Middle molecular weight substances<\/strong> (MMW, 0.5\u201340 kDa): Examples are \u03b22M (11.8 kDa) and parathyroid hormone (9.5 kDa). MMWs are less effectively cleared by low-flux membranes and require HF with high-flux membranes for efficient removal from the patient\u2019s blood. The removal of MMW solutes relies on convection, where solute transport occurs via solvent drag caused by the TMP gradient.<\/p>\n Protein-Bound Uremic Toxins<\/em><\/strong> (PBUTs): This category includes molecules with low molecular weights, such as indoxyl sulfate and p-cresyl sulfate, which are characterized by their strong affinity for plasma proteins, particularly albumin, with binding rates exceeding 80%. Despite their low molecular weight, these PBUTs present a significant challenge for removal, as only the free (i.e., unbound) fraction can pass through the dialysis membrane (68).<\/p>\n By combining the mechanisms of HD and HF, HDF harnesses the enhanced clearance of larger solutes provided by HF while maintaining the high clearance of smaller solutes achieved through HD. This integration allows HDF to offer superior solute removal across a broader range of molecules than any other dialysis modality except for kidney transplantations (Figure 4.1).<\/p>\n HVHDF is particularly effective at removing middle molecules\u2014solutes with molecular weights typically between 500 and 60,000 Da\u2014that are inadequately cleared by conventional HD. Removing these molecules improves patients’ clinical profiles, addressing systemic inflammation, cardiovascular risks, dialysis-related amyloidosis, and other long-term complications associated with CKD, as outlined in Chapter 6. \u00a0HVHDF is critical in ensuring these solutes are efficiently cleared.<\/p>\n Figure 4.1 | Schematic weekly clearance of urea, creatinine, vitamin B12, and \u03b22-microglobulin with different kidney replacement therapies.<\/span><\/p>\n Urea<\/em><\/strong> (60 Da): Urea clearance is significantly enhanced with HVHDF compared to conventional HD, depending on the volume of substitution fluid utilized (30, 69-73). For instance, the European Dialysis Outcomes and Practice Patterns Study (DOPPS) demonstrated that patients undergoing thrice-weekly HVHDF with substitution fluid volumes between 15.0 and 24.9 liters per session achieved higher Kt\/V urea levels compared to patients receiving conventional HD (30). These findings highlight the potential of high-efficiency HDF to optimize small-molecule removal through adequate convective volume delivery, providing an effective alternative to conventional dialysis methods.<\/p>\n It is crucial to emphasize that the enhanced urea removal and the consequently higher Kt\/V observed in patients treated with HVHDF, compared to those receiving high-flux HD,<\/em> should never be used as a rationale for reducing dialysis treatment time<\/em>.<\/p>\n Dialysis duration remains an independent risk factor for mortality, irrespective of the efficiency of solute clearance (74-76). While HVHDF provides superior clearance of uremic toxins compared to conventional HD, optimizing patient outcomes requires a holistic approach beyond Kt\/V values. Shortening treatment time may counteract the benefits of HDF by compromising hemodynamic stability, fluid balance, and overall metabolic control (77, 78). Thus, implementing HVHDF should enhance both clearance and treatment adequacy rather than being used as a justification for shortening dialysis duration.<\/p>\n In addition, Canaud et al. highlighted that, by optimizing HDF prescriptions by incorporating automated ultrafiltration and substitution control, HVHDF can deliver a higher dialysis dose for small- and middle-molecule uremic compounds without increasing dialysis fluid consumption. This can be achieved by maintaining a high blood flow rate (i.e., >350 ml\/min) and reducing the dialysate flow\/blood ratio to 1.2 rather than 1.4, 1.5, or higher (29). Additionally, at equal dialysis doses, dialysis fluid consumption is significantly reduced, showing that HVHDF offers greater efficiency and environmental sustainability compared with high-flux HD (29).<\/p>\n Phosphate<\/strong> (95 Da): Phosphate removal during HDF is increased by 15\u201320%, enabling a reduction in the required doses of oral phosphate binders compared to conventional HD (56, 79-84). However, the impact on predialysis phosphatemia is modest, with reductions typically less than 15%. This limited effect is attributed to several factors, including increased dietary protein and phosphorus intake in patients transitioning to HDF due to improved appetite (37, 83, 85, 86). Additionally, phosphorus exhibits distinct removal kinetics compared to urea, as its clearance reaches a plateau phase beyond which further reductions in serum phosphate levels do not occur (86-88). A rebound in plasma phosphorus levels is also observed following the end of the dialysis session, a phenomenon seen in both HD and HDF (89).