| Abstract|| |
In recent years, methods of on-line urea concentration and on-line urea clearance monitoring have been proposed for control of dialysis dose (Kt/V) and protein catabolic rate (PCR) in patients on regular dialysis therapy; these offer an alternative to the established methods of urea kinetics based on pre- and post-dialysis measurements of urea concentration. In contrast to such conventional urea kinetics, the new methods deliver results in real time and treatment parameters can be changed instantly. Three on-line measurement methods are to be distinguished: monitoring of urea concentration in ultrafiltrate, monitoring of urea concentration in dialysate (both yield Kt/V and PCR), and monitoring of urea clearance based on conductivity measurements. Some of these approaches are already applied commercially. Here, these methods are compared using results obtained from laboratory and clinical studies. The on-line methods are found to be more accurate than methods based on pre- and post-dialysis urea concentrations, and to be better suited for clinical routine. This paper outlines the principal methods, reviews the present literature, gives an overview of the applications and compares them to conventional pre- and post-dialysis concentration-based methods of urea kinetics. It is concluded that these methods are likely to find a widespread application in the control of dialysis adequacy.
Keywords: Hemodialysis, Urea clearance, Quality control, Urea kinetics.
|How to cite this article:|
Stiller S, Al-Bashir A, Mann H. On-line Urea Monitoring during Hemodialysis: A Review. Saudi J Kidney Dis Transpl 2001;12:364-74
|How to cite this URL:|
Stiller S, Al-Bashir A, Mann H. On-line Urea Monitoring during Hemodialysis: A Review. Saudi J Kidney Dis Transpl [serial online] 2001 [cited 2021 Apr 18];12:364-74. Available from: https://www.sjkdt.org/text.asp?2001/12/3/364/33560
| Introduction|| |
It is generally accepted that in dialysis therapy efficiency can be described in terms of low molecular weight solute clearance.
Therefore, for about 20 years, the individual dose of dialysis (Kt/V) has been defined as the product of urea clearance (K) times the length of treatment time (t) in relation to the urea distribution volume (V) of the patient. Hereby urea is accepted as a marker of low molecular weight solutes; it is distributed in whole body water and can be measured easily. In addition to this, urea is an end product of protein metabolism and thus enables us to determine protein catabolic rate (PCR), a measure of patient nutrition.
Both parameters, Kt/V and PCR, are normally derived from pre- and post-dialysis plasma urea concentrations by formal urea kinetics or by using one of the available short formulae. Formal urea kinetics is defined as the solution of the mass balance equations for the considered session. Despite the drawbacks which are outlined later, consequent monitoring of Kt/V and PCR and adherence to recommended ranges for both (Kt/V between 1.2 and 1.6, PCR > 1.0 g/kg/day) has been shown to decrease morbidity and mortality and enhance overall well-being of patients on regular dialysis treatment.
Methods of on-line urea concentration and on-line urea clearance monitoring that claim to avoid the disadvantages of the conventional pre- and post-dialysis measurement methods have been proposed over the last 10 years. Meanwhile, complete systems for on-line monitoring which are integrated into the dialysis machine have become commercially available. In this paper, the different approaches to on-line urea monitoring are described and relevant publications are reviewed.
| Methods for measuring urea concentration on-line|| |
Urea monitoring during dialysis is done by passing either ultrafiltrate or dialysate at a small flow rate through a conductivity cell, a cartridge containing urease and a second conductivity cell in series [Figure - 1]. Urea is broken down to NH 4 + ,OH - and HCO3 - in the cartridge, which contains immobilized urease. This results in a change in electric conductivity ∆S, which is calibrated against the urea concentration. In order to avoid exhausting the urease in the cartridge, urea is measured discontinuously at suitable intervals; the cartridge is disposed of after a session.
The urea concentration can be measured on-line either directly or indirectly via the dialyzer clearance (K). In the direct measuring approach, urea can be measured at given intervals (ti) either in filtrate or in dialysate [Figure - 2]a and b).
