Introduction

Chronic kidney disease (CKD) affects approximately 11%–13% of the global population.¹ Ultimately, it can progress to end‑stage renal disease (ESRD), which requires renal replacement therapy, such as hemodialysis (HD), among other options. The life expectancy of patients treated with HD remains relatively poor. Cardiovascular disease (CVD) is present in more than 50% of individuals with ESRD undergoing HD,² and these patients face a significantly higher risk of cardiovascular mortality. This may be due to the joint presence of risk factors common in the general population along with those specific to ESRD, such as inflammation, accumulation of certain uremic toxins, oxidative stress, and underlying diseases (eg, hypertension and diabetes mellitus).³

Oxidative stress is a condition caused by an imbalance between excessive oxidant production—including reactive oxygen species (ROS) and nitric oxide—and an insufficient antioxidant defense system.⁴ The accumulation of these oxidants in CKD patients triggers the activation of inflammatory mediators, which subsequently promote the oxidation and modification of lipids (including oxidized low‑density lipoproteins [oxLDLs]), proteins (including homocysteine and myeloperoxidase), carbohydrates, and nucleic acids.⁵ Oxidative stress and inflammation in HD patients are closely linked,⁶ as phagocytic cells produce various reactive free radicals in response to inflammatory signals. Additionally, both are strongly associated with endothelial dysfunction.⁴

Elevated plasma osmolality and osmotic stress may significantly impair both cardiac and renal function,⁷ particularly in HD patients. To our best knowledge, the potential link between changes in plasma osmolality and oxidative stress markers in ESRD patients treated with HD has not yet been widely investigated.

Therefore, this study aimed to evaluate the effect of a single HD session on oxidative stress markers and to assess the role of osmolality as a potential early indicator of oxidative stress risk associated with HD.

Patients and methods

A prospective observational study was conducted between March and May 2022. The study was approved by the Bioethical Commission at the Jan Kochanowski University of Kielce (6/2022) and adhered to the principles outlined in the Declaration of Helsinki.

Exclusion criteria comprised HD duration of less than 3 months, myocardial infarction or stroke within 6 months prior to the study, neoplastic disease, and exacerbation of any chronic disease within 1 month before the study. All adult patients with ESRD undergoing HD who did not meet the exclusion criteria were enrolled (n = 44). All participants provided written informed consent prior to enrollment.

During the study period, all patients attended 4‑hour HD sessions 3 times a week, with a dialysate flow rate maintained at a minimum of 500 ml/min and a blood flow rate of at least 300 ml/min. HD was performed using a high‑flux polysulfone membrane dialyzer with a surface area ranging from 1.4 to 2.2 m² (Fresenius Medical Care, Bad Homburg, Germany). Anticoagulation was achieved with unfractionated heparin.

Blood samples were collected from the arteriovenous fistula or vascular catheter immediately before and after the HD session. Other parameters, such as weight and blood pressure, were measured at the same time points. The volume of ultrafiltration—indicating the fluid removed during HD—was recorded post‑HD. Plasma osmolality was calculated using the Osmometer 800 CLG (Trident‑Med s.c., Warsaw, Poland). The serum total oxidant status / total oxidant capacity (TOS/TOC) and total antioxidant status / total antioxidant capacity (TAS/TAC) were measured photometrically using ready‑to‑use test kits, following the original manufacturer’s instructions (KC5100 and KC5200, respectively; Immundiagnostik AG, Bensheim, Germany). The TAS/TAC kit evaluates antioxidant capacity by measuring the reaction between sample antioxidants and a known amount of exogenously added hydrogen peroxide (H₂O₂), while the TOS/TOC kit quantifies total lipid peroxides. Interleukin‑6 (IL‑6) and oxLDL levels were measured using ready‑to‑use test kits, also following manufacturers’ instructions (Diaclone SAS; Besancon, France for IL‑6 and Immundiagnostik AG for oxLDL). Optical density (OD) was measured using a Synergy HTX multi‑mode reader (BioTek, Santa Clara, California, United States).

