Oxalate clearance by haemodialysis a comparison of seven dialysers

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1 Nephrol Dial Transplant (2005) 20: doi: /ndt/gfh971 Advance Access publication 5 July 2005 Original Article Oxalate clearance by haemodialysis a comparison of seven dialysers Casper F. M. Franssen Dialysis Centre Groningen and Department of Internal Medicine, Division of Nephrology, University Medical Centre Groningen, Groningen, The Netherlands Abstract Background. In patients with primary hyperoxaluria (PH) and oliguric end-stage renal disease, oxalate removal is largely dependent on the clearance by dialysis. Data on the oxalate clearance of newer dialyser types are scarce or absent. Therefore, we measured oxalate clearances of seven dialysers in a single 52-yearold female patient with PH. Since haemodiafiltration (HDF) has been advocated to increase oxalate clearance, we also assessed the effect of different predilution flows. The goal of the study was to select the dialyser and pre-dilution flow combination with the highest oxalate clearance. Methods. Oxalate clearances were assessed by simultaneously taking afferent blood and efferent dialysate samples at 30, 60, 120 and 180 min after the start of haemodialysis. Blood flow and dialysate flow were 350 and 500 ml/min, respectively. All dialysers were tested at a pre-dilution flow of 2 l/h. Six dialysers were also tested at either a pre-dilution flow of 4.5 l/h or without HDF, depending on the ultrafiltration coefficient of the dialyser. Results. Oxalate clearances differed markedly between the tested dialysers, ranging from 144±10 to 220±12 ml/min. The highest oxalate clearances were achieved with HdF100S (219±10 ml/min) and Sureflux FB-210U (220±12 ml/min) at a pre-dilution flow of 2 l/h. Higher pre-dilution flows (2 l/h vs no HDF or 4.5 vs 2.0 l/h) yielded similar oxalate clearances. Conclusion. The highest oxalate clearances were achieved with a high-flux polysulfone and a cellulose triacetate dialyser with a large surface area. Higher pre-dilution flows did not augment oxalate clearance. Keywords: clearance; dialysis; haemodiafiltration; haemodialysis; oxalate; primary hyperoxaluria Correspondence and offprint requests to: C. F. M. Franssen, PhD, Department of Internal Medicine, Division of Nephrology, University Medical Centre Groningen, Hanzeplein 1, 9713 GZ Groningen, The Netherlands. c.f.m.franssen@int.umcg.nl Introduction Primary hyperoxaluria type 1 (PH1) is a rare autosomal recessive disorder caused by absent or a functional defect of the liver-specific enzyme alanine:glyoxylate aminotransferase (AGT) [1]. AGT deficiency results in increased synthesis of the metabolic end-product oxalate, most of which is excreted in the urine as long as the renal function is preserved. Many PH1 patients develop progressive renal failure over the years as a result of parenchymal oxalate deposition and renal stone formation. As renal function deteriorates, an increasing fraction of the daily oxalate production is retained and deposited in renal and extrarenal tissues, e.g. bones, nerves, retina and the heart [2]. Except for those patients who receive a kidney transplant or a combined liver and kidney graft before dialysis is started, most PH1 patients with end-stage renal disease are dependent on short- or long-term renal replacement therapy. In these patients, oxalate is removed from the body only by residual renal function and by dialysis. Unfortunately, no form of dialysis is able to keep up with endogenously produced oxalate [1,2]. However, assuming a constant oxalate production rate, it is obvious that it is worthwhile to remove as much oxalate as possible by dialysis in order to limit tissue oxalate accumulation. The weekly oxalate removal can be enhanced by increasing the haemodialysis intensity, especially by increasing the frequency of the haemodialysis sessions or by combining haemodialysis with peritoneal dialysis [3 6]. Oxalate removal can be enhanced further by increasing the oxalate clearance during