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Suitability of α_{1}microglobulin reduction rate as a biomarker of removal efficiency of online hemodiafiltration: a retrospective cohort study
Renal Replacement Therapy volume 7, Article number: 10 (2021)
Abstract
Background
Online hemodiafiltration (OLHDF), whether in predilution OLHDF (preHDF) or postdilution OLHDF (postHDF), is conducted to efficiently remove low molecular weight proteins from the blood of patients requiring dialysis. β_{2}microglobulin (β_{2}MG) and α_{1}microglobulin (α_{1}MG) are used as biomarkers to evaluate removal efficiency of OLHDF.
We aimed to evaluate the relationship between β_{2}MG and α_{1}MG reduction rates and the amount of albumin leakage. Furthermore, we statistically analyzed the relationship between the α_{1}MG reduction rate and α_{1}MG removal amount, and its suitability as a biomarker for evaluating the removal efficiency of OLHDF.
Methods
We collected the results of regularly conducted routine evaluations to assess the efficiency of OLHDF from cases of patients undergoing maintenance dialysis at our clinic from 2018 to 2019. Data on was collected on both preHDF and postHDF sessions. β_{2}MG and α_{1}MG reduction rates were analyzed. Regression analysis on reduction rates showed a significant correlation between the α_{1}MG reduction rate and the α_{1}MG removal amount.
Results
We conducted 435 tests on OLHDF efficiency in 87 cases undergoing maintenance dialysis at our clinic in 2018 and 2019. There were 80.7 ± 4.5% for the β_{2}MG reduction rate, 33.8 ± 9.4% for the α_{1}MG reduction rate, and 3.9 ± 1.8 g/s for the amount of albumin leakage. There was no correlation between the β_{2}MG reduction rate and the α_{1}MG reduction rate, or between the amount of albumin leakage and β_{2}MG reduction rate.
Conclusion
α_{1}MG reduction rate was found to correlate with its removal amount, demonstrating its suitability as a biomarker for evaluating the removal efficiency of OLHDF.
Trial registration
Retrospectively registered.
Background
Online hemodiafiltration (OLHDF), whether in predilution OLHDF (preHDF) or postdilution OLHDF (postHDF), is conducted to efficiently remove low molecular weight proteins (LMWP) (molecular weight [MW]: 10,000–55,000), particularly larger LMWPs (MW > 30,000), from blood of dialysis patients. This is accomplished by increasing the convection volume from highvolume replacement fluid. Biomarkers are important for evaluating the efficiency of OLHDF. The small molecular size of β_{2}microglobulin (β_{2}MG) (MW: 11,800, Stokes radius: 16A) allows for removal dependent on both diffusion and convection. Therefore, β_{2}MG cannot be used as a biomarker to accurately assess OLHDF removal efficiency.
α_{1}Microglobulin (α_{1}MG) (MW: 33,000, Stokes radius: 28.4A) is mainly removed by convection in dialysis therapy with a more suitable molecular size; hence, it has often been used as a biomarker to assess the efficiency of removal of middle and largeMW solutes in Japan for over 30 years. Similarly, we reported a strong correlation between changes in the clinical symptoms of dialysis patients and changes in the α_{1}MG reduction rate [1, 2]. We also reported that OLHDF should aim for a β_{2}MG reduction rate of 80% and an α_{1}MG reduction rate of over 35% when performed to treat complications in longterm dialysis patients [3, 4].
There is a twofold difference in molecular weight between albumin (alb) (MW: 66,000, Stokes radius: 35.5A) and α_{1}MG, with a difference in Stokes radius of 20% [5]. Thus, it is impossible to separate α_{1}MG and alb based on differences in molecular size alone and remove them using current dialysis membranes. Some degree of alb leakage is inevitable when removing α_{1}MG at high efficiency. There is no report examining the appropriateness of the reduction rate of the α_{1}MG reduction rate as a biomarker using statistical analysis. In this study, we evaluated the relationship between β_{2}MG and α_{1}MG reduction rates and the amount of alb leakage, as well as the relationship between the reduction rates of β_{2}MG and α_{1}MG based on the results of OLHDF removal efficiency tests regularly conducted as routine examinations in 2018 and 2019. Furthermore, we statistically analyzed the relationship between the α_{1}MG reduction rate and the α_{1}MG removal amount, and we investigated whether this was a suitable biomarker for evaluating the removal efficiency of OLHDF.
Methods
Four hundred thirtyfive tests for evaluating OLHDF efficiency in 87 patients undergoing maintenance dialysis at our clinic between January 2018 and December 2019 (male/female: 60/27, age: 60.5 ± 12.0 years, dry weight: 60.5 ± 13.0 kg, dialysis vintage: 146.0 ± 130.9 months). Pre and postHDF comprised 304 and 131 sessions, respectively. Blood flow rate (Qb) was 261.5 ± 28.5 mL/min, total dialysate flow rate (Qd total) was 400 mL/min for 22 cases and 500 mL/min for 413 cases, and replacement fluid volume was 48.7 ± 8.2 L/session(s) for preHDF and 14.2 ± 3.1 L/s for postHDF. Treatment time was 4.1 ± 0.2 h/s (Table 1). Different types of hemodiafilters commercially available in Japan were used. The obtained test values were used to investigate the relationship between β_{2}MG and α_{1}MG reduction rates and the amount of alb leakage, as well as the relationship between the β_{2}MG reduction rate and α_{1}MG reduction rate.
Thereafter, the relationship between the α_{1}MG reduction rate and its removal amount was analyzed, and whether the reduction rate was suitable for evaluating the removal efficiency was investigated. This was done by setting a 95% prediction interval of the regression curve created from the α_{1}MG removal amount (horizontal axis, mg) and reduction rate (vertical axis, %), and extracting test values that were outliers from this interval from the regression analysis. Values with reduction rates that were lower than the lower limit or higher than the upper limit of the calculated 95% prediction interval were defined as outlier test values. A histogram and QQ plot was used to assess if a set of data have a normal distribution. Continuous variables were expressed as the mean ± standard deviation (SD) and categorical variables as frequencies. A P value of < 0.05 was considered statistically significant, and all P values were twosided. All statistical analyses were performed using SPSS statistics ver. 23.0 (IBM Japan, Ltd., Tokyo, Japan) software.
Postdialysis values used to calculate the β_{2}MG and α_{1}MG reduction rates were values that were corrected by the hematocrit level to exclude concentration effects. The waste dialysate was partially stored throughout the dialysis session at a rate of 2 L/h, after which the total amount stored was wellmixed, and part of this was used to measure the β_{2}MG and α_{1}MG amounts removed and the amount of alb leakage.
This study has been approved by our institutional committee on human and/or animal research. All patients provided informed written consent.
Results
