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QUALITY ASSURANCE IN PERITONEAL DIALYSIS

Department of Nephrology and Medical Intensive Care, Charité-Universitätsmedizin Berlin, Campus Virchow–Klinikum, Berlin, and Fresenius Medical Care,¹ Bad Homburg, Germany

Correspondence to: A. Jörres, Department of Nephrology and Medical Intensive Care, Charité-Universitätsmedizin Berlin, Campus Virchow–Klinikum, Augustenburger Platz 1, Berlin D-13353 Germany. achim.joerres{at}charite.de

	ABSTRACT

TOP ABSTRACT CHOOSING THE TIME FOR... SETTING TREATMENT TARGETS ON... ACHIEVING TREATMENT TARGETS OVER... CAUTION REGARDING EARLY AND... REFERENCES

 

The present article reviews current treatment targets for peritoneal dialysis (PD) and the various methods for evaluating adequacy with time on PD.

KEY WORDS: Adequacy; treatment targets; peritoneal function tests.

Successful treatment of uremic patients with peritoneal dialysis (PD) encompasses several important quality targets. Probably the most important are these:

• Patient survival
  
• Technique survival
  
• Quality of life
  
• Nutrition status
  
• Hypertension control
  
• Correction of anemia
  
• Control of renal osteodystrophy
  
• Reduction of cardiovascular events
  

To achieve these targets, treatment must be adapted to the individual needs of the patient.

	CHOOSING THE TIME FOR PD START

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Adaptation obviously begins with choice of the optimal point at which to initiate dialysis, followed by selection of the ideal mode of treatment: continuous ambulatory PD (CAPD) or automated PD (APD). In making this decision, the personal preferences and the living and working environment of the patient both need to be considered.

Current guidelines advocate a timely (that is, early) start to treatment. The Dialysis Outcomes Quality Initiative (DOQI) guidelines from 2000 (1) stated that dialysis should be initiated when the weekly renal Kt/V_urea falls below 2.0, which approximates a kidney urea clearance of 7 mL/min and a kidney creatinine clearance (CCr) between 9 mL and 14 mL/min/1.73 m². The European guidelines from 2005 (2) advocated that dialysis should be instituted whenever evidence of uremia is present, when blood pressure or hydration status cannot be controlled, or when nutrition status deteriorates. It was recommended that dialysis should be initiated at a CCr level between 8 mL and 10 mL/min/1.73 m². The IDEAL (Initiating Dialysis Early and Late) Study—a prospective, multicenter, randomized controlled trial to compare a broad range of outcomes in patients starting dialysis therapy with a Cockcroft–Gault glomerular filtration rate of 10 mL to 14 mL/min/1.73 m² versus 5 mL to 7 mL/min/1.73 m²—is currently underway in Australia and New Zealand and, hopefully, will help to further clarify the issue of timely initiation.

Once the start of PD has been decided, the nephrologist needs to keep in mind that patients can be very different not only in terms of body weight, size, and composition, but also in the intrinsic transport properties of the peritoneal membrane. Finally and importantly, residual renal function (RRF) and nutrition status at the time of PD initiation both need to be assessed to arrive at an optimal individualized PD prescription (Figure 1).

View larger version (9K): [in this window] [in a new window]   Figure 1 — Factors influencing peritoneal dialysis prescription.

	SETTING TREATMENT TARGETS ON PD

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In the ADEMEX study (3), 965 incident PD patients were randomly assigned, in a 1:1 ratio, to an intervention or a control group. Subjects in the control group continued to receive their existing PD prescription (4 daily exchanges with 2 L standard PD fluid). The subjects in the intervention group were treated with a modified prescription, so as to achieve a weekly peritoneal CCr of 60 L/1.73 m². The primary endpoint was death, and the minimal follow-up period was 2 years. The separation of the two treatment groups according to delivered dialysis dose was successful; however, the study found no difference in survival.

Another prospective randomized study assessed the effect of Kt/V on survival and clinical outcomes (4). A total of 320 new CAPD patients with baseline renal Kt/V < 1.0 were recruited from six centers in Hong Kong and were randomized to three Kt/V target groups:

• Group A: 1.5 – 1.7
  
• Group B: 1.7 – 2.0
  
• Group C: >2.0
  

Every 6 months, Kt/V and nutrition status were assessed, and the dialysis prescription was adjusted accordingly. The primary endpoint was death, and the minimum follow-up period was 2 years. Again, no significant survival difference between the three groups was observed; however, patients with a Kt/V < 1.7 had more clinical problems and more severe anemia. No further difference was observed between a Kt/V of 1.7 – 2.0 and a Kt/V of >2.0.

