/V using the Watson formula for body water. Peritoneal Kt/V and renal Kt/V were estimated separately. The dialysate/plasma creatinine (D/P) ratio was calculated from the concentrations in 4-h dialysate and plasma creatinine. The peritoneal equilibration test proposed by Twardowski et al. (28) was used as a standardized procedure.
Oxalate balance measurements. The concentration of oxalic acid (POx) in plasma was measured spectrophotometrically using a commercially available kit (MAK315, Sigma, Barcelona, Spain) according to the manufacturer’s recommendations (29). The data were measured in μmol/L. However, the measurements were presented in both μmol/l and mg/l for better visualization of oxalate turnover.
Daily urinary oxalate (UOx) excretion and the PDE oxalate (PDEOx) concentration were determined using an oxalate oxidase/peroxidase reagent (BioSystems, Barcelona, Spain). To assess oxalate, 24-h PDE was collected on the day preceding the test.
The oxalate transport status was determined as the ratio of 4-h dialysate to plasma oxalate (D/P Ox). Renal oxalate clearance (ROxCL) and peritoneal oxalate clearance (PerOxCL) were calculated using the following formulas:
Statistical analysis. Statistical analysis was performed and all graphs were generated on MedCalc Statistical Software version 19.2.6 (Ostend, Belgium). The means (M), the standard deviations (SD), the median (Me), and the interquartile ranges [Q25-Q75] were calculated according to the distribution of the data. The Student’s t-test and the nonparametric Mann-Whitney test (U-test) were used for the differences between groups. The Kruskal-Wallis (ANOVA) test was used to compare PerOxCL depending on the number of peritonitis episodes. Categorical variables were expressed as proportions and the Chi-square (χ2) test was applied to compare the two groups. The Spearman’s correlation test was used to evaluate the association between POx concentration, oxalate transport status, and PerOxCL.
Univariate linear regression analysis was performed to explore the independent effect of ROxCL and PerOxCL on POx concentration. The model’s F-ratio, significance level, and residuals were presented using the Shapiro-Wilk test (W) for normal distribution. The data of ROxCL and PerOxCL were log-transformed before inclusion into the model because they did not fit the normal distribution.
Receiver operating characteristic (ROC) curve analysis and logistic regression analysis were performed to assess the overall discriminative ability of the number of peritonitis episodes on the predicting evaluation of the above-average POx concentration and calculate the odds ratios (OR) and 95% confidence interval (CI).
Results
Patient data. Among 62 enrolled PD patients, there were 41 (66%) patients who had experienced at least one episode of peritonitis and 21 (34%) who had never had peritonitis before inclusion and completion of the study. The peritonitis group had suffered 79 peritonitis episodes (1-5 episodes per patient). Twenty-three (56%) patients had experienced more than one episode throughout the PD treatment period of 2108 patient-months. The overall incidence of peritonitis was 0.45 episodes per patient-year or 1 episode per 26.7 patient-months during the PD treatment. Table I shows the baseline characteristics of the participants according to their peritonitis status. These data were obtained during routine clinical practice immediately after enrollment of the patients in the study.
As seen in Table I, both groups had similar ages, percentages of males and diabetics, and ESKD causes. No differences were observed between the groups in most clinical characteristics. However, the patients of the peritonitis group had a significantly lower estimated glomerular filtration rate (eGFR) and 24-h urine volume, and higher serum phosphate and parathyroid hormone levels compared to the peritonitis-free group. In addition, the peritonitis group had lower PD adequacy indicators, a larger number of anuric patients, and more Icodextrin users compared to the peritonitis-free group.
Oxalate balance patterns according to peritonitis status. The PD patients of the peritonitis group had a significantly lower level of oxalate transport status (4-h D/P Ox), PerOxCL, daily PDEOx excretion, and overall oxalate removal levels compared with the peritonitis-free patients (Table II). It should be noted, that the peritoneal oxalate transport status has a direct association with creatinine transport status in the PD patients (r=0.57, p<0.001). Moreover, the peritonitis group tended to have less UOx excretion with statistically higher ROxCL compared to the peritonitis-free group.