the haemodialysis session. Oxalate clearances of dialysers have been studied previously in PH patients [1,6 8] and in non-ph patients [9 13]. The mean dialyser surface areas in these studies was 1.2 m 2 (range m 2 ) and reported oxalate clearances varied between 51.9 and 143 ml/min. In recent years, new dialysers have become available, especially with larger surface areas and higher ultrafiltration coefficients (K UF ), but data on the oxalate clearance of these dialysers are not ß The Author [2005]. Published by Oxford University Press on behalf of ERA-EDTA. All rights reserved. For Permissions, please journals.permissions@oupjournals.org

2 Oxalate clearance by haemodialysis 1917 available. To optimize oxalate removal in a 52-year-old female PH1 patient with end-stage renal disease, we measured oxalate clearances of seven dialysers which differed in membrane material, surface area and K UF. Since haemodiafiltration (HDF) has been advocated to increase oxalate clearance [9,12], we also assessed the influence of HDF in pre-dilution mode on the oxalate clearance. The ultimate goal of the study was to be able to select the dialyser type and pre-dilution flow combination with the highest oxalate clearance. Methods Oxalate clearances were measured in a 52-year-old female patient with PH (patient no. 1). She had recurrent kidney stone formation from the age of 10 years. PH1 had been diagnosed on the basis of increased urinary excretion of both oxalate and glycolate in combination with pyridoxine responsiveness of oxalate levels. PH1 was not confirmed by liver biopsy. In the 10 years before she started dialysis, urinary oxalate excretion averaged 1500 mmol/24 h (range mmol/24 h). At the time of the clearance studies, she had been on renal replacement therapy for 14 months. Signs of systemic oxalate deposition included peripheral neuropathy and mild renal calcinosis. During the clearance studies, post-dialysis body weight varied between 64.5 and 65.5 kg; her height was 174 cm. The inter-dialytic weight gain ranged from 0.8 to 2.0 kg. Diuresis averaged 1000 ml/24 h and residual creatinine clearance was 2 ml/min. Urinary oxalate excretion ranged from 590 to 680 mmol/24 h during the clearance studies. The haemodialysis sessions were haemodynamically stable. Haematocrit ranged from 0.32 to Blood access was obtained by an Brescia-Cimino arteriovenous fistula. There was no recirculation. The haemodialysis schedule was four times a week for 4 h. All sessions were performed with an AK-200 dialysis apparatus (Gambro, Lund, Sweden). Blood flow and dialysate flow were 350 and 500 ml/min, respectively. Dialysate composition was sodium 140 mmol/l, potassium 1.0 mmol/l, calcium 1.5 mmol/l, magnesium 0.5 mmol/l, chloride 108 mmol/l, bicarbonate 34 mmol/l, acetate 3.0 mmol/l, glucose 1.0 g/l. Dialysate temperature was 36.0 C. In this patient, the oxalate clearances of seven dialysers (Table 1) were measured during separate haemodialysis sessions. All dialysers were tested at a pre-dilution flow of 2 l/h. Six of the seven dialysers were also tested at a second pre-dilution flow during a separate haemodialysis session: dialysers with K UF <20 ml/h mmhg were also tested without HDF, and dialysers with K UF 20 ml/h mmhg were also tested with a pre-dilution flow of 4.5 l/h. The high-flux dialysers were not tested without HDF to avoid significant back-filtration. After completion of the studies in patient no. 1, the oxalate clearance of one of the dialysers that performed best in this patient (Sureflux FB-210U) was also measured in three additional patients with PH1. These patients (patient nos 2, 3 and 4) were referred to our hospital for transplantation (kidney transplantation or combined kidney and liver transplantation) and were dialysed pre-transplantation in our centre. One of these patients (patient no. 4) was also dialysed once post-transplantation. Blood flow and dialysate flow in these three patients differed from those in patient no. 1. In patient no. 2, blood flow was 350 ml/min and in patient nos 