In hemodialysis (HD), the composition of the dialysis fluid (DF) is key to optimize the balance of fluids, electrolytes and acid-base.1 HD machines produce the DF by mixing its three components: purified water, concentrated acid and bicarbonate. Acid concentrate receives this name because it must contain an acid which, when mixed with bicarbonate in the hydraulic circuit of the machine, prevents the precipitation of calcium and magnesium carbonate. The acid most commonly used is acetic acid, in concentrations ranging from 3 to 10 mmol/L, most often 3−4 mmol/L. This concentration is about 30 times higher than the observed in plasma, 0.1 mmol/L, which results in a net transfer of acetate from the DF to plasma during the HD session, increasing the plasma concentration of acetate.2 The concentration of acetate during and after HD are higher in those patients with a lower capacity to metabolize acetate, generally with decreased muscle mass, and will produce adverse effects.

The DF with citrate (CDF) was developed to improve the biocompatibility of the dialysis with bicarbonate plus acetate.3 Citrate is a calcium chelating agent and as such it has anticoagulant properties, by reducing ionic calcium. The benefits that have been reported with CDF include a reduction of thrombogenicity,4 inflammation, an increased clearance of low and medium molecular weight molecules,5,6 improvement in nutrition,7 tolerance8 and acid-base control, with less predialysis acidosis9 and without severe complications even with the hemodiafiltration.10

The clinical evidence of these advantages in randomized studies is scarce. The use of DF with citrate has increased slowly in daily clinical practice.

In 2019 we published the first part of the ABC-treat study11 (NCT03319680), which demonstrated that the use of citrate instead of acetate has an acute effect on electrolytes, reducing the levels of bicarbonate, calcium and magnesium post-dialysis and increasing PTH. A lower frequency of hypotension was also observed, 10.8% with CDF as compared to 14.1% with ADF.

This second part of the ABC-treat study investigates the chronic effect of HD with citrate, without acetate, compared with the standard DF with respect to changes in acid-base status, parameters of mineral-bone metabolism, dialysis efficacy, tolerance, nutrition and inflammation.

Although benefits of citrate dialysis fluid compared to that of acetate have been published, the information from randomized studies is scarce. This study was designed to compare the effect of citrate with acetate in a controlled and randomized manner. Dialysis with citrate versus acetate causes changes in parameters of bone-mineral metabolism, and acid-base, acutely and chronically, and also improves hemodynamic tolerance and probably, the nutritional status.

As previously described,11,15 citrate improves acid-base control with respect to Acetate; it decreases the peak of alkalemia post-HD while maintaining the same acid-base balance, as demonstrated by no change in preHD bicarbonate after 16 weeks.

Severe postHD alkalemia has been associated with hemodynamic instability16 and arrhythmias.17 HD with a high and inadequate concentration of bicarbonate can increase mortality.18 The metabolism of citrate19 is probably slower than that of acetate.10 About 25% of buffers transferred to the patient during HD depend on the metabolization of acetate or citrate.

The present study shows that, the CDF, as compared to the ADL, modifies some parameters of calcium-phosphorus metabolism both acutely, as previously described,4,5,9 and at a long term. Citrate is associated with lower levels ionized calcium and magnesium, pre and postHD and an increase in the serum concentration of PTH. All this despite the higher concentration of calcium used in the LDC as compared with ADF. Citrate has a high affinity for calcium and magnesium, which modifies the mass transfer through the dialyzer of these two elements. Low calcium concentrations in DF may cause hypocalcemia, that would stimulate PTH, bone turnover, and the appearance of cramps, arrhythmias and increasing the risk of hypotensive episodes. For all of the above, we and other authors,6,20 increase the calcium concentration in the DF when patients are dialyzed with citrate. As previously observed,6 the increase in calcium of 0.15 mmol/L does not prevent the decrease in ionic calcium. As described Gabutti et al.,20 the CDF, despite using a higher concentration of calcium, prevents the increase in post-HD ionic calcium that appears with ADF, which in turn inhibits the secretion of PTH. In this work we observe that the postHD values ​​of ionic calcium and magnesium are slightly lower (∼0.1 mmol/L) with LDC than with LDA, which has been previously described.10 After 16 weeks with citrate, preHD PTH level is significantly increased.

Observational studies have shown that the risk of death is increased in HD patients with values of iPTH <2 or> 9 times the upper limit of normal for the assay.21 The spectrum of patients with CKD stage 5 has changed over the past 20 years, from a bone with high-remodeling to low-remodeling form of bone disease which is currently present in 40 and 70% of patients, and it is characterized by relatively low levels of iPTH. This change is due to a progressive aging of the HD population with more diabetics and a high prevalence adynamic bone disease, which is associated with an increased risk of fractures and vascular calcifications,22–24 that together with aging and diabetes are important risk factors for death.25,26 In this context, less alkalosis, the reduction of ionized calcium levels with an increase in PTH produced by the CDF could be an advantage to reduce vascular calcifications and adynamic bone disease. In addition, citrate would decrease the propensity to calcifications as measured by the T50 test at 3 months in a preliminary study,27 in a short study duration, multicenter, randomized and crossover28 and in another performed in rats with ex vivo cultured aortic rings.29

In our study, the serum magnesium concentration decreased when patients were on CDF.