Effects of renal and peritoneal oxalate clearance on pox concentration in PD patients with preserved diuresis. ROxCL was correlated with residual diuresis (r=0.71, p<0.001) and weekly renal Kt/V (r=0.57, p<0.001), while PerOxCL was directly associated with the daily peritoneal ultrafiltration rate (r=0.57, p<0.0001), CrCL (r=0.6, p<0.0001), and D/P Ox ratio (r=0.46, p=0.0003). In addition, residual diuresis was directly correlated with the overall oxalate removal levels (r=0.66, p<0.0001). Low UOx excretion was associated with high PDEOx concentration (r=–0.36, p=0.007). However, neither residual diuresis (r=–0.13, p=0.27) nor overall oxalate removal levels (r=–0.11, p=0.34) were associated with POx concentration in PD patients. It is worth noting that POx was not correlated with eGFR (r=0.008, p=0.95) or the daily peritoneal ultrafiltration rate (r=0.23, p=0.12). However, it was inversely correlated with oxalate transport status (Figure 1a) and the weekly PerOxCl (Figure 1b).
Bearing in mind that there was no statistically significant association between PerOxCL and ROxCL (r=–0.17, p=0.24), univariate linear regression analysis was performed to determine their particular contribution to oxalate balance. Only PerOxCL was found to be an explanatory factor for POx in PD patients regardless of age and gender (W=0.99, F-ratio 17.7, p=0.0001) (Figure 2).
Oxalate balance in anuric PD patients. Among the enrolled PD patients, 21 (34%) patients presented anuria and 17 (81%) of them had experienced at least one episode of dialysis-related peritonitis (see Table I). The anuric PD patients independently of the peritonitis status had lower PerOxCL [20.2 (11.2-35.7) versus 40.6 (15.0-53.8) L/week/1.73m2, p=0.009] and overall oxalate removal levels [33.7 (14.9-56.6) versus 46.9 (25.3-62.4) mg/d, p=0.04] compared to the patients with preserved residual diuresis. In anuric conditions, POx concentration had a direct strong association with PDEOx levels (r=0.84, p<0.0001) and 24-h D/P Ox ratio (r=0.52, p=0.003) and was inversely correlated with the weekly peritoneal Kt/V (r=–0.45, p=0.01).
Association between the number of peritonitis episodes and oxalate balance in PD patients. A direct association was observed between the number of peritonitis episodes and POx concentration (r=0.52, p<0.001). However, a negative association was found between the number of peritonitis episodes and the D/P creatinine ratio (r=–0.37, p=0.01), CrCL (r=–0.35, p=0.02), D/P Ox (r=–0.58, p<0.001), and PerOxCL (r=–0.58, p<0.001). Figure 3 clearly demonstrates the association between the decreasing tendency in PerOxCL and the increasing number of PD-related peritonitis episodes. Moreover, an inverse association was found between the number of peritonitis episodes and the weekly plasma Kt/V (r=–0.51, p<0.001) and, consequently, the weekly total Kt/V (r=–0.33, p=0.04).
ROC analysis was performed for further assessment of the impact of dialysis-related peritonitis on POx elevation above the average level in our cohort (>39.2 μmol/l or 3.5 mg/L). We observed that the area under the curve (AUC) of the number of peritonitis episodes was 0.86 [95% CI=0.75-0.94] demonstrating the ability to discriminate different POx concentrations. The optimal cut-off value of the number of peritonitis episodes as a predictor of POx concentrations >3.5 mg/l was determined to be >1 (sensitivity of 72.4% and specificity of 85.7%) (Figure 4).
The logistic regression analysis confirmed a 4-fold risk of elevation in POx in the peritonitis group compared with the peritonitis-free group (χ2=27.5, p<0.001; OR=4.06, 95% CI=2.0-8.3).