Minimal and optimal Kt/V targets were studied (5) using a retrospective analysis of survival in a cohort of anuric PD patients (90 women, 42 patients with diabetes). Survival was analyzed according to baseline Kt/V at the time of documentation of anuria. The results indicated that a baseline Kt/V of <1.67 was associated with poorer survival after documentation of anuria. This effect was observed mainly in female patients. The best survival was observed when Kt/V was between 1.67 and 1.86. The study concluded that, in anuric PD patients, the minimal Kt/V target should be set to 1.70, and the optimal target to about 1.80.

As a consequence, the 2005 European guidelines (2) advocated a minimum peritoneal target for weekly Kt/V_urea of 1.7 in anuric patients and suggested an additional target for minimum peritoneal net ultrafiltration of 1.0 L daily. Patients on APD, with frequent short exchanges and slow transport status, may fulfill these targets, but may have a low peritoneal CCr. Such patients should therefore meet the additional target of a weekly CCr of more than 45 L.

More recently, the International Society for Peritoneal Dialysis (ISPD) published similar recommendations. Importantly, the ISPD recommendations state that adequacy of dialysis should be interpreted clinically rather than by targeting only solute and fluid removal. For small-solute removal, the total (renal + peritoneal) Kt/V_urea should not be less than 1.7 at any time. A separate target for CCr is not required in CAPD. In APD, because of the more variable relationship between urea and CCr, an additional target of 45 L/1.73 m² for weekly CCr is recommended. The guidelines also suggest that attention be paid to both urine volume and the amount of ultrafiltration, with the goal of maintaining euvolemia. However, in contrast to the European guidelines, no numerical target for ultrafiltration is set. Also, the new 2006 U.S. National Kidney Foundation Kidney Disease Quality Outcomes Initiative guidelines suggest a minimum Kt/V target of 1.70 and otherwise generally follow the lines of the European and ISPD guidelines (6).

	ACHIEVING TREATMENT TARGETS OVER TIME ON PD

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Successful PD depends on the functional integrity of the peritoneal membrane as the dialyzing organ. However, with time on PD, morphology changes such as fibrosis and thickening, vasculopathy, and angiogenesis occur in the peritoneal membrane. In some patients, these changes are accompanied by progressive functional changes that ultimately lead to therapy failure.

The timing and extent of these changes in peritoneal function vary greatly from patient to patient and can also change significantly over time within the same individual. Following a patient's peritoneal membrane transport status longitudinally and systematically is therefore of considerable importance. Numerous techniques are available for this purpose.

The most widely used technique is the peritoneal equilibration test (PET), which was developed by Twardowski and colleagues (7). The PET was the first standardized method to quantify individual peritoneal membrane characteristics. Because PET results have been found to be relatively stable and reproducible, they were also used as the first reliable tool for researchers comparing peritoneal transport properties within larger PD populations.

The PET test is conducted as follows:

After an overnight exchange involving an 8- to 12-hour dwell:

• Glucose solution (2 L, 2.5% concentration, 2.3% anhydrous) is instilled and allowed to dwell for 4 hours.
  
• Several times during the dwell, the patient is requested to roll from side to side.
  
• Dialysate urea, glucose, sodium, and creatinine are measured at 0, 2, and 4 hours.
  
• A blood sample is taken after 2 hours.
  
• The drain bag is weighed to assess both drain and net ultrafiltration volume.
  
• Dialysate-to-plasma (D/P) ratios are calculated for creatinine, urea, and sodium at 0, 2, and 4 hours.
  
• The ratio of glucose at drain time (t) to the dialysate glucose concentration at time 0 (D_t/D₀) is measured.
  

Based on population studies, patients thus tested are categorized as being low, low-average, high-average, or high transporters.

A disadvantage of the PET is that it is rather labor-intensive, requiring the repeated taking of samples from peritoneal effluent, which is then re-infused, constituting an infection risk. More recently, the "fast PET" was developed as a simpler alternative (8).

The fast PET requires only a single dialysate sample:

• After draining the overnight dwell, the patient starts an exchange at home and arrives at the PD unit in time for drainage after a 4-hour dwell.
  
• At the end of the exchange, a blood sample is taken.
  

The analysis of the fast PET is otherwise identical to that of the standard PET. However, only two measures of peritoneal membrane permeability are determined: D/P creatinine and dialysate glucose after 4 hours. Should those two measures yield contradictory results, the test cannot be interpreted accurately.