This occurred despite that both CDF and ADF contained the same concentration of magnesium in the DF (0.5 mmol/L). This has been described in other studies,10 and it is not surprising since citrate chelates the magnesium in addition to calcium. Elevated serum magnesium levels are associated with a decrease in vascular calcifications.30,31 However, low serum magnesium on dialysis citrate does not adversely affect the propensity for calcification.27 Although optimal serum levels in HD patients remain controversial,32 patients with low levels of Mg have a worse prognosis than those with somewhat high levels, as we have recently published33; similar results have been observed in other studies.34–40 The type of DF could influence the concentration of magnesium and the risk of death.33 Jefferies et al.41 in a study based on DF with acetate, compares the usual concentration of magnesium, 0.5 mmol/L with a high, 1.0 mmol/L, without observing adverse effects with DF with high magnesium concentrations. It has been suggested that magnesium in the CDF should be increased, an idea that we share. The increase in magnesium concentration in dialysis fluid has been associated with improvement of vascular calcifications,42 a reduction in total and cardiovascular disease mortality,43 as well as a reduction in inflammation markers (TNF alpha and IL6).44

Although some authors have published an improvement in the effectiveness of dialysis with the CDF, as measured by Kt/V or Kt, we have not confirmed this finding. This improvement has been explained by the anticoagulant effect of citrate. In the present study, as previously described,11 we did not observe differences in the degree of coagulation of chambers or dialyzers, the degree of coagulation with both CDF and ADF was 0, absent, or 1, minimum in 80% of the sessions. Our explanation is that the differences in the coagulation of the dialyzer and therefore in the dialysis efficiency only becomes apparent when heparin doses are low.

The most relevant effects of acetate in HD are hemodynamic instability secondary to the vasodilation it causes. This is mediated by nitric oxide45 and activation of pro-inflammatory cytokines.46,47 The plasma acetate increases above 1 mmol/L after HD with a DF containing 4 and 8 mmol/L of acetate.48 In patients with a slower rate of acetate metabolism, such as those with a reduced muscle mass, the concentrations of acetate would be higher and for a longer period of time. Previous studies found a lower frequency of hypotension in 44 patients in HD, especially the symptomatic and more severe episodes.8 With ADF there would be a greater fall in peripheral vascular resistance and systolic blood pressure.20 In the same line of these observations, we found that the hypotension episodes are more frequent during ADF than with CDF, our study which compile the information from 4416 HD sessions would be the largest evaluating this aspect.

In the present work there are no significant changes in CRP. The beneficial effect of citrate and the removal of acetate on inflammation49 could be counteracted, in some patients, due to the appearance of hypomagnesemia.50

One of the most interesting results of our study is the significant increase in Lean Mass Index (LTI) after 16 weeks with citrate, suggesting that CDF could improve nutritional status. In this sense, there is evidence previously published about an increased phase angle and body cell mass measured by monofrequency bioimpedance after 12 weeks of with citrate.51 The serum albumin would increase after 12 weeks with citrate,51 sometimes this effect would be limited to patients with hypoalbuminemia,52 as is the case in our patients with an albumin below 3.8 g/dl. These studies attribute the improvement in nutritional status to a reduction of inflammation or to the presence of citrate in the CDF.6 The anabolic impact suggested by the increase of the LTI could be related by the calories contributed by citrate53 and its incorporation into the Krebs cycle.54 Acetate to be metabolized in that precise way of needs Coenzyme A, pyruvate dehydrogenase and NAD+. The incorporation of citrate into cells do not require insulin.55 Mitochondria play a key role in the skeletal muscle function56 and citrate can be incorporated directly if other substrates are scarce.57 These data have been collected in patients with citrate as an anticoagulant, in which citrate is used in higher concentrations than in our study. However, the infusion of the CDF into OL-HDF provides a much greater amount of citrate to patients than conventional HD. Mass muscle and its function are adversely affected by several factors in patients with CKD in HD,58 the use of citrate instead of acetate could improve some of them.

The sample of patients studied is small and includes only patients with an AVF. Because one of the objectives in the study design was to assess the efficacy of dialysis and the duration of the study was 32 weeks, it was decided to exclude patients with a catheter to avoid the possibility of dysfunction and influence on dialysance. For this reason, the etiology of CKD of the patients included in the study is not representative of the entire dialysis population, since the first cause was glomerulonephritis, followed by diabetes, while both in the Spanish population and in most countries, the main cause of CKD in dialysis patients is diabetes. A positive aspect of the study was the use of a crossover design: thus, we eliminate patient variation over time, and served as his own control. Although we did not analyze the impact of long-term use of CDF in key objectives, such as mortality,59–61 in our knowledge, this is the longest randomized trial using citrate, allowing the assessment and exploration of long-term consequences.

In conclusion, DF with citrate modifies most of the metabolic parameters of bone mineral, not only acutely, but also in the long term. As far as we know, this it is the first time this has been addressed in a randomized controlled trial. This observation has two important practical implications: the magnesium concentration should be increased in DF with citrate in most cases, and citrate is the treatment of choice in patients with adynamic bone disease or low PTH levels. Compared to acetate, citrate offers greater hemodynamic stability reducing episodes of hypotension, and may improve nutritional status. Long-term randomized controlled trials are needed to confirm the potential nutritional benefits and their impact on the morbidity and mortality.

The sponsor of this study has been the Foundation of the Spanish Society of Nephrology (SEN) and an Independent Research grant from Baxter.

P. de S. has received consultancy or speaker fees or travel support from Vifor Pharma, Amgen, Fresenius, Astra Zeneca, Nipro, Alexion, Astellas, Braun and Baxter. R.P.G., M.M. have received fees for the participation as speakers at Fresenius and Baxter meetings and aids for travel to Scientific Congresses from Nipro, Fresenius and Baxter.