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NDT Advance Access originally published online on April 27, 2007
Nephrology Dialysis Transplantation 2007 22(7):2006-2012; doi:10.1093/ndt/gfm065
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© The Author [2007]. Published by Oxford University Press on behalf of ERA-EDTA. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Effect of the super-flux cellulose triacetate dialyser membrane on the removal of non-protein-bound and protein-bound uraemic solutes

Rita De Smet1, Annemieke Dhondt1, Sunny Eloot2, Francesco Galli3, Marie Anne Waterloos1 and Raymond Vanholder1

1Renal Division, Department of Internal Medicine, University Hospital Ghent, 2Institute of Biomedical Technology, Hydraulics Laboratory, Ghent University, Gent, Belgium and 3Department of Internal Medicine, Section of Applied Biochemistry and Nutritional Sciences, University of Perugia, Perugia, Italy

Correspondence and offprint requests to: Rita De Smet, Renal Division, Department of Internal Medicine, University Hospital, De Pintelaan 185, 9000 Gent, Belgium. Email: rita.desmet{at}Ugent.be



   Abstract
Top
 Abstract
Introduction
 Patients and methods
 Results
 Discussion
 References
 
Background. Uraemic solutes accumulate in haemodialysis (HD) patients and interfere with physiological functions. Low-flux (LF) HD does not efficiently remove all uraemic compounds. We investigated whether large pore super-flux (SF) cellulose triacetate membranes (CTA) result in a better removal of uraemic solutes.

Methods. Eleven patients were dialysed consecutively with LF-CTA and SF-CTA during 3 weeks. Urea (UR), creatinine (CR), uric acid (UA), 3-carboxy-4-methyl-5-propyl-2-furanpropionic acid (CMPF), indole-3-acetic acid (IAA), indoxyl sulfate (IS), hippuric acid (HA), pentosidine (PENT), low-molecular weight (MW) AGEs (AGEs) and albumin were determined in pre-HD, post-HD blood and in dialysate. Reduction rate (RR), dialytic clearance and mass transfer-area coefficient (KoA) were calculated.

Results. SF-HD resulted in a higher RR than LF-HD for IS and AGEs. Urea RR correlated with HA (r = 0.59), IS (r = 0.68) and IAA (r = 0.67), (P < 0.05) for SF. Dialytic clearance ranged from 20 ± 5 to 179 ± 20 ml/min for LF and from 24 ± 6 to 191 ± 24 ml/min for SF; being higher with SF for UA, HA, IS and IAA (SF vs LF, P < 0.05). KoA was higher for most compounds with SF-HD. Albumin loss per SF session was 3.4 ± 1.3 g. The retrieved amount of uraemic solutes in dialysate with LF and SF was comparable.

Conclusions. In conventional HD, SF-CTA was superior to LF-CTA for removal of most protein-bound compounds, especially IS. Reduction rate, dialytic clearance and KoA were higher with SF. The SF-CTA membrane is albumin-leaking; however, this property could not completely explain the amount of retrieved protein-bound compounds in dialysate.

Keywords: albumin; dialysate; haemodialysis membranes; removal; uraemic toxins



   Introduction
Top
 Abstract
 Introduction
Patients and methods
 Results
 Discussion
 References
 
Uraemic solutes accumulate in end-stage renal disease patients and interfere with various physiological functions [1]. Up till now, 90 uraemic compounds have been listed by the European Uremic Toxin (EUTox) Work Group and classified in three groups according to biochemical properties: (i) small water soluble non-protein-bound compounds such as urea and creatinine, (ii) protein-bound solutes such as indoxyl sulfate and (iii) middle molecules [2]. Urea is generally used as a marker molecule for dialysis efficiency; its kinetic behaviour, however, is not representative, not even for other water soluble uraemic compounds, such as guanidino compounds [3]. Although several modifications in dialytic strategies were proposed, none of those seemed to affect substantially the removal of protein-bound solutes. In a haemodialysis setting, high-flux membranes did not allow additional removal of protein-bound compounds compared with low-flux membranes [4]. To enhance the elimination of middle molecules and protein-bound uraemic solutes, new protein-leaking-dialysis membranes [5], new devices such as mid-dilution dialysers [6], and/or new strategies such as daily dialysis [7] and on-line haemodiafiltration [8] and sequential haemofiltration–haemodiafiltration [9] became available and were evaluated.