<\/p>\n \u03b22-microglobulin<\/strong> (~11,800 Da): In patients with ESKD, serum \u03b22M accumulates and precipitates, forming fibrillary structures and amyloid deposits in bones, periarticular tissues, vessel walls, and internal organs, especially the heart. Elevated levels of \u03b22M have been strongly associated with dialysis-related complications, such as amyloidosis, which can contribute to joint pain and carpal tunnel syndrome, and with adverse cardiovascular and infectious outcomes. HDF demonstrates superior efficacy in removing \u03b22M, when compared to conventional HD (59, 90-92) with a significant reduction in its circulating concentrations over a mid-term period (59, 93, 94). By lowering \u03b22M concentrations, HDF addresses a key biomarker of uremic toxin accumulation and mitigates associated inflammatory and structural complications.<\/p>\n HDF is the most efficient KRT method to remove \u03b22M and middle molecules, twice as much compared with high-flux HD. Ward et al. showed that, by highly efficient HDF operative conditions,\u00a0 the \u03b22M clearance obtained was 73 ml \/ min, which means that the \u03b22M mass removed during the 4-hour session was close to 200 mg per session (600 mg per week) (95). Considering that the clearance of \u03b22M exhibits a linear relationship with convective volume, it can be anticipated that HVHDF offers the most effective means of removing \u03b22M\u00a0 (96). This approach may be recommended for patients with \u03b22M levels \u2265 27 mg\/L to reduce the risk of mortality (59). It is also recommended for those with symptomatic manifestations of amyloidosis that significantly affect their quality of life, and cases such as arthropathy, bone cysts with pathologic fractures, carpal tunnel syndrome, systemic involvement, and symptomatic autonomic dysfunction (97).<\/p>\n A comprehensive meta-analysis of the effectiveness of high-flux HD and convective dialysis modalities, including HDF and HF, in removing \u03b22M, was published in 2018 (59). Given the discrepancies in the literature regarding \u03b22M clearance and the clinical benefits of different dialysis modalities, the authors of this meta-analysis sought to evaluate the determinants of effective \u03b22M removal based on a systematic review of published studies. The analysis included 69 studies spanning from 2001 to 2017, incorporating data from 1,879 patients and 6,771 clearance measurements. Using a random effects meta-analysis and meta-regression model, the authors examined dialysis-related parameters such as membrane composition, modality, blood and dialysate flow rates, and substitution fluid rates (59). They found that while conventional high-flux HD achieved an average \u03b22M clearance of 48.75 mL\/min, convective therapies significantly outperformed this, with an average clearance of 87.06 mL\/min. HDF, in particular, provided enhanced clearance, underscoring its potential superiority in removing MMW toxins. Notably, membrane material emerged as a key determinant of \u03b22M clearance. High-flux dialyzers composed of polyarylethersulfone exhibited superior \u03b22M clearance in high-flux HD, whereas polysulfone (PS) membranes were associated with better performance in convective therapies such as HDF. The study also highlighted the role of blood flow and substitution fluid rates in optimizing \u03b22M removal. Higher substitution fluid rates in post-dilution HDF resulted in superior clearance, while dialysate flow rates were not found to be a significant factor in enhancing \u03b22M removal.<\/p>\n One intriguing finding was the substantial contribution of adsorption to \u03b22M clearance, particularly when comparing blood-side versus dialysate-side measurements. Adjusted dialysate-side \u03b22M clearances were significantly lower than whole blood clearances, suggesting that membrane adsorption plays a crucial role in trapping \u03b22M beyond diffusive and convective mechanisms. The study found no clear secular trend indicating improved \u03b22M clearance over time, indicating\u00a0 that, notwithstanding improved dialysis efficiency, limitations persist in removing \u03b22M effectively (59).