1430 Volume 2 . Number 9 ‘ 1992

Clinical Outcome of Continuous Ambulatory PeritonealDialysis Predicted by Urea and Creatinine Kinetics1’2

James C. Brandes,3 Walter F. Piering, Joseph A. Beres, Samuel S. Blumenthal, and Claire Fritsche

J.C. Brandes. W.F. Piering, J.A. Beres, S.S. Blumenthal,

C. Fritsche. Department of Medicine, Nephrology Sec-

tion, Medical College of Wisconsin, Milwaukee, WI

(J. Am. Soc. Nephrol. 1992; 2:1430-1435)

ABSTRACTThe effectiveness of urea kinetics (Kt/V, where K isurea clearance, t is treatment time, and V is thevolume of distribution for urea) to assess the ade-quacy of continuous ambulatory peritoneal dialysis(CAPD) and clinical outcome has not been estab-lished prospectively, and cross-sectional clinical

studies have been inconclusive. A minimum weekly

creatinine clearance of 40 to 50 L is recommended,but the adequacy of this dose is unproven. We intro-duced a simpler approach to creatinine kinetics inthe form of an efficacy number (EN) calculated fromdata obtained in a standardized 4-h dwell ex-

change. To determine the most effective model forpredicting CAPD adequacy, residual renal function,weekly Kt/V urea, weekly creatinine clearancestandardized to body surface area, and EN (liters per

gram of creatinine per day) were measured in 18

stable CAPD patients followed prospectively for atleast 12 months. Patients were divided into threegroups, good (G), intermediate (I), and poor (P), onthe basis of uremic symptoms, mortality, hospitaldays, biochemical indices, and the need for transferto hemodialysis. When comparing groups G (N = 6)and P (N = 8), weekly Kt/V were 2.3 ± 0.2 versus I .5± 0. 1 (P< 0.005), weekly creatinine clearances were

71.5 ± 8.6 versus 35.1 ± 1.3 L (P< 0.001), and EN

were 7.4 ± 0.8 versus 3.6 ± 0.2 L/g of creatmnmne/day

(P < 0.005). Creatinine kinetics (weekly clearance

and EN) but not urea kinetics could differentiate

group I (N = 4) from groups G or P. Both urea andcreatinine kinetics predict clinical outcome in CAPD.

I Received March 25, 1991. Accepted October 1 1. 1991.

2 Presented In part at the American Society of Nephrology 1990 meeting in

Washington, DC.

3 Correspondence to Dr. J.C. Brondes, Froedtert Memorial Lutheran Hospital.9200 W. Wisconsin Avenue, Milwaukee, WI 53226.

1046-6673/0209- 1430$03.00/0

Journal of the American Society of NephrologyCopyright C 1992 by the American Society of Nephrology

However, creatinine kinetics are a more sensitivepredictor of CAPD adequacy with less overlap be-tween the outcome groups compared with weeklyurea Kt/V values. Therefore, creatinine kinetics aremost suitable for determining the adequacy of and

the need for adjusting CAPD prescriptions.

Key Words: Urea clearance, creatinine clearance, continuous

ambulatoryperitonealdialysis adequacy, Kt/V, efficacynum-

ber

A dequate dialysis is crucial for the well-being ofpatients with ESRD. In 1 98 1 . the first report

from the National Cooperative Dialysis Study (NCDS)

offered guidelines for effective hemodialysis (1 ). Thestudy concluded that the hemodiabysis prescriptionwas related to the occurrence of morbid events. In-

creased morbidity as defined by hospitalizations oc-curned in patients with high serum urea concentra-tions. On the basis of the results of that study. Gotch

and Sargent (2) introduced Kt/V, where K is ureaclearance, t is treatment time, and V is the volumeof distribution for urea. This index of dialysis ade-quacy has now become the accepted method by which

to determine the hemodialysis prescription (3). butwhat indices of adequacy for penitoneal dialysis ex-1st?

Until recently, the standard, daily continuous am-

bulatony penitoneal dialysis (CAPD) prescription hasbeen quite empiric: four 2-L exchanges. Unfortu-nately, this limited prescription does not take into

account differences in body size on penitoneal trans-

fer kinetics and may be responsible for the 34%transfer nate from CAPD to other dialysis modalitiesafter only 2 yr of therapy (4). Several investigatorshave tried to correlate urea on cneatinine kinetics inCAPD patients with clinical outcome.