The urea concentration in the filtrate is equal to the plasma water concentration. From the measured urea concentration profile the distribution volume (V) and the amount of urea removed (UR) during one dialysis session and the clearance (K) can be derived using the 2-pool kinetic modeling; Kt/V and PCR can then be calculated. Although many studies have been performed using this method, ,,, filtrate-based measurements of urea are not employed in any of the commercially available systems.
Shunt and cardiopulmonary recirculation both decrease urea concentration in the ultrafiltrate as compared to arterial plasma concentration. While the influence of cardiopulmonary recirculation is small (some 3%), shunt recirculation may seriously falsify the results.
Measurement of urea in the effluent dialysate makes it possible to calculate the amount of urea removed during one session directly (i.e. without using kinetic modeling). This is termed 'true removed urea' or TRU. TRU per session is not the same in each of the three weekly sessions: it is highest at the beginning of the week and lowest at the end of the week. This must be taken into consideration when calculating PCR.
Kt/V can be determined from pre-dialysis plasma urea concentration but can also be derived from the analysis of the urea concentration profile in the dialysate.  To avoid blood sampling at the start of a session, dialysate is recirculated for 5 minutes to get equilibrated with and C pre is then measured in the dialysate.
Urea concentration in the effluent dialysate reflecting the actual removal of urea is always correct, independent of recirculation. Therefore the influence of recirculation on the pre-dialysis concentration is avoided, thus, urea monitoring in dialysate gives correct results.
Dialysate-based urea measurements are employed in the Urea Monitor 1000 (UM 1000® ) from Baxter  and the DQM 200° device from Gambro.  In both cases, the unit is a separate module (i.e. not integrated in the machine). The results of the urea monitoring as presented by the DQM 200 are:
TRU total removed urea (g)
URR urea reduction ratio
Kt/V, eKt/V dialysis dose, equilibrated Kt/V
PCR, nPCR protein catabolic rate (g/day) and normalized PCR (g/kg/day).
In the UM 1000, results are presented as cumulative urea removal (equal to TRU at the end of the session), Kt/V during dialysis, Kt/V at the end of the session and PCR at the end of the session.
| Methods for measuring urea clearance on-line|| |
Another method measures the dialyzer clearance, whereby urea clearance is not measured directly, but an ionic dialysance which is closely related to urea clearance is measured. This requires the addition of a second conductivity cell in the effluent dialysate. Normally only one conductivity cell positioned in the dialysate flowing into the dialyzer is present [Figure - 3]. Online clearance measurement is deployed in DIASCAN® system (Hospal,  ) and OCM® (Fresenius Medical Care,  ).
In order to derive the ionic dialysance from two conductivity measurements, a short variation of solute concentration (∆S 1 ) is superimposed on the normal conductivity of the dialysis fluid flowing into the dialyzer [Figure - 4]. This pulse may have a simple rectangle form [Figure - 4]a or an alternating positive-negative shape [Figure - 4]b. As in dye dilution methods for flow measurements, the height of the pulse ∆S 1 is decreased to ∆S 2 respectively during the passage of the fluid through the dialyzer to a magnitude which depends on the clearance of low molecular weight solutes. The conductivity pulse also becomes elongated and somewhat smoothed (as seen when the pre- and post-dialyzer pulses are superimposed on each other, see frame on the right side of [Figure - 4]. It is important that a plateau is reached, for only then electrolyte clearance (KEl) is given by the simple equation 1). The OCM-device features a dynamic measurement, i.e. a short pulse that does not have to reach a stable plateau is followed by a second pulse in the antagonizing direction in order to provide for neutral net sodium transfer [Figure - 4]b.