Results

Discussion

Our study generated 2 main findings. To the best of our knowledge, this was the first study identifying changes in plasma osmolality as a potential key factor influencing oxidative stress during a HD session. Moreover, we observed a marked exacerbation of oxidative stress following HD.

Osmolality appears to play a critical role in modulating the oxidative balance. Increase in plasma osmolality—whether due to fluid shifts, solute retention, or impaired clearance during HD—may exacerbate oxidative stress by depleting antioxidant defenses, as shown by the significant drop in TAS/TAC. Hyperosmolality induces osmotic stress, which can damage cell membranes and disturb the redox state. Under such stress, cells activate pathways that boost ROS production and overwhelm antioxidant systems by depleting TAS/TAC. Moreover, antioxidants are lost through neutralization of ROS and passage across the dialyzer membrane, further intensifying oxidative stress.⁸ Monitoring osmolality in HD patients could serve as an early indicator of oxidative stress risk, and strategies to stabilize osmolality may help mitigate oxidative damage and reduce cardiovascular risk.^9,10 Interestingly, we noted a negative correlation between changes in osmolality and TOS/TOC after HD. The postdialysis reduction in TOS/TOC may suggest that increased osmolality alters the redox balance, leading to oxidative stress by lowering antioxidant defenses rather than directly increasing oxidant levels.

Notably, there were no significant changes in the levels of one of the main osmoles—sodium—after the HD session. Similarly, changes in another major osmole—urea—did not correlate with oxidative stress markers and were weaker prognostic indicators for antioxidative markers than changes in osmolality. These findings imply that unmeasured osmoles within the osmotic gap—such as organic solutes or anions not routinely assessed—may be pivotal in driving redox imbalance during HD. Identifying these uremic toxins could broaden our understanding of oxidative stress in ESRD and guide future interventions.

Patients with early‑stage CKD already face heightened oxidative stress, which worsens as renal function declines and with HD itself.¹¹ We confirmed that HD itself increases the oxidative burden—evidenced by the increase in TOS/TOC and oxLDL levels—and simultaneously depletes antioxidant capacity, as reflected by lower TAS/TAC. This likely stems from factors such as membrane bioincompatibility, removal of antioxidant molecules, and activation of inflammatory pathways.¹² The resulting oxidative imbalance may contribute to cellular damage, chronic inflammation, and elevated cardiovascular risk—hallmarks in ESRD patients treated with HD.³

Elevated CVD mortality in HD patients has been closely linked with endothelial dysfunction and systemic inflammation.¹³ Additionally, Jaroszyński et al¹⁴ found that heat shock protein 27 appears to be a marker linking cardiovascular mortality to oxidative stress in patients undergoing HD treatment. Impaired kidney function perpetuates chronic inflammation by weakening antioxidant and anti‑inflammatory defenses and impairing toxin clearance, leading to higher levels of acute‑phase proteins (eg, CRP, fibrinogen) and proinflammatory cytokines (eg, IL‑6, IL‑1β, and tumor necrosis factor α).¹⁵ Similarly to other studies, we observed an increase in CRP levels after a single HD session.^16,17 The positive association between the CRP level and TAS/TAC suggests a compensatory upregulation of antioxidant defenses in response to inflammation‑driven oxidative stress, though sustained or excessive stress may eventually overwhelm these defenses. This underscores the importance of controlling inflammation as a therapeutic target in HD patients.

The observed increase in oxLDL levels after the HD session supports the thesis that ESRD patients are particularly prone to atherosclerosis.¹⁸ The disease is defined by the presence of atherosclerotic plaques, lipid build‑up, and inflammatory cell accumulation within the arterial wall. These changes can reduce stability, potentially causing rupture and resulting in ischemic events. Multiple mechanisms contribute to the development of atherosclerosis, and the accumulation of oxLDLs in the arterial intima is thought to be the first stage in its development. Additionally, oxidative DNA damage has also been implicated in atherosclerosis progression and in the elevated all‑cause and cardiovascular mortality seen in HD patients.^19,20