Recent reports evaluating urea kinetics as a usefulpredictor of CAPD outcome are conflicting (5-7). Theaverage urea Kt/V for a CAPD patient adjusted to theequivalence of thrice-weekly hemodialysis is approx-imately 0.6, a value associated with overt uremicsymptoms in the hemodialysis patient (8). Yet, pa-

tients on CAPD do as well as those on HD. Therefore,urea Kt/V values considered adequate in hemodi-

alysis may not apply in CAPD.Twardowski and Nobph (9) have suggested that a

weekly creatinine clearance of 40 to 50 L pen 1.73m2 �y surface area probably results in adequatedialysis. However, clinical evidence to support this

Brandes et al

Journal of the American Society of Nephrology 1431

recommendation is backing. As a simpler alternative

to 24-h collections of urine and dialysate, we intro-duced the method of an efficacy number (EN). whichcan be calculated from a standardized 4-h dwell cx-change (10).

The purpose of this study is to compare weeklyurea Kt/V, weekly creatinine clearance, and the EN

as models to predict the clinical adequacy of CAPDin 1 8 patients followed prospectively for at least 1 yr.

METHODS

General

Eighteen patients on standard CAPD regimenswere monitored for 1 2 months. They were ennobled

consecutively as they were seen in the CAPD clinic.Patients were excluded only if they were unwilling to

participate by bringing in their 24-h urine and diaby-sate collections. Clinical outcome as defined belowwas determined during this study period. Timed di-abysates. urine, and serum were collected to deter-mine urea clearance, Kt/V, creatinine clearance (li-tens pen week). and protein catabolic nate (PCR; gramsper day) normalized to dry weight. All patients under-went a standardized penitoneab equilibration test

(PET) with 2.5% glucose dialysate (1 1). and EN were

calculated. Urea and cneatinine kinetic determina-

tions were completed within the first month of the12-month study interval. Compliance was monitoredwith monthly serum creatinine/urea concentrations.

The daily diet, restricted to 88 mEQ of Na� and 1,800

mL of fluid, was monitored by the Nephrobogy Dieti-

cian.

Study Subjects

Nine women and nine men (mean age. 53 ± 1 3 yn;

(mean ± SD) were studied. Their underlying kidneydiseases included diabetic nephnopathy (N = 10).chronic gbomerubonephnitis (N = 3), polycystic kidneydisease (N = 2), focal and segmental glomeruboscle-

rosis (N = 1), and unknown (N = 2). The mean lengthof time on CAPD at the completion of the 1 2-monthstudy period was 25 ± 14 months (mean ± SD).

Calculations

Urea Kinetics. Urea Kt/V was determined from thetimed dialysate volume (Vd) and the diabysate-to-

plasma ratio of urea, (D/P)urea. The volume of ureadistribution (V), which is equivalent to total bodywater, was estimated from sex, age, height, andweight by the Watson nomognam (12):

Kt124 h) Vd X (D/P)

V total body water

Endogenous urea clearance was added to the Kt term

above to obtain the total 24-h urea Kt/V value. Thisnumber was multiplied by 7 to yield the weekly urea

Kt/V.

The PCR was calculated from the urea generationrate (G��; milligrams pen minute) by the method ofRandenson et at. (13):

PCR = 10.76 x (G�� + 1.46)

� was determined from the diabysate and urinecollection. The PCR was normalized to the dry bodyweight of the patients (NPCR; grams pen kilogram perday).

Creatinine Kinetics. Creatinine clearance in literspen week was extrapolated from the collection of

dialysate and urine as described elsewhere (1 4) andwas standardized pen 1 .73 m2 body surface area.

The EN was determined from the data collected in

a standardized 4-h PET:

ICntn,p x V1L/d)]EN (L/g of creatmnmne/day) =ACPPD

where Cr1D,Pl is the diabysate-to-plasma ratio of crc-

atinine at 4 h, VIL/di is the prescribed volume of cx-changes per day in liters, and ACPPD was calculated

as the sum of penitoneal dialysis (PD) creatinine ap-

peanance over 24 h (estimated from the PET cx-change as 4-h diabysate creatinine concentration inmilligrams per deciliter Per] X the drain volume indeciliters [V] x 6) and creatinine generation at a

constant extranenal clearance (0.4 dl/kg/day x serum

creatinine [Scr; milligrams pen deciliterj x body wt

Ikibognams]) (15):

ACPPD (g/24 h) = I(D�r X V X 6) + (0.4

x �Cr X body wt)] #{247}1.000

Clinical Outcome

Clinical outcome was defined as good (G), interme-diate (I), or poor (P). based on the following five cate-gonies.