Discussion
Hyperoxalemia in PD patients has been described in early clinical studies (6, 7, 14). However, current reports in the field of oxalate balance in ESKD patients are limited and mostly focused on the hemodialysis population (5, 30). Furthermore, the association between dialysis-related peritonitis and oxalate balance in PD patients had never been analyzed before. We have suggested 2 possible pathways of how dialysis-related peritonitis affects oxalate balance in PD patients: 1) the alteration of the peritoneal membrane could decrease the peritoneal clearance of oxalate and, consequently, its removal level, and 2) the use of antibiotics could increase intestinal oxalate absorption and hyperoxalemia (31-34). The present study focused on the effect of dialysis-related peritonitis on oxalate balance through the reduction of the peritoneal oxalate removal rate in PD patients. For this, we evaluated the contribution of residual renal and peritoneal oxalate clearances, as well as the effect of peritonitis on oxalate balance in PD patients, providing several new findings. First, this research showed significantly higher concentrations of POx in PD patients with a history of peritonitis compared with the peritonitis-free patients. Also, the peritonitis patients demonstrated a significantly lower 4-h D/P oxalate ratio, PerOxCL, daily PDEOx excretion, and overall oxalate removal levels compared with the peritonitis-free patients. Second, we identified that PerOxCL but not ROxCL largely affected oxalate balance in PD patients. Third, a strong association was observed between the number of peritonitis episodes, PerOxCL, and POx concentration. According to our results, PD patients with a history of peritonitis had a 4-fold risk of POx increase, and even a single episode of peritonitis was associated with an increase in POx concentration (>3.5 mg/l).
In accordance with previous results, the present study demonstrated that UOx excretion or overall oxalate removal levels in all PD patients were within reference intervals for healthy subjects and similar to the previously reported average of UOx excretion in ESKD patients (3, 5, 6, 8, 13, 35). Nonetheless, the levels of urine output in the participants of the present study were at least four times lower than those of healthy subjects (2, 35), meaning that PD patients had hyperoxaluria. This phenomenon is a result of inadequate Ox clearance, which balances between the peritoneal and renal routes depending on the peritoneal and residual kidney functions (RKF). Therefore, high POx concentrations in PD patients could not be explained by insufficient renal excretion only. Decreased PerOxCL due to dialysis-related peritonitis could potentially result in hyperoxaluria. Our data on the negative association between UOx excretion and PDEOx levels confirms this hypothesis. Moreover, in a recent study, Perinpam et al. (35) have demonstrated increased POx concentrations and UOx excretion concomitantly with eGFR decline in patients with primary hyperoxaluria, enteric hyperoxaluria, and kidney stone disease. These are in agreement with the present study, where we reported increased ROxCL in patients with high POx concentration. In our view, the higher ROxCL in the peritonitis group compared with the peritonitis-free group originated in decreased PerOxCL and elevated POx concentrations. The decreasing tendency of the UOx excretion despite high ROxCL in this group can be explained by the lower levels of daily urine volume compared to the peritonitis-free group.
It has been demonstrated that the number of peritonitis episodes is correlated with RKF decline (18, 36). Our results are in agreement with this since RKF measured by both renal urea clearance (renal Kt/V) and the presence of anuria was significantly lower in the peritonitis group compared with the peritonitis-free patients. However, contrary to our expectations, neither residual diuresis nor overall oxalate removal levels and eGFR were associated with POx concentration in PD patients. Our findings are in agreement with data provided by Marangella et al. (6) that demonstrated a higher level of dialysate oxalate excretion compared to urinary excretion. This means that the effect of peritoneal clearance on POx concentration might become evident in the absence of RKF. In other words, we believe that not only does the impaired kidney function lead to an increase in POx but also PerOxCL plays a crucial role in the oxalate dyshomeostasis in PD patients. In this context, peritoneal transport and altered functioning associated with peritonitis could be the main cause of hyperoxalemia in PD patients.