<\/p>\n Pre-dialysis serum \u03b22M concentrations may not differ between HDF and high-flux HD<\/em> (60). The primary limitation in \u03b22M removal during post-dilution HDF is not due to the clearance capacity of the hemodiafilter itself but to the resistance within the patient’s body to mass transfer. This resistance arises from the interaction between the patient’s physiology and the dialysis system, which restricts the mobilization of \u03b22M from tissue stores into the bloodstream for removal (98). During dialysis, a concentration gradient develops between well-perfused areas, where \u03b22M is readily available for clearance, and deeper, poorly perfused compartments, where its movement is restricted. This phenomenon, known as the compartmentalization effect<\/em>, results in an apparent sequestration of \u03b22M within the body, limiting the efficiency of its removal even in HVHDF (98). Following dialysis, the \u03b22M that remained in remote compartments begins to redistribute into circulation, causing a post-dialysis rebound in serum levels. The extent of this rebound reflects the imbalance created during the session and underscores the challenge of eliminating \u03b22M (98).<\/p>\n Parathyroid Hormone Fragments<\/strong> (~9,000 Da), and Fibroblast Growth Factor 23<\/em> (~32,000 Da) are implicated in CKD bone metabolism alterations. Elevated levels of these molecules contribute to CKD-related mineral and bone disorders (CKD-MBD), leading to complications such as renal osteodystrophy and vascular calcification. By effectively reducing their plasma concentrations, HVHDF may play a role in mitigating the progression of CKD-MBD and its associated complications and may attenuate the need for calcimimetics (86, 99-103).<\/p>\n Leptin <\/strong>(~16,000 Da) is elevated in CKD and contributes to metabolic dysregulation, appetite suppression, and inflammation. It is implicated in ESKD patient malnutrition and anorexia (104, 105). A study by Kim et al. reported that HVHDF significantly reduces serum leptin levels compared to conventional low-flux HD, resulting in reduced circulating concentrations in HDF-treated patients (105, 106).<\/p>\n Inflammation is believed to contribute to the development and progression of other common complications in ESKD patients, including atherosclerosis, protein-energy wasting, and heart-related conditions. By lowering circulating levels of inflammatory markers<\/strong>, HDF may help mitigate the systemic inflammation commonly seen in CKD in adults and children (67, 79, 100, 107-113).<\/p>\n Advanced Glycation End Products<\/strong> (AGEs, >10,000 Da) are associated with oxidative stress, vascular damage, and cardiovascular complications. Their plasma values are reduced in diabetic and non- diabetic ESKD patients treated by HVHDF (79, 116).<\/p>\n HVHDF effectively removes various MMW substances<\/strong>, which may help mitigate comorbidities commonly associated with adverse clinical outcomes in patients with ESKD. \u00a0By targeting these uremic toxins, HVHDF offers a therapeutic advantage in improving the clinical profile and overall prognosis for ESKD patients.<\/p>\n Protein-Bound Toxins<\/strong>: Conventional HD primarily targets small, water-soluble solutes through diffusion, whereas HDF, due to its convective transport, facilitates superior clearance of protein-bound and larger uremic toxins. Ronco et al. showed a more significant reduction in both free and PBUTs in post-dilution online HDF compared to pre-dilution HDF (133). Toxins such as indoxyl sulfate and p-cresyl sulfate, implicated in cardiovascular morbidity and progression of CKD, are more effectively reduced with HDF. Evidence suggests modest improvements in the removal of protein-bound solutes like p-cresyl sulfate and indoxyl sulfate (134). 3-Carboxy-4-methyl-5-propyl-2-furanpropionic acid, a protein-bound erythropoietic inhibitor, can be reduced in HDF, mainly when using protein-leaking high-flux membranes (129, 130).<\/p>\n <\/p>\n","protected":false},"featured_media":0,"template":"","format":"standard","meta":{"_acf_changed":false},"categories":[128],"tags":[250],"language":[41],"articles":[215],"class_list":["post-8456","article","type-article","status-publish","format-standard","hentry","category-articles","tag-handbook-hdf","language-english","articles-hemodiafiltration","entry","no-media"],"acf":[],"yoast_head":"\n![\"\"](\"https:\/\/ami.advancedrenaleducation.com\/wparep\/wp-content\/uploads\/sites\/11\/2025\/07\/figure4.1.jpg\")
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