1) K El =(QD+Q UF ) * (1-∆S 2 /∆S 1 )
Here QD is the dialysate flow and QUF the ultrafiltration rate. Equation 1) correctly describes the influence of ultrafiltration on ionic dialysance. Urea clearance Kurea is simply given by:
2) K urea = a * K el
where a is a factor which relates the diffusive clearance of urea to ionic dialysance measured. While in vitro ionic dialysance (K EL ) and urea (K urea ) is equal,  in vivo clearance of urea is somewhat higher. Experimental data published differ to some extent. For example, Kuhlmann et al  found K urea to be 2.4% higher than K El (a = 1.024) with a correlation coefficient of 0.87, while the guide to the DIASCAN® system  states that K urea is greater than K El by a factor of 10 to 15% (i.e. a = 1.1 to 1.15). Ionic dialysance in dialysis is given by the differences in the concentrations between plasma and dialyzing fluid of sodium, potassium, chloride, bicarbonate, and phosphate ions. The different size of the respective gradients across the dialyser membrane may explain the various values published for the factor a
Kt/V is then calculated from K urea , the elapsed time t and the distribution volume V; the last is approximately equal to total body water, and this may be estimated from the patient's dry weight using either the Watson formulae  (employed in the DIASCAN® and OCM® devices) or whole body impedance measurements.
The DIASCAN system utilizes a single positive or negative pulse of 1 mS/cm (length 2 min) every 30 minutes [Figure - 4]a; the oCm system employs an alternating pulse every 10 minutes, and the pulse frequency can be selected with a minimum spread of 25 minutes [Figure - 4]b. The OCM device is able to calculate possible influences of dialysis parameter alterations during the session (Qb, Qd, UF-rate) within one minute. It exactly measures the effective dialysis time t of the treatment, taking in account that system checks and other treatment interruptions do not count as effective treatment time.
Shunt recirculation and cardiopulmonary recirculation decrease urea clearance and also ionic dialysance measured as described. The actual decrease depends on extracorporeal blood volume, blood flow, and length of the conductivity variation. Both systems (DIASCAN and OCM) account correctly for the influence of recirculation.
All three methods are applicable to single needle dialysis.
In order to determine Kt/V as well as PCR, at least one direct measurement of the pre-dialysis plasma urea concentration is necessary to calculate the PCR. Note: URR is the urea redaction ratio, K is the dialyzer clearance (also termed effective body clearance), V is the distribution volume, RU (removed urea) is the kinetically derived urea removal, TRU is the true (measured) urea removal, and SRI is the solute removal index defined by equation 3). 1 if Cpre is determined avoiding the influence of shunt recirculation.
3) SRI =1 -V post . C post /(V pre . C pre )
[Table - 1] provides a summary of the three urea monitoring methods under discussion, i.e. urea concentration measurements in ultrafiltrate, urea concentration measurements in dialysate, and clearance measurements using conductivity variation. Urea concentration measurements provide the important parameters for assessing adequacy of dialysis, i.e. Kt/V, URR (urea reduction ratio) and PCR. Clearance measurements, on the other hand, do not deliver PCR values; hence measurement of the pre-dialysis urea concentration is necessary to determine PCR. The clearance-based systems employed in the DIASCAN and OCM devices do not include an algorithm to calculate PCR.
In addition to Kt/V and PCR, on-line urea concentration measurements can provide other parameters, such as urea distribution volume (V), the amount of urea removed per session (UR), the solute removal index (SRI) and actual dialyzer clearance K (also termed effective body clearance). Urea concentration measurements made in ultrafiltrate require kinetic modeling to derive all parameters (including the key parameters Kt/V and PCR) except URR. The parameters UR, PCR, K, V, Kt/V and SRI can be directly calculated from dialysate-based urea concentration measurements. Therefore, UR is also indicated as true removed urea (TRU). Urea concentration measurements have the advantage that the underlying kinetic modeling can be validated: the basic model parameter V and the mass transfer coefficient for the individual patient can be calculated from the measured data. Urea concentration measurements, therefore, provide a perfect tool for the detailed study of urea kinetics.
Commercially available devices, the UM 1000 and DQM 200,  use dialysate-based urea concentration measurements, and the DIASCAN  and OCM  employ urea clearance measurements.
Application of urea monitors.
Several papers have been published on the subject. The first steps to urea monitoring started with serial measurements of urea concentration in dialysate started in the early 90 s . ,,,,
The described methods for on-line urea concentration and urea clearance monitoring have been tested by several authors in-vitro and in-vivo under clinical conditions [Table - 2].
Canaud  et al (1998) compared measurements of urea concentration in effluent dialysate with laboratory data using a selfdesigned urea sensor. They also performed a clinical validation of their method.