3 and 4 blood flow was 400 ml/min. Dialysate flow was 700 ml/min in all three patients. Dialysate composition and temperature were identical to the regimen in patient no. 1. During a test haemodialysis session, the oxalate clearance was assessed four times (at 30, 60, 120 and 180 min after the start of the haemodialysis session) by simultaneously taking an afferent blood and efferent dialysate sample. The oxalate clearance was calculated as follows: clearance ¼ Q do (C do /C bi ). Q do denotes the efferent dialysate flow. C do and C bi denote the oxalate concentrations in the efferent dialysate and the afferent blood sample, respectively. The efferent dialysate flow Q do was calculated as: Q di þ UF rate þ pre-dilution flow rate. Q di is the afferent dialysate flow. The four independent clearance calculations during each test dialysis session were averaged to obtain the mean (±SD) oxalate clearance of the dialyser. In patient no. 1, the total amount of oxalate removed by a single haemodialysis session was assessed by continuous partial dialysate sampling (on ice) and multiplication of the oxalate concentration in this sample by the total waste dialysate volume [13]. All blood and dialysate samples were immediately put on ice and processed within 30 min. Samples were deproteinized using hydrochloric acid and sodium chloride. After centrifugation, samples were stored at 20 C for a period up to 2 months [14 16]. Oxalate concentrations in plasma and dialysate were determined using isotope dilution mass spectrometry essentially as described [14 16]. The normal upper plasma reference value in our laboratory is 5.0 mmol/l. Table 1. Specifications of the tested dialysers Dialyser Manufacturer Material K UF (ml/h mmhg) Surface area (m 2 ) Crystal 4000 Hospal Dasco, Medolla, Polyacrylonitrile (AN69XS) Modena, Italy Nephral 500 Hospal Dasco Polyacrylonitrile (AN69XT) HdF100S Fresenius Medical Care, Polysulfone Bad Homburg, Germany HG-700 Cobe, Lakewood, CO, USA Hemophane Sureflux 150L Nipro, Nissho Corporation, Cellulose triacetate Osaka, Japan Sureflux 210E Nipro Cellulose triacetate Sureflux FB-210U Nipro Cellulose triacetate

3 1918 C. F. M. Franssen All data are presented as mean±sd. Clearances were compared using the non-parametric Mann Whitney U-test. Correlations between different parameters were calculated with the non-parametric Spearman test. A P-value <0.05 was considered to be statistically significant. Results Pre- and post-dialysis plasma oxalate levels, total oxalate removal and oxalate clearances in patient no. 1 are shown in Table 2. The oxalate clearance of each dialyser was stable during the 2.5 h period (from 30 to 180 min after the start of the dialysis session) during which the clearances were measured (data not shown). Oxalate clearances differed markedly between the tested dialysers, ranging from 144±10 to 220±12 ml/min for the Crystal 4000 (HDF 4.5 l/h) and the Sureflux FB-210U dialyser (HDF 2.0 l/h), respectively. Higher pre-dilution flows (2 l/h vs no HDF or 4.5 l/h vs 2.0 l/h) yielded similar oxalate clearances. The oxalate clearance at HDF 4.5 l/h did not differ from the clearance at HDF 2 l/h in the five dialysers that were tested at both these pre-dilution flows in patient no. 1 (P ¼ 0.55). The highest oxalate clearances were achieved with HdF100S (219±10 ml/min) and Sureflux FB-210U (220±12 ml/min) at a pre-dilution flow of 2 l/h. Figure 1a and b shows the relationship between the oxalate clearance and the dialyser surface area and between the oxalate clearance and the dialyser K UF, respectively. The Spearman correlation coefficient between surface area and oxalate clearance for the seven dialysers that were tested at HDF 2 l/h was There was a trend towards a statistically significant correlation (P ¼ 0.10). There was no significant correlation between K UF and the oxalate clearance for the seven dialysers which were tested at HDF 2 l/h (r ¼ 0.1). As shown in Figure 2, in patient no. 1 there was a significant correlation between the intra-dialytic oxalate removal