SF-CTA membrane haemodialysers have been developed. Compared with standard CTA, the new SF membrane is more permeable and has a larger pore size, a higher ultrafiltration coefficient and allows higher clearances of urea and creatinine (Table 1). Those membranes might remove other, especially protein-bound uraemic compounds more efficiently, and this could be clinically important since associations between protein-bound uraemic compounds and uraemic symptoms have been demonstrated [10–12].


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  		Table 1. Technical information of the two studied dialysers based on in vitro experiments (Qb: 200 ml/min, Qd: 500 ml/min) (Manufacturer data)

 
In the present study, we assessed whether dialysis with SF-CTA membranes resulted in a better removal and clearance of uraemic solutes with various characteristics compared with LF-CTA membranes. We evaluated pre-HD and post-HD concentrations, reduction rate, dialytic clearance and mass transfer area coefficient (KoA) of the two membranes in a single session after 3 weeks of treatment. Nine uraemic compounds with different biochemical properties, molecular weight and protein binding were evaluated and the concentration was measured: (i) in the group of water soluble non-protein-bound molecules: urea (UR), creatinine (CR) and uric acid (UA); (ii) in the group of protein-bound molecules: 3-carboxy-4-methyl-5-propyl-2-furanpropionic acid (CMPF), indole-3-acetic acid (IAA), indoxyl sulfate (IS), hippuric acid (HA) and pentosidine (PENT); (iii) in the group of middle molecules: low molecular weight advanced glycation end products (AGEs).



   Patients and methods
Top
 Abstract
 Introduction
 Patients and methods
Results
 Discussion
 References
 
We started the study with 12 equilibrated haemodialysis patients. One patient was withdrawn because the SF-HD session was performed in single-needle modus due to vascular access problems. The age of the remaining 11 patients (6 females/5 males) varied from 37 to 83 years. The residual CR clearance ranged from 0 to 1.5 ml/min and the time on dialysis was 4–380 months. The patients had an arteriovenous fistula (AVF) as vascular access. The study was approved by the Ethics Committee of the University Hospital Ghent and patients gave written informed consent. During at least 1 month before the study, patients had been dialysed with low-flux dialysers 3 times 4.5 h/week. The medication and food intake did not change during the study period. In all treatments the same dialysis machine, a 4008H haemodialysis machine (Fresenius Medical Care, Bad Homburg, Germany) was used in a two needle mode. The dialysis sessions were carried out first with LF-CTA (Sureflux-150L, 1.5 m2, Nissho Nipro, Osaka, Japan) and then with SF-CTA (Sureflux-150FH, 1.5 m2, Nissho Nipro, Osaka, Japan) each for 3 weeks. Dialysate flow was set at 500 ml/min and the intradialytic weight loss was noted. Dialysate was prepared as proposed by the European Best Practice Guidelines and the European Pharmacopoeia [13]. Pre-HD, post-HD blood samples and spent dialysate were collected from the last mid-week dialysis session with both LF and SF. At the end of dialysis, blood was collected after reducing the flow (QB) to 50 ml/min for 2 min followed by stopping the blood pump [14]. Spent dialysate was collected during the whole session by a T-piece with a plastic 22-gauge needle placed on the dialysate drainage tube in a polyethylene container of 2 l as previously described [15]. Equilibrated eKt/V was determined using the following formula, eKt/V = spKt/V – 0.4 x K/V + 0.02 [16], with spKt/V being determined using the second-generation Daugirdas formula [17]. Normalized protein catabolic rate (nPCR) was calculated according to Kloppenburg et al. [18].

Reagents
HPLC-grade water and methanol were purchased from Acros Organics (Fairlawn, NJ, USA). The reagents for the determination of UR, serum albumin and all products for preparing standard solutions were obtained from Sigma Chemical (St Louis, MO, USA). Creatinine reagent was obtained from Analis (Namur, Belgium); CMPF was a kind gift of H. Liebich (Tuebingen, Germany).

Biochemical assays
Urea nitrogen, CR, total protein and serum albumin were measured by standard laboratory techniques. Albumin in dialysate was quantitated by nephelometry (Dade–Behring Analyzer II, Dade–Behring, Brussels, Belgium). The haematocrit was obtained with the capillary centrifugation technique. Analyses of CMPF, HA, IAA, IS and UA were performed by HPLC (Waters, Milford, MA, USA) [7]. The dialysate samples were applied such on the RP-C18 column, serum or plasma were deproteinized by heat denaturation to determine the total (unbound plus bound) or ultrafiltered for the free (unbound) concentrations [19]. AGEs were detected by measuring the total fluorescence [20]. PENT was determined by ion-pair RP–HPLC [21].