Urea concentration measured with the urea sensor and in the laboratory showed a linear relation characterized by (R 2 is called the concordance, R is not identically to the correlation coefficient r):
4) C Sensor = 1.01 CLab + 0.33 with R 2 = 0.95
The urea concentration C given in mmol/l.
Clinical validation was performed in 7 acutely ill patients: 5 on hemodiafiltration and 2 on hemofiltration. Urea removal and nPCR (n is used by some authors to indicate that protein catabolic rate is related to body weight: g/kg/day) were calculated and compared to direct measurements in the ultrafiltrate and the collected dialysate.
In a second study Canaud et al  analysed urea dynamics in 13 patients by means of a continuous on-line urea monitoring system interfaced with a two-pool kinetic model hosted in a microcomputer. Three consecutive sessions of each patient in one week were analysed. They found that a two-pool model structure is perfectly applicable for urea dynamics during hemodialysis.
The characteristic parameters of a twopool model were calculated: intra- and extracellular distribution volumes (V 1 , V 2 ) and mass transfer coefficient K 12 . The total distribution volume was 38.1 ± 7.8 l (53 ±
8% of dry body weight), with V 1 29.2 ± 6.6 l and V 2 9.0 ± 2.4 l. The average mass transfer coefficient K 12 was 912 ± 255 (857± 235 ml/min normalized to 1.73/body surface area), and ranged from 363 to 1289 ml/min.
Kt/V calculated from pre- and post-dialysis urea concentrations using bedside formulae (Garred,  Daugirdas 1 ,  Daugirdas  tended to overestimate Kt/V by about 30% compared to Kt/V urea ; while formulae based on equilibrated post-dialysis urea concentrations (Daugirdas and Depner,  Tattersall et al  ) compared well with Kt/V urea . This study provides the most comprehensive comparison of laboratory data, bedside formulae and formal urea kinetics.
Navino et al  used a self-developed urea sensor system (shown in [Figure - 1]) along with the Bellco SPA system, which consists of an hemofilter and a dialyzer in series. The system was compared in vivo with preand post-dialysis urea concentration measurements using a two-pool model. Measured and predicted values of predialysis urea concentrations, post-dialysis urea concentrations and urea rebound concentrations correlated well; r = 0.95, r = 0.89 and r = 0.97, respectively.
Navino using the Bellco system compared two dialyzers as second filter (1.8 sqm polysulfon high flux and 1.95 sqm benzyl substituted cellulose low flux) and two modes of post-dilution hemodiafiltration (reinfusion before and after the dialyzer clinically. The post-dilution mode with reinfusion after the dialyzer was found to yield a higher Kt/V than with reinfusion before the dialyzer, and a low permeability dialyzer was found to be more effective than a high permeability dialyzer.
Yanai et al  guided ultrafiltrate through an electrolyte (Na + , K + , Cl - ) and a urea sensing unit at a low rate (0.3-1.0 ml/min) and used a self-designed tiny ultrafilter (surface area 23.3 cm 2 ). All concentrations were measured every 5 minutes. System safety and accuracy was tested in 7 patients and 72 sessions in total. In the experimental phase, only Kt/V as a function of the elapsed dialysis time was determined. The system is not yet commercially available.
Both Keshaviah et al  and Ronco et al  applied a prototype of the UM 1000 clinically. In a clinical validation involving 31 patients, Keshaviah compared the results with laboratory data and a modified direct dialysis quantification based on a two-pool model, and found a good agreement. Urea concentration in dialysate measured with the UM 1000 and with the Beckman Syncron Analyzer CX-3 were highly correlated (r=0.991), so were the equilibrated pre-dialysis concentrations by the UM 1000 and the concentrations in plasma by the CX-3 (r=0.971). Comparison of the following parameters derived from urea concentration measured by the UM 1000 and from collected dialysate showed the following correlations: Urea removed (r=0.983), Kt/V (r=0.89), PCR (r=0.94), solute removal index SRI (r=0.87), V (r=0.87), and effective body clearance identical with the actual dialyzer clearance K (K eff ) (r =0.74).