1. Clinical Assessment Survey. During the studyperiod, patients completed a survey with the CAPDnurse on symptoms potentially resulting from mad-equate dialysis. Nurses were unaware of the urea andcreatinine kinetic results at the time the survey was

completed. Patients scored the following symptomson signs on a three-point scale: insomnia, weakness,

anorexia, nausea, vomiting. itching, “restless legs.”tremor, and confusion. The absence of a symptom

would score a three, and the presence of a symptomat a higher frequency would scone a one. Hence, ifthe patient had no symptoms, a total score of 9 x 3= 27 would result. The category was positive if thepatient scored less than on equal to 1 8 points andnegative if greater than 1 8 points were scored.

1.5 2.0 2.5 3.0

Weekly Urea Kt/V

A

B

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2 4 6 8 10 12

Efficacy Number

CAPD Adequacy

1432 Volume 2 . Number 9 ‘ 1992

2. Hospitalizations. Positive is defined as more

than 1 2 hospital days for NCDS-abbowed diagnoses(1 6) over the 1 2-month observation period.

3. Death.4. BIochemical Indices. The category was positive

if any one of the following was present when aver-aged over the 1 2-month study period: serum albumin,:�3 g/dl; BUN. �lOO mg/dL; NPCR. sO.6 g/kg/day.

5. Failure of CAPD. Positive is defined as transfer

to hemodialysis for reasons other than patient pref-erence.

Clinical outcome was G if no categories were posi-

tive; outcome was I if only one category (other thancategory 3) was positive; on outcome was P if eithercategory 3 or any two categories were positive.

Statistical Analysis

One-way analysis of variance followed by unpairedt tests was used to compare differences between thethree outcome groups. Linear regression analysiswas used to delineate any correlations among param-

etens. Mubtivariate analysis was used to determinethe effects of several factors on specific outcome

parameters, controlling for the comorbid conditions

of diabetes meblitus, hypertension, and cardiovascu-

bar disease. Data are reported as means ± SE.

RESULTS

Clinical outcome was G in six patients. I in fourpatients. and P in eight patients. Two patients with Ioutcome transferred to hemodiabysis, and the othertwo had low index scores on the clinical assessmentsurvey. In the P outcome group. two of eight patientsdied, one of sepsis and the other of a gastrointestinalbleed. Six had low scores on the clinical assessmentsurvey. Four had more than 1 2 hospital days oven

the study period. and four had abnormal biochemicalindices. There was no significant difference in age,cause of end-stage renal disease, sex, length of time

on dialysis. or other underlying diseases, includingdiabetes, among the three groups.

The clinical assessment survey score correlatedwith urea and creatinine kinetics (P < 0.00 1 ; Figure1) independently. Multiple regression yielded a sig-nificant model for the clinical assessment score (P <

0.00 1 ) where the clinical assessment score was equalto (urea Kt/V + EN + age + constant). In a similaranalysis. the number of hospital days correlated with

the urea Kt/V, EN, and weekly creatinine clearance(P = 0.021). Serum albumin as an indicator of ade-

quate dialysis did not correlate with any kineticmodel (univariate or multivariate), although the con-relation with weekly creatinine clearance and the ENapproached significance (Figure 2). The NPCR corre-bated independently with the weekly urea Kt/V butnot with weekly creatinine clearance, weekly dialytic

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20 y= 11.6 +O.14x24 R = 0.73, p = 0.00122 U U.U U

18

16U U

14 .

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12 . ‘ , . -‘

20 40 60 80 100 120

Normalized Weekly Creatinine Clearance

Figure 1 . The clinical assessment score determined from thepatient survey significantly correlated with weekly urea Kt/V (A), normalized creatinine clearance (liters per week per1.73 square meters surface area) (B) and the efficacy num-ber (liters per grams of creatinine per day) (C).