We conducted 435 tests on OLHDF efficiency in 87 cases undergoing maintenance dialysis at our clinic in 2018 and 2019 (male/female: 60/27, age: 60.5 ± 12.0 years, dry weight: 60.5 ± 13.0 kg, dialysis vintage: 146.0 ± 130.9 months). PreHDF comprised 304 sessions, and postHDF comprised 131 sessions. Qb was 261.5 ± 28.5 mL/min, Qd total was 400 mL/min for 22 cases and 500 mL/min for 413 cases, and replacement fluid volume was 48.7 ± 8.2 L/session(s) for preHDF and 14.2 ± 3.1 L/s for postHDF. Treatment time was 4.1 ± 0.2 h/s (Table 1). Ten types of hemodiafilters commercially available in Japan were used (Table 2). The membrane surface area was 2.1 ± 0.1 m^{2}.
The results of all 435 tests (mean ± SD) were 80.7 ± 4.5% for the β_{2}MG reduction rate, 33.8 ± 9.4% for the α_{1}MG reduction rate, and 3.9 ± 1.8 g/s for the amount of alb leakage. There were six cases in ten sessions where the α_{1}MG reduction rate was > 50%, and the maximum α_{1}MG reduction rate value was 60.0%. No significant correlation was observed between the amount of alb leakage and β_{2}MG reduction rate (Fig. 1). There was a slightly strong significant correlation between the amount of alb leakage and α_{1}MG reduction rate. The removal dynamics of alb leakage and α_{1}MG removal were almost identical (Fig. 1). There was no correlation between the β_{2}MG reduction rate and the α_{1}MG reduction rate (Fig. 2).
Results of the regression analysis on reduction rates showed a significant correlation between the α_{1}MG reduction rate and the α_{1}MG removal amount (Table 3). Additionally, 17 out of the 435 total sessions (3.9%) were outlier values from the 95% prediction interval of the regression curve (Fig. 3). It was shown from these analyses that it was possible to appropriately evaluate the removal efficiency of OLHDF by investigating the reduction rate without determining the removal amount.
Discussion
In this study, we evaluated the relationship between β_{2}MG, α_{1}MG reduction rates, and amount of alb leakage, as well as the relationship between the reduction rates of β_{2}MG and α_{1}MG. There was no correlation between the β_{2}MG reduction rate and the α_{1}MG reduction rate, nor between the amount of alb leakage and β_{2}MG reduction rate. We found that alb leakage was inevitable when removing α_{1}MG at high efficiency. Further, we revealed that there was a significant correlation between the α_{1}MG reduction rate and the α_{1}MG removal amount. Our study showed that α_{1}MG reduction rate was a suitable biomarker for evaluating the efficiency of removal of OLHDF.
In Japan, α_{1}MG is widely used as a biomarker for evaluating the performance of removal efficiency of OLHDF for several reasons: (1) optimal molecular size, (2) low likelihood of measurement errors due to its nonnegligible blood concentration, (3) low likelihood of concentration changes under physiological conditions, (4) stable synthesis rate, (5) accumulation in renal failure, and (6) convectionbased removal. α_{1}MG is primarily synthesized in the liver. Under physiological conditions, 50% of it is present in the blood in free form, while 50% is bound with dimeric immunoglobulin A (IgA) [6]. It is estimated that free α_{1}MG increases to around 70% because its excretion from the kidneys is reduced during renal dysfunction, but there are no detailed reports on these dynamics. α_{1}MG has strong antioxidant activity and has been reported as a protective molecule by scavenging free radicals, binding to heme, and undergoing reduction reactions when exposed to oxidative stresses [7,8,9,10,11]. The antioxidant activity of α_{1}MG in dialysis patients may be a topic of future studies.
The MW of IgA is 160,000; hence, the combined MW of the resulting α_{1}MGIgA complex is 350,000, which is impossible to remove using dialysis. Since free α_{1}MG is the target in dialysis therapy and total α_{1}MG is usually measured, the reduction rate is affected by the binding affinity of α_{1}MG and IgA. Consequently, the α_{1}MG reduction rate calculated with the pre and postdialysis values do not appropriately indicate the removal efficiency of OLHDF.
Therefore, we investigated whether the α_{1}MG reduction rate and the α_{1}MG removal amount obtained from the 435 OLHDF sessions were significantly correlated. Results showed that there was a significant correlation between the two measures. However, there were 17 test values outside the 95% prediction interval of the regression curve. Specifically, outliers were observed below the lower limit in seven patients across seven sessions, whereas outliers were observed over the upper limit in five patients across ten sessions. Test conditions when outlier values were obtained may be a topic of future studies.
Measuring the amount of a given solute removed during dialysis is complicated due to the need for the installation of a device that is capable of continuously collecting waste fluid from a drainage line of the patient monitoring system. However, the reduction rate can be calculated simply by collecting blood before and after dialysis and calculating the reduction rate using the following equation:
Where Ht is hematocrit and C is concentration. This enables the evaluation of removal efficiency of middle and largeMW solutes.
It is essential in the evaluation of highefficiency OLHDF to investigate the removal efficiency of solutes with a MW of 30,000–50,000 using α_{1}MG as a biomarker. Our study supported the use of α_{1}MG reduction rate to be an appropriate evaluation method indicating that past accumulated data can be applied to future studies.
Several reports investigated the effects of OLHDF on a patient’s survival [12,13,14]. Kikuchi et al. [15] reported that preHDF with highreplacement fluid volume had a more favorable effect on patient’s survival than hemodialysis and preHDF with lowreplacement fluid volume. However, both reports only investigated the impacts of replacement fluid volume on a patient’s survival, and neither mentioned specific numerical values for removal efficiency. In Europe, α_{1}MG has been used for over five years as a biomarker for removal performance when evaluating OLHDF or highperformance dialyzers, and the evaluation of dialysis efficiency using α_{1}MG reduction rate has become increasingly common [16, 17]. The groundbreaking JAMREDS study, which was started in Japan this spring, examined the effect of not only replacement fluid volume but also α_{1}MG reduction rate on patient survival of OLHDF. Hence, the usefulness of removing middle and largeMW toxins is anticipated to become even more apparent in the future.
The limitation of this study was that it was performed in a single facility, which can lead to selection bias.
Conclusions
We found that α_{1} reduction rate may be used as a valid biomarker to evaluate the removal efficiency of OLHDF. Furthermore, some alb leakage is inevitable when removing α_{1}MG at high efficiency.
Availability of data and materials
The datasets during and/or analyzed during the current study available from the corresponding author on reasonable request.
Abbreviations
 alb:

Albumin
 HD:

Hemodialysis
 IgA:

Immunoglobulin A
 LMWP:

Low molecular weight protein
 MW:

Molecular weight
 OLHDF:

Online hemodiafiltration
 preHDF:

Predilution online hemodiafiltration
 postHDF:

Postdilution online hemodiafiltration
 SD:

Standard deviation
 Qb:

Blood flow rate
 Qd total:

Total dialysate flow rate
 α_{1}MG:

α_{1}microglobulin
 β_{2}MG:

β_{2}microglobulin
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Acknowledgements
This study received no external funding and was selffunded by Kenji Sakurai.
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KS designed the study and drafted the manuscript. HH and YK acquired and analyzed the data. TS designed the study. All authors read and approved the final manuscript.
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This study has been approved by our institutional committee on human and/or animal research. All patients provided informed written consent.
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Sakurai, K., Hosoya, H., Kurihara, Y. et al. Suitability of α_{1}microglobulin reduction rate as a biomarker of removal efficiency of online hemodiafiltration: a retrospective cohort study. Ren Replace Ther 7, 10 (2021). https://doi.org/10.1186/s4110002100326y
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Keywords
 α_{1}microglobulin
 Reduction rate of α_{1}microglobulin
 Online hemodiafiltration
 Biomarker for evaluation of removal efficiency