Peritonitis is an established risk factor for structural and functional alterations in the peritoneal membrane, eventually leading to peritoneal fibrosis, technique failure, and increased mortality (20, 37-39). In the early study by Wiggins et al. (40), the acceleration in membrane transport was observed one year after peritonitis. Recently, van Esch and van Diepen et al. demonstrated in a series of studies (19-21) that even a single episode of peritonitis leads to an increase in small solute transport and a decrease in the ultrafiltration rate. In turn, high peritoneal small solute transport status is associated with increased glucose absorption, protein loss, peritoneal and capillary surface area, inflammation, mortality, and technique failure in PD patients (37, 38). Similarly, in the present study, we observed worse dialysis adequacy in PD patients with peritonitis history compared with the peritonitis-free patients. In our view, both low CrCL and PerOxCL could result in high POx concentrations. The observed identity between creatinine and oxalate peritoneal clearance could be explained by their similar molecular weights (113 g/mol for creatinine and 90.3 g/mol for oxalic acid). The absence of a difference in daily ultrafiltration levels between the groups was most likely caused by a high percentage of Icodextrin users among peritonitis patients.
Other reasonable explanations for the strong association between high POx concentration and peritonitis history, which we were unable to analyze in the present study, are: 1) the effect of peritonitis on gut microbiota and oxalate gastrointestinal absorption due to the long-term use of antibiotics, and 2) dietary restrictions, malnutrition, and high protein intake due to the loss of protein associated with PD-related peritonitis. Further research on the effects of gut microbiota correction or dietary restrictions on oxalate balance may open up a new path to improve PD-related peritonitis outcomes.
The findings presented here are subject to at least three limitations. First, it was a cross-sectional study, and the oxalate balance profile was measured at only one point during the research period; therefore, causality could not be established. Second, the sample size was relatively small and the study population represented a group of patients in a stable clinical condition within the target levels of Kt/V. Contrary to expectations, the selection criteria could render our findings exaggerated. Third, we did not evaluate the effects of other factors potentially associated with POx elevation: different PD modalities or PD solution composition, glucose absorption, oxalate and/or protein dietary intake, phosphate binders, use of antibiotics and other medications that could influence intestinal oxalate absorption.
Notwithstanding these limitations, our study is the first to our knowledge that reports a strong association between dialysis-related peritonitis and elevated POx in PD patients. Although the clinical significance of hyperoxalemia in PD patients remains unclear, recent studies have demonstrated that oxalate is involved in the activation of oxidative stress, chronic inflammation, and atherogenesis (1, 8, 9, 11), which may explain the high cardiovascular and overall mortality in PD-related peritonitis. Quite possible that further research to investigate the effect of dietary oxalate restriction or the gut microbiota correction may open up a new pathway to improve PD-related peritonitis outcomes. Thus, we believe that our hypothesis would be reflected in further research to clarify the interaction mechanisms and the role of PD-related peritonitis in impaired oxalate balance in PD patients.
Conclusion
The results of the present pilot study indicate an important role of PerOxCL in oxalate balance in PD patients. Moreover, dialysis-related peritonitis was associated with decreased peritoneal oxalate transport status, PerOxCL, daily PDEOx excretion, and overall oxalate removal levels. Finally, dialysis-related peritonitis was a significant predictor of POx elevation. This research has resulted in new questions that need to be addressed in future investigations. Therefore, further controlled clinical trials are necessary to deeply understand the association between peritonitis and oxalate balance in PD.
Footnotes
Authors’ Contributions
NS conceived the presented concept, designed the study, analyzed and interpreted the patient data, wrote and edited the manuscript. LK performed the laboratory oxalate measurements. LL and LS collected the data and contributed to the manuscript preparation. SS performed the laboratory measurements. All Authors read and approved the manuscript.
Data Availability Statement
The datasets generated and analyzed for the current study are available from the corresponding author on reasonable request.
Conflicts of Interest
The Authors have no potential conflicts of interest, financial or otherwise.
- Received November 2, 2021.
- Revision received November 23, 2021.
- Accepted December 10, 2021.
- Copyright© 2022, International Institute of Anticancer Research (Dr. George J. Delinasios), All rights reserved