creatinine clearance, on EN (Figure 3). Multipleregression for the NPCR yielded a significant model(P = 0.026) in which NPCR = urea Kt/V - age +

constant.All three kinetic models differentiated between P

and G outcome groups (P < 0.05; Figure 4). Weeklyurea Kt/V was 2.3 ± 0.2 in the G group and 1 .5 ± 0.2in the P group. Weekly cneatinine clearance was 71± 8 L/wk in the G group and 35 ± 1 L/wk in the Pgroup. The EN in the G and P groups were 7.4 ± 0.8and 3.6 ± 0.2 L/g of cneatinmne/day. respectively.

y = 2.34 + 0.44xR=0.36,p=0.l4

A

4,

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01�

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U U. U

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Weekly Urea KtIV

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t

20 40 60 80 100 120

Normalized Weekly Creatinine Clearance

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2

y = 2.46 + 0.14x

R = 0.47, p = 0.052

a

Brandes et al

Journal of the American Society of Nephrology 1433

2 4 6 8 10 12

Efficacy Number

Figure 2. The correlation of serum albumin with weekly Kt/V(A) normalized weekly creatinine clearance (liters perweek) (B) and the efficacy number (liters per grams ofcreatinine per day) (C).

Both creatinine models (weekly creatinine clearance

and the EN) differentiated the intermediate groupfrom the P on G outcome groups (P < 0.05).

DISCUSSION

Our study provides further insight into the use ofurea and creatinine kinetics for predicting clinicaloutcome in CAPD patients. There are few studiesthat have analyzed clinical outcome as a function ofurea or creatinine kinetics. Teehan et at. (6) retro-spectively analyzed the clinical outcome of 5 1 stableCAPD patients over 5 yr with urea kinetic parameters

���:L0 1.5 2.0 2.5 3.0

Weekly Urea KiN

Figure 3. The protein catabolic weight normalized to dryweight (grams per kilogram per day) correlated with theweekly urea Kt/V.

and found both death and transfusion requirement

were partially predicted by the level of urea Kt/V.B The NPCR. Kturea, and serum urea level were all sig-

nificantly higher in patients who remained unhos-

pitalized throughout the 5-yr study period.In a cross-sectional study. Keshaviah et at. (7)

found that weekly urea Kt/V correlated fairly wellwith a clinical assessment score, on the basis of

unemic signs and symptoms, serum albumin, hema-tocrit, and bean body mass. Lack of correlation wasfound in 26% of patients studied.

Blake et at. (5) investigated the predictive value of

the urea kinetic model on clinical outcome in 76

stable CAPD patients. The clinical outcome parame-C ters, similar to those in our study. included hospital-

izations, transfer from CAPD. peritonitis. transfu-sions, fatigue. pruritus. and insomnia. Unlike ourfindings. oven a 6-month study interval. the ureakinetic model parameter did not correlate with clini-

cab outcome. The conflicting results between thestudy by Blake et at. and our study may be because

of the range of urea Kt/V values, the analysis ofdifferent outcome parameters. and the length of thestudy period. We found a wide range of weekly ureaKt/V values (range. 1 .04 to 2.75) with a fairly even

distribution between the extreme values. Blake et at.do not fully describe the range in values of the ureakinetic parameters used. We included serum albu-mm, a powerful predictor of clinical outcome (1 7), in

the analysis; this parameter was not used in thestudy by Blake et at. (5).

The PCR was used in this study to assess clinicaloutcome because the PCR is correlated with proteinintake and nutrition, which are ultimately affectingthe serum albumin concentration. Retrospectiveanalysis by Lindsay and Spanner (1 8) has suggestedthat the PCR may be dependent upon the amount oftreatment, i.e. , urea Kt/V. In that study. increasing

the urea Kt/V by changing the dialysis prescriptionresulted in an increase in the PCR in 20 hemodialysisand 2 CAPD patients. Our data support this hypoth-

2.3±0.2

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Good Intermediate Poor

Clinical Outcome Group

71±8

#{149}44±3I 35±1

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Good Intermediate Poor

Clinical Outcome (;roup

7.4±0.8

#{149}4.9±0.4

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Clinical Outcome Group

Figure 4. All three kinetic methods differentiated betweenthe G and P outcome groups (P < 0.005 for all panels).There was no significant difference between the I and G orP outcome groups in terms of weekly urea Kt/V (A). Clinicaloutcome compared with (B) weekly creatinine clearance(liters per week per 1.73 square meter body surface area)or (C) the efficacy number (liters per grams of creatinineper day) was significantly different in all three outcomegroups (P< 0.05).

esis that the PCR may depend on urea Kt/V (Figure3).

The absolute urea