and the pre-dialysis plasma oxalate levels (r ¼ 0.9; P<0.001). In this patient, there was no significant correlation between the oxalate clearance and intra-dialytic oxalate removal. As shown in Table 3, the oxalate clearances with the Sureflux FB210U dialyser in patient nos 2, 3 and 4 were slightly higher in comparison with the clearance of the Sureflux FB-210U dialyser in patient no. 1. Of note, these three patients were dialysed with higher blood and/or dialysate flows than patient no. 1. Patient no. 4 was tested at a pre-dilutional HDF flow of both 2 and 4.5 l/h. In this patient, pre-dilutional HDF of 4.5 l/h did not result in a higher oxalate clearance in comparison with HDF 2 l/h. Discussion This study shows that oxalate clearances differ markedly between dialysers. Of the tested dialysers, the highest oxalate clearances were achieved with a Table 2. Plasma oxalate levels, oxalate removal and oxalate clearance during haemodialysis in patient no. 1 Dialyser Pre-dialysis plasma oxalate Post-dialysis plasma oxalate Oxalate reduction ratio (%) Oxalate removal (mmol/ session) Mean (SD) oxalate clearance (ml/min) (mmol/l) (mmol/l) No HDF HDF 2 l/h HDF 4.5 l/h Crystal 4000 HDF 2 l/h ±10 HDF 4.5 l/h ±10 Nephral 500 HDF 2 l/h NA 173±2 HDF 4.5 l/h NA 184±4 c HdF100S HDF 2 l/h ±10 a,b HDF 4.5 l/h ±54 HG-700 No HDF ±7 HDF 2 l/h ±31 Sureflux 150L HDF 2 l/h ±6 a Sureflux 210E HDF 2 l/h NA 187±12 a HDF 4.5 l/h ±4 c Sureflux FB-210U HDF 2 l/h ±12 a,b HDF 4.5 l/h ±19 c In all sessions, blood flow and dialysate flow were 350 and 500 ml/min, respectively. HDF ¼ haemodiafiltration in pre-dilution mode; NA ¼ data not available. a P<0.05 in comparison with Crystal 4000 at HDF 2 l/h. b P<0.05 in comparison with Nephral 500 at HDF 2 l/h. c P<0.05 in comparison with Crystal 4000 at HDF 4.5 l/h.

4 Oxalate clearance by haemodialysis 1919 (a) Oxalate clearance (ml/min) Oxalate removal (µmol/session) (b) Oxalate clearance (ml/min) Dialyser surface area (m 2 ) K UF (ml/hxmmhg) Fig. 1. (a) Relationship between the dialyser surface area and oxalate clearance. See text for statistics. Filled squares, no HDF; filled circles, HDF 2 l/h; filled triangles, HDF 4.5 l/h. (b) Relationship between the ultrafiltration coefficient (K UF ) of the dialyser and oxalate clearance. See text for statistics. Filled squares, no HDF; filled circles, HDF 2 l/h; filled triangles, HDF 4.5 l/h. high-flux polysulfone and a high-flux cellulose triacetate dialyser, both with a large surface area. Higher predilution flows up to 4.5 l/h did not augment oxalate clearance in comparison with no HDF in the low-flux dialyser or compared with a pre-dilution flow of 2 l/h in the high-flux dialysers. This finding is in keeping with the expected limited role of convection in the clearance of this relatively small (mol. wt 90 Da) molecule. Although convection itself is expected to result in a slight increase of the clearance, the overall oxalate clearance did not increase with higher HDF flows probably because pre-dilutional HDF limits clearance by dilution of the afferent blood. The observation of a lack of effect of higher pre-dilution flows on oxalate clearance contrasts with the results of two other studies that reported a favourable effect of HDF in comparison with standard haemodialysis [1,2]. These studies involved only non-ph patients Pre-dialysis plasma oxalate level (µmol/l) Fig. 2. Correlation between pre-dialysis plasma oxalate levels and intra-dialytic oxalate removal (r ¼ 0.9; P < 0.001). Marangella et al. compared standard haemodialysis using a cuprophane dialyser and HDF using a dialyser with a polysulfone or AN69 membrane and reported a mean oxalate clearance of 82±25 during standard haemodialysis and 113±45 during HDF [3]. However, this study is difficult to interpret since details on the dialyser surface areas and K UF are not presented. Dell Aquila et al. compared the weekly oxalate removal by standard haemodialysis using a 1.3 m 2 cuprophane dialyser with HDF using a 1.2 m 2 AN69 dialyser [2]. These authors found a higher