Calculations
Reduction rate (%) of each compound was calculated from the pre-HD and post-HD serum or plasma concentrations. Dialytic clearance (ml/min) was calculated from the log mean of the pre-HD and post-HD concentrations and the retrieved amount of compound in the dialysate [22]. KoA (ml/min) values were calculated from the clearances, blood water flow and dialysate flow [23–24]. Protein binding (%) was determined from the total and free serum or plasma concentrations. The haemoconcentration during dialysis was calculated as the ratio pre-HD/post-HD of total protein [4].

Statistics
Data were analysed using SPSSTM 12.0 statistical software (SPSS Inc., Chicago, IL, USA). The data were expressed as mean values with SD. The comparisons between the two membranes were made with the Wilcoxon matched-pairs test. Association between reduction rates of uraemic compounds was examined using the non-parametric Spearman's correlation coefficient. P-values less than 0.05 were considered significant.



   Results
Top
 Abstract
 Introduction
 Patients and methods
 Results
Discussion
 References
 
Haemodialysis sessions
No significant difference in duration of the dialysis sessions and QB was observed between the two treatments. Pre-HD body weight, weight loss during dialysis, eKt/V and the normalized protein catabolic rate (nPCR), an index of protein intake, remained unchanged (Table 2). The pre-HD serum albumin level decreased from 3.6 ± 0.4 g/dl after 3 weeks LF dialysis, to 3.4 ± 0.5 g/dl after 3 weeks SF (P < 0.05); post-HD albumin remained unaltered. The haematocrit increased significantly post-HD vs pre-HD for both LF and SF; however, the values were not different for both membranes (Table 3). In spent dialysate collected during the SF session 3.4 ± 1.3 g albumin was found, whereas no albumin was detected in dialysate of the LF membrane. As the haemoconcentration was equal for both membranes, no correction was made for that parameter.


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  		Table 2. Characteristics of the LF-HD and SF-HD sessions and amount of albumin in dialysate

 

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  		Table 3. Patient characteristics during the LF-HD and SF-HD sessions

 
Total concentration and reduction rate
The total pre-HD and post-HD concentrations of the compounds and their reduction rate during the LF and SF session are given in Table 4. Pre-HD concentration decreased significantly (P < 0.05) after 3 weeks SF for IS (2.57 ± 0.84 vs 2.07 ± 0.66 mg/dl) and CMPF (0.88 ± 0.50 vs 0.84 ± 0.36 mg/dl) and was lower, but not significantly, for all other compounds except IAA. The post-HD level behaved in the same way. The reduction rates for LF and SF ranged for the non-protein-bound compounds (UR, CR, UA) from 69.5% to 75.1% and from 71.2% to 75.7%, respectively. For protein-bound compounds (HA, IAA, IS, CMPF, PENT) the RR ranged between –17.7% and 69.0% and between –10.2% and 69.7% for LF and SF, respectively. Compared with UR, CR and UA, the reduction rate for each other compound was significantly lower (P < 0.05), except for HA which had the same RR as CR. SF resulted in a significantly (P < 0.05) better reduction rate for IS and AGEs compared with LF; the reduction rate was also enhanced for all other compounds, although not significantly. Correlations of reduction rates of non-protein-bound compounds with protein-bound compounds were observed with the SF membrane only: for UR with HA (r = 0.59), IS (r = 0.68) and IAA (r = 0.67), (P < 0.05) as illustrated in Figure 1, for CR with HA (r = 0.67) and IS (r = 0.81), and for UA with HA (r = 0.63) and IS (r = 0.70) (P < 0.05), indicating a comparable removal mode of the non-protein-bound solutes with protein-bound solutes. In contrast, no correlations were found with the LF membrane.


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  		Table 4. Total pre-HD and post-HD concentrations in mg/dL (unless mentioned otherwise) and reduction rate (%) of uraemic compounds with LF and SF

 

Figure 1
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  		Fig. 1. Correlation analyses (n = 11) of the urea reduction rate with the reduction rate of (A) hippuric acid (r = 0.59, P = 0.050), (B) indoxyl sulfate (r = 0.68, P = 0.02) and (C) indole-3-acetic acid (r = 0.67, P = 0.02) for SF.