Originally, the PET was standardized to be performed after a long overnight exchange. At the time, almost all patients were on CAPD, and that approach was convenient. However, recent studies have confirmed that the preceding long dwell exerts only a minimal impact on small-solute equilibration during the test. As a consequence, the "short PET" was introduced, allowing for preceding dwell times of between 3 and 12 hours, and accepting that the test itself can be performed with either a 2- or 4-hour dwell time.

It has to be kept in mind, however, that the PET may actually overestimate peritoneal membrane transport and underestimate the variation in peritoneal transport that may occur under actual clinical conditions. Moreover, the PET does not allow for assessment of total solute removal, because it does not assess residual kidney function. To overcome this problem, the PET can be combined with a 24-hour collection for renal and peritoneal solute clearance.

As a more sophisticated alternative, the peritoneal function test (PFT) has been developed and evaluated in a number of multicenter studies (9). Instead of requiring standardized exchange conditions, the PFT is applied to the patient's regular dialysis regime to calculate the peritoneal mass transfer area coefficient. In addition to peritoneal transport and fluid balance, the PFT measures total delivered dose for urea and creatinine and also supplies information on protein and calorie nutrition.

The PFT requires these inputs:

• Sampling of each dialysate exchange
  
• A written record of the exchange: inflow and outflow volumes, and glucose concentration
  
• Documentation of the length of each exchange in the 24 hours preceding the clinic visit
  
• A timed urine collection
  
• A blood sample taken at the end of the collection
  
• An exchange drained in the clinic at the time of the visit [a quality assurance (QA) exchange, serving as a control]
  

The dialysate samples are analyzed for urea, creatinine, glucose, and total protein. The urine volume is recorded, and a urine sample is analyzed for urea and creatinine. The blood sample is tested for urea, creatinine, glucose, total protein, and albumin. The results of these tests are usually entered into a computerized kinetic modeling program [Pack PD or Patient-On-Line (Fresenius Medical Care, Bad Homburg, Germany)], which determines the mean mass transfer area coefficient (MTAC), which is given as the Pt50, that is, the time required for the dialysate concentration of urea and creatinine to reach 50% of the respective plasma concentration.

For further follow up visits, a simplified PFT (sPFT) can be used. The sPFT requires:

• A record of the glucose concentration used
  
• A record of the fill and drain volumes and the duration of each exchange over 24 hours
  
• An aliquot of a single 2- to 3-hour exchange, timed so that it can be drained in the clinic (the QA exchange)
  
• A blood sample
  
• A 24-hour urine collection
  

A new Pt50 is calculated from three D/P ratios of the previous standard PFT and the D/P ratio of the QA exchange from the current sPFT.

A good measure of the delivered dialysis dose can also be obtained by performing a 24-hour batch dialysate test in combination with a simultaneous timed urine collection and a blood sample (10). The 24-hour D/P creatinine ratio obtained as a result correlates well with the information on peritoneal transport that is obtained by a PET. The main disadvantage of this test is that patients need to be well trained and reliable if they are to accurately collect individual drains. In case of doubt, it might be advisable to perform the collection and sampling at the clinic.

Another system that is rather widely employed for analysis of peritoneal membrane function is the personal dialysis capacity (PDC) test, which is based on the three-pore model from Rippe et al. (11). The PDC evaluates peritoneal membrane characteristics by means of three parameters:

• Area parameter A_0/DX, which determines the diffusion of small solutes
  
• The final reabsorption rate of fluid from the abdominal cavity to blood when the glucose gradient has dissipated (JV_AR)
  
• The large-pore fluid flux (JV_L), which determines loss of protein to the PD fluid
  

In this procedure, the patient is asked to perform 5 exchanges, starting with a short dwell (2 – 3 hours), followed by two intermediate dwells (4 – 6 hours), another short exchange (2 – 3 hours), and finally a long overnight dwell. The glucose concentrations used are varied so that the short dwells are performed with different glucose concentrations.

The patient needs to weigh each bag before and after drain, note the exact time of infusion and drainage, and take a sample from all drained bags. In addition, a 24-hour urine collection is required. The dialysate samples are analyzed for urea, creatinine, glucose, and albumin (or total protein). The urine sample is analyzed for urea, creatinine, and protein. Blood samples are drawn at the beginning and at the end of the test procedure for determination of sodium, urea, creatinine, glucose, and albumin (or total protein).

The information obtained is computed on the basis of the three-pore-model using a specific software program that yields comprehensive information on peritoneal membrane function, transport characteristics, and ultrafiltration. A recent study in 135 incident PD patients indicated that the PDC test is superior to the PET in discriminating inflammation as the cause of fast transport status (12).