Even 90 minutes after the start of a session, a reliable estimation of the time necessary to obtain the prescribed Kt/V could be derived.
Ronco et al  evaluated nine consecutive hemodialysis sessions for each of six patients. Kt/V (UM 1000) and Kt/V (Daugirdas 1 ) correlated well (r=0.91) if the urea concentration after equilibration (30 min after the end of dialysis) was used.
Total urea removal obtained with the UM 1000 and by collecting the total amount of dialysate was in excellent agreement (r=0.98); PCR from the UM 1000 device was in good agreement (r=0.92) with values from direct quantification (dialysate + urine).
Stemby tested the DQM 200 in-vitro and under clinical conditions. The calculated initial urea mass (m o ) and the true value in six in vitro tests showed a mean difference of 1.2 + 0.9%. Clinical test involved 83 treatments in four centers including 33 patients. Kt/V measured with the DQM 200 correlated well (r=0.947) with Kt/V from Daugirdas2 corrected for equilibration and the respective mean values showed no significant difference. Distribution volume V=33.4 + 0.71 derived by the DQM 200 was not statistically different from V Watson =33.9 + 0.5 L given by the Watson formulae and V ed =32.8 + 0.71 from collected dialysate. Correlation was low (r=0.76, n=81) with V Watson but good with V ed (r=0.91, n=83).
Peticlerc et al  measured conductivity in dialysate by switching the conductivity cell, which is normally positioned at the dialysate inlet, to the dialysate outlet for 10 seconds. The conductivity pulse superimposed to the normal dialysate conductivity is 1 mS/cm and lasts two minutes. They compared the dialysis dose obtained with the conductivity method (Kt/V cond ) with the dialysis dose derived from collected dialysate (Kt/V cd ) and found the relation:
Kt/V cond =0.11+0.89* Kt/V cd with r=0.94
They also studied the influence of ultrafiltration and shunt recirculation on the ionic dialysance.
Di Filippo et al  employed the DIASCAN system in 44 patients on a thrice weekly hemodialysis regime. They compared the dialysis dose derived from the ionic dialysance Kt/V EL , to the dose Kt/V eq (equilibrated) derived from the pre-dialysis urea concentration, the concentration 30 minutes after the start of dialysis, and the pre-dialysis concentration of the subsequent session. The difference (Kt/V EL -Kt/Veq ) was found to be smaller than ±0.15 (2 SD), showing a good agreement.
Kuhlmann et al  tested the on-line clearance monitor (OCM) from Fresenius Medical Care in a study including 20 hemodialysis patients by comparing the results with conventionally acquired values. OCM data correlated very well with the conventional measurements of urea clearance corrected for recirculation (blood side: r=0.87, dialysate side: r=0.82, p<0.001). Conductivity based measurements of Kt/V correlated well with Kt/V values based on a single pool model (r=0.940, p<0.001), values from the equilibrated single pool variable volume kinetic model (r=0.982, p<0.001), values calculated using Daugirdas' formula (r=0.951, p<0.001) and values from direct quantification of spent dialysate (r=0.900, p<0.001). Serum sodium, body weight, arterial PO2 and blood pressure before hemodialysis remained unaffected in OCM measurements as compared to the respective baseline values.
In summary, different studies have shown that Kt/V and PCR determined with on-line methods correlate well with laboratory data obtained under in vitro and clinical conditions, and that they are superior to methods based on pre- and post-dialysis urea concentrations. The time interval analysed in each of the studies was too small to provide clinical results on patient morbidity and mortality.
| Discussion|| |
On-line monitoring of urea concentration and conductivity allows low molecular weight solute clearance to be measured at any time during hemodialysis therapy. Compared to the commonly used method which involves post-dialysis blood sampling, and which is particularly sensitive to the sampling technique, iterative measurements during the whole dialysis session yield more reliable data.
The differences between the on-line methods and conventional kinetics are shown in [Table - 3].
All pre- and post-dialysis based methods which involve extra blood sampling induce additional costs, certain logistic requirement (chain of cold storage units) and demand stringent correctness of staff work. Use of bedside formulae limits the control to the midweek session, although the session after the long dialysis-free interval is of more interest clinically because concentrations reach peak values then. Bedside formulae are also limited to regular dialysis therapy with three sessions per week.