 
Unbound (free) concentration and protein binding
The behaviour of the unbound concentrations of HA, IAA, IS and AGEs during the study was similar with that of the total concentrations (Table 5, upper panel). A significant decrease in pre-HD and post-HD level was observed for IS with the SF membrane. The protein binding increased after LF-HD as well as after SF-HD for all four protein-bound compounds (Table 5, lower panel).


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  		Table 5. Unbound (free) pre-HD and post-HD in mg/dL or Au and protein binding of four protein-bound uraemic compounds with LF and SF

 
Amounts of retrieved solutes and albumin in spent dialysate
Measured amounts of solutes in spent dialysate were comparable for LF and SF (Table 6). Transfer into dialysate depends in part on the blood concentration; in our study, pre-HD levels were lower after 3 weeks HD with SF compared with LF (Table 4), potentially resulting in a lower mass transfer into dialysate with SF and equal concentrations in both dialysates.


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  		Table 6. Measured amounts of compounds in spent dialysate of LF and SF

 
In spent dialysate of SF 3.4 ± 1.3 g, albumin was found, whereas in dialysate of LF no albumin was retrieved (Table 2). No correlation was found between the amount of albumin and HA, IS, IAA or AGEs in dialysate of SF.

It was not possible to make a distinction between total and free protein-bound compounds in dialysate. Due to the extreme dilution of albumin the protein-bound solutes were released from their protein binding sites (unpublished personal observation).

To evaluate a possible adsorption of albumin on the SF membrane, we calculated the amount of albumin loss from the plasma [3] and compared it with the amount of albumin retrieved in the SF dialysate. The albumin loss from plasma was 2.3 ± 5.5 g, whereas 3.4 ± 1.3 g albumin was found in dialysate (P = NS).

Dialytic clearance and mass transfer area coefficient (KoA)
The dialytic clearance ranged for LF from 20 ± 5 to 179 ± 20 ml/min and for SF from 24 ± 6 to 191 ± 24 ml/min (Table 7). A significantly higher clearance with SF vs LF was found for the non-protein-bound compound UA (127 ± 11 vs 131 ± 22 ml/min, P < 0.05) and for three protein-bound compounds: HA (91 ± 11 vs 100 ± 19 ml/min, P < 0.05), IS (20 ± 5 vs 24 ± 6 ml/min, P < 0.05) and IAA (52 ± 39 vs 65 ± 48 ml/min, P < 0.05), and a similar trend was observed for UR and AGEs (P < 0.1). It is of note that no dialytic clearance could be calculated for CMPF and PENT since none of them were detectable in dialysate.


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  		Table 7. Molecular weight (MW), dialytic clearance and mass transfer area coefficient (KoA) of uraemic compounds

 
We found an enhancement of the dialytic clearance for most compounds with SF (Table 7). The lowest KoA was observed for IS and was 21 ± 6 and 26 ± 7 ml/min for LF and SF, the highest KoA was for urea 671 ± 258 and 771 ± 356 ml/min (LF and SF) (Table 7).



   Discussion
Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
References
 
This study compared, in a conventional dialysis set up, the effect on the removal of uraemic compounds of an LF-CTA membrane and a SF-CTA membrane with larger pore size than the former, both membranes have the same electric charge and same hydrophilicity and hydrophobicity.

The concentration, removal, dialytic clearance and mass transfer area coefficient (KoA) of nine uraemic retention compounds were evaluated. We found that protein-bound solutes, especially IS, were removed more efficiently with SF than with LF and observed an enhancement of dialytic clearance and KoA for most solutes.

Diffusion and convection are the principal mechanisms for solute removal during dialysis and they depend on the interaction of removable solutes, membrane and dialysate. One of the key characteristics that influences those processes, is the pore size of the dialysis membrane [25], and the membrane porosity is a function of pore size and number of pores [26]. In addition, for the removal of protein-bound compounds, the albumin loss through membranes with large pores may also play a role.

The equilibrated eKt/V values obtained with both CTA membranes were above the threshold proposed by the Dialysis Outcome Quality Initiative (K/DOQI) guidelines and the European Best Practice Guidelines (EBPG) [27,28]. In agreement with this observation, we obtained relatively high reduction rates and clearances for UR and the other water soluble non-protein-bound solutes; removal rates of protein-bound solutes and AGEs were lower, as expected. The concentrations of most compounds decreased during the dialysis sessions with both membranes (pre-HD vs post-HD); however, CMPF increased due to ultrafiltration and haemoconcentration. Nevertheless, SF resulted in a significantly better RR for IS and AGEs than LF, also with PMMA, protein-leaking dialysers, a reduction of glycation markers was demonstrated [21]. In the SF-setting, in contrast to LF, correlations of reduction rate were observed for UR with HA, IS and IAA (Figure 1), indicating that protein-bound solutes behave more like non-protein-bound solutes with SF. As demonstrated in vitro by Meyer et al. [23] we also found low KoA values for the protein-bound compounds compared with non-protein-bound compounds for both membranes in the present in vivo setting.