A less complex procedure is the dialysis adequacy and transport test (DATT) that was introduced by Rocco et al. (13). For the DATT, patients perform their regular dialysis exchanges. A blood sample and a single 10-mL aliquot from a pooled and well-mixed 24-hour dialysate collection are used to calculate the 24-hour D/P ratio. It must be noted that the DATT has been validated only for patients using a CAPD regime of 4 2-L exchanges, and therefore this test should not be used for patients on APD.

The accelerated peritoneal examination (APEX) test (14) uses a procedure similar to the PET, but peritoneal permeability for both glucose and urea are integrated in a single figure representing the time at which the equilibration curves for glucose and urea cross. A shorter APEX time indicates a higher peritoneal permeability. The APEX time may be used to establish the optimal contact time between the functional peritoneal membrane surface area and the dialysate in the individual patient. In most patients, the APEX time will be 2 hours or less, and so the test procedure usually takes less time than a PET does.

Another more sophisticated method to assess peritoneal function (15) is the standard peritoneal permeability analysis (SPA). The SPA looks at fluid kinetics during a 4-hour dwell with dextran 70. This test also obtains the MTAC of small solutes, the percentage of absorbed glucose, and the peritoneal loss of serum protein. Standard PET parameters can also be calculated from the SPA parameters. Conversely, the D/P creatinine and the D_t/D₀ glucose can be used with the drained volume to calculate the MTAC of creatinine and the percentage of glucose absorbed. Performing the SPA with a high dialysate glucose concentration provides more reliable information on ultrafiltration, because the larger drain volume minimizes sampling errors. Moreover, the sodium sieving associated with a hypertonic glucose solution enables estimation of aquaporin-mediated water transport.

	CAUTION REGARDING EARLY AND PERIODIC FUNCTION TESTS IN PD

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A general note of caution to keep in mind is that, independent of the chosen test procedure, peritoneal transport properties may undergo significant changes— particularly within the first few weeks of PD. Thus, any peritoneal function test performed during this initial phase yields only preliminary information that must be confirmed by a repeat test about 4 weeks later (16). Moreover, significant errors may underlie any of the described test procedures, either because of sampling mistakes or imprecision in laboratory methods. For example, creatinine measurements based on the Jaffé method are sensitive to glucose, yielding falsely high dialysate creatinine levels that need to be corrected for high dialysate glucose concentrations.

In any case, a reassessment of peritoneal transport properties and RRF should be performed at regular intervals during the course of PD for potential modification of the therapy prescription. With progressive loss of RRF, peritoneal dialysate volumes, dwell time, and glucose concentration have to be adapted, and even the form of therapy—for example, CAPD, continuous cycling PD, PD Plus, tidal PD, or nightly intermittent PD—may have to be modified. In addition, the more sophisticated test procedures may serve as a basis for valuable recommendations concerning nutrition.

The 2001 DOQI guidelines originally recommended that a test procedure be repeated every 4 months. However, because most tests are time-consuming for patients and staff alike, the actual frequency that is chosen will usually be determined by local circumstances and the facilities of the PD program.

	REFERENCES

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1. II. NKF-K/DOQI Clinical Practice Guidelines for Peritoneal Dialysis Adequacy: update 2000. Am J Kidney Dis2001; 37(Suppl 1):S65 -136.[Medline]
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3. Paniagua R, Amato D, Vonesh E, Correa–Rotter R, Ramos A, Moran J, et al. Effects of increased peritoneal clearances on mortality rates in peritoneal dialysis: ADEMEX, a prospective, randomized, controlled trial. J Am Soc Nephrol 2002;13 : 1307-20.[Abstract/Free Full Text]
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10. Busch S, Schreiber M, Bodnar D, Buchler N, Fuchs J, Jackson–Bey D, et al. The 24-hour D/P ratio is a convenient screen for identifying altered peritoneal transport rates. Adv Perit Dial 1993; 9:119 -23.[Medline]
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12. Van Biesen W, Van der Tol A, Veys N, Dequidt C, Vijt D, Lameire N, et al. The personal dialysis capacity test is superior to the peritoneal equilibration test to discriminate inflammation as the cause of fast transport status in peritoneal dialysis patients. Clin J Am Soc Nephrol 2006; 1:269 -74.[Abstract/Free Full Text]
13. Rocco MV, Jordan JR, Burkart JM. Determination of peritoneal transport characteristics with 24-hour dialysate collections: dialysis adequacy and transport test. J Am Soc Nephrol1994; 5:1333 -8.[Abstract]
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