Blood sampling can be done in any session during the week only when 2-pool formal urea kinetics is applied  (e.g. solving the mass balance equation for each individual session). Sampling can also be done in the session which follows the long dialysis-free interval when plasma concentrations are highest (urea, potassium. creatinine, base deficit, uric acid). Urea kinetic calculations can also be applied for weekly frequencies of up to 7 sessions and, if blood sampling is conducted according to the NKF guidelines,  recirculation can also be taken into account. Such a urea kinetic program has been developed by Stiller & Mann.  Urea kinetic calculations provide mean weekly values, whereas on-line Kt/V and on-line PCR data only refer to single dialysis treatments. The effect of residual renal clearance Kres on Kt/V and PCR, which is accounted for by on-line urea monitoring, is not covered by the bedside formulae. However in on-line clearance monitoring this effect has not been described.
On-line monitoring is, in principle, more accurate than pre- and post-dialysis based methods because random errors can be eliminated by appropriate algorithms from repeated measurements in short time intervals. On the other hand, unless special precautions are meticulously observed in drawing pre- and post-dialysis samples, the urea concentrations attained may well differ from the actual values. 
Differences between on-line urea concentration monitoring and on-line urea clearance measurements are summarized in [Table - 4].
While urea concentration monitoring provides both dialysis dose (Kt/V) and PCR, urea clearance monitoring yields only Kt/V. In clearance monitoring, PCR can be derived from a single additional determination of pre-dialysis urea concentration, but this option is not provided in the available DIASCAN and OCM systems.
Present constructions are such that urea concentration monitoring is always conducted in a separate module (UM 1000, DQM 200), while urea clearance monitors are fully integrated into the dialysis machine (DIASCAN, OCM).
An important feature regarding the practicability and acceptance of any system for dialysis quality control is its costeffectiveness in both money and staff time terms. Separate monitoring devices (i.e. those not integrated into the dialysis machine, as in the case of the present UM1000 and DQM 200 devices must be connected to the extracorporeal system and be calibrated; this might pose an obstacle for a regular application. Extra costs per session may also be a deterrent to purchasing such a system, despite the fact that these systems provide both Kt/V and PCR. However, there seems to be no principle problem for the manufacturers to integrating these monitoring systems into the dialysis machine and to automating calibration procedures.
The presently available systems based on ionic dialysance measurements (DIASCAN and OCM) are already fully integrated into the dialysis machines and regular application cause no additional costs. They are therefore more easily accepted and employed more frequently
A warning should be added: Kt/V and PCR are only two aspects of quality control in treating dialysis patients. Kt/V > 1.2 and PCR >1.0 g/kg/day are necessary, but not the only sufficient requirements for adequate dialysis therapy. Phosphate, electrolyte and acid-base regulation, elimination of middle and high molecular solutes, avoidance of side effects of treatment, a dedicated staff and a competent physician are indispensable requirements for good dialysis therapy.
| Conclusions|| |
A number of conclusions can be drawn from the published studies on on-line urea monitoring.
The advantages of using the described online methods to obtain Kt/V or both Kt/V and PCR are their good accuracy, no need for blood sampling, and real time availability of data.
However, it must be mentioned that the present commercially available systems for on-line urea concentration monitoring have not yet reached a final state of development. Stand alone devices, requiring space, connection to the dialysate flow and manual calibration, have serious obstacles for their acceptance in clinical dialysis. This might explain why production of the UM 1000 has been stopped. Presuming these systems will eventually exploit the full potential for easy application and low cost, a wide acceptance in regular dialysis seems probable.
On the other hand, systems based on urea clearance monitoring are already integrated into the dialysis machines. This state of development justifies a recommendation for routine application as, perhaps, the best clinically available methods for dialysis quality control and a general acceptance in routine dialysis seems most likely. However output of PCR should be supplemented.
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Arzt Fur Innere Medizin Nephrologie, Schurzelter Str 564, D-5100 Aachen
[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4]
[Table - 1], [Table - 2], [Table - 3], [Table - 4]