weekly oxalate removal during HDF compared with standard haemodialysis. This study should be interpreted with caution since the authors did not provide any data on plasma oxalate levels or oxalate clearances (see below). As we only tested the effect of HDF in pre-dilution mode, we cannot rule out that HDF in post-dilution mode may yield higher oxalate clearances at higher postdilution flows. There was a trend towards a significant correlation between the dialyser surface area and oxalate clearance. The lack of a significant correlation is probably explained by the relatively small number of dialysers tested. In addition, factors other than dialyser surface area play a role in the diffusion process. For instance, the charge of the dialyser membrane may be important. The polyacrylonitrile fibres of the Crystal 4000 have a negative charge, as has the oxalate molecule [17]. This may, in part, explain the significantly lower oxalate clearance of this dialyser in comparison with a cellulose triacetate dialyser (Sureflux 150L) that has almost the same surface area. A limitation of the comparative dialyser study in patient no. 1 is that each dialyser and pre-dilution flow combination has been tested during only one dialysis session. Therefore, more data are necessary before firm conclusions can be drawn for specific dialysers.

5 1920 C. F. M. Franssen Table 3. Oxalate clearances of the Sureflux FB-210U dialyser in patient nos 2, 3 and 4 with primary hyperoxaluria type 1 Patient no. Patient characteristics Pre-dialysis plasma oxalate (mmol/l) Blood Mean (±SD) oxalate clearance (ml/min) flow/dialysate flow (ml/min) HDF 2 l/h HDF 4.5 l/h 2 38 year-old female; weight 52 kg; height 168 cm; 10 years on dialysis 3 25 year-old male; weight 71 kg; height 176 cm; 2 years on dialysis 4 35 year-old male; weight 68 kg; height 178 cm; 4.5 years on dialysis / ± / ± (HDF 2 l/h) 400/ ±28 50 a (HDF 4.5 l/h) 400/ ±9 a Post-transplantation. There was a strong correlation between the intradialytic oxalate removal and the pre-dialysis plasma oxalate levels, an observation that has already been described by Hoppe et al. [2]. As a practical consequence, intra-dialytic oxalate removal cannot be used as a surrogate marker for oxalate clearance since it is highly dependent on the pre-dialysis plasma level. The assessment of clearance has the advantage that it is not influenced by the plasma level of the substance that is being studied. Therefore, the efficiency of different haemodialysis settings should primarily be compared by clearance and not by total removal. Assessment of the total weekly oxalate removal, however, may give useful additional information on the overall effectiveness of the haemodialysis regime. Although the exact oxalate production in this patient is unknown, it is probably more than the 1500 mmol/day that was being excreted in the urine before she developed end-stage renal disease since she already had signs of oxalate deposition at the start of renal replacement therapy. In PH1, daily oxalate production has previously been estimated at mmol/day [18]. Patient nos 2 and 4 were on dialysis for many years before they were transplanted. There were various reasons for this long interval to transplantation including significant co-morbidity and anti-hla (human leukocyte antigen) antibodies in patient no. 2. Such a long interval is not recommened since it is associated with ongoing tissue oxalate deposition and carries a great risk of post-transplantation renal failure due to calcium oxalate deposition in the kidney graft. Therefore, early transplantation, preferably a combined liver and kidney transplantation, is warranted if ever possible. The oxalate removal per unit time of haemodialysis can be increased by using a dialyser with a higher oxalate clearance. However, this does not compensate for an insufficient dialysis schedule. In primary hyperoxaluria, more frequent dialysis sessions are needed to limit oxalate deposition as much as possible. 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