The question arose what the role was of albumin in removal of protein-bound compounds during SF dialysis: (i) At least in part these molecules could be dragged out of the bloodstream together with albumin. (ii) A second mechanism for this removal of protein-bound solutes could be that albumin molecules are retained within the large dialyser pores having contact with the dialysate without passing through it [29], resulting in a hence and forth retrieval of molecules from the plasma and other body compartments and their subsequent release into the dialysate. The ellipsoidal shape of albumin of 104 x 40 Å [30] and the super-flux pores with an average diameter of 78 Å, might favour such a process. The retention of albumin in the membrane and its potential role in the carry over of uraemic solutes is of course speculative. The driving force to release solutes from albumin in the absence of albumin solution in the dialysate is the concentration difference of the free fraction of the solute between dialysate and plasma which results in the removal of the free fraction and the subsequent displacement of the remnant compound from the serum albumin–compound complex, resulting in a continued displacement of protein-bound compounds from the serum albumin–compound complex during dialysis. (iii) A third possible mechanism to clear protein-bound compounds is the adsorption on the membrane of albumin to which the compounds are bound. A monolayer of albumin is formed on cellulose triacetate; adsorption of albumin occurs on the inner surface of the membranes and probably also in the pores of the SF [31]. Only limited data are available concerning the degree of albumin adsorption on these membranes. Clark et al. [32] reported an adsorption of only 30.8 ng on CTA membranes after contact with a 50 mg/l bovine serum albumin solution. Since the albumin loss from plasma and the amount of albumin in dialysate were not significantly different in the present study, we consider the adsorption of albumin as a minor component in the elimination of protein-bound compounds by SF.

The decrease in pre-HD concentrations and the better removal of uraemic compounds with SF might contribute to a clinical benefit for patients, since a relation of the concentration of protein-bound uraemic solutes with indices of clinical condition has been demonstrated [33]. Of note, however, differences in concentration between SF and LF, even if significant, are relatively small. The observed differences in clearance may indeed not imply biological significance; only long-term clinical studies could demonstrate such an effect.

Protein binding of HA, IS, IAA and AGEs was increased post-HD vs pre-HD for both membranes. This can in part be explained by the ultrafiltration of plasma water, resulting in an increased serum albumin for a decrease in the total available compound. In addition, several protein-bound solutes compete all together for the same binding sites, which are liberated by their simultaneous removal; also, this effect makes available extra binding sites, resulting in an increase of protein binding. Concerns might be raised with regard to albumin loss across the SF-CTA membrane, in this way contributing to malnutrition. Similar, or even more important, losses have been observed with peritoneal dialysis (3.2 g/day) [34,35]. In addition, one might consider that the uraemic condition provokes the structural degradation of albumin by oxidative and other processes [36]. It might be hypothesized that a part of the lost albumin that had been oxidized is functionally inferior [37], while it might be replaced by newly generated albumin of superior quality if metabolism is intact. Nevertheless, it remains desirable to withhold this SF technique in patients with hypoalbuminaemia until the safety in this condition has been demonstrated.

Post-dialysis samples were collected after reducing the flow to 50 ml/min for 2 min followed by stopping the blood pump. As in our unit several patients are included in the Membrane Permeability Outcome study (MPO) (principal investigator F. Locatelli), we uniformly use the sampling method advocated by this study. To overcome the potential flaw related to this approach, eKt/V was calculated out of the spKt/V values obtained with the above approach.

It is concluded that, in a conventional HD setting, SF-CTA was superior to LF-CTA for several protein-bound compounds, especially IS; reduction rate, dialytic clearance and KoA were increased. Protein-bound solutes behaved more like non-protein-bound solutes with SF-CTA membranes. SF-CTA leaked some albumin, but this could not explain the amount of protein-bound compounds in the dialysate.

Conflict of interest statement. None declared.



   References
Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 References
 

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Received for publication: 9. 2.06
Accepted in revised form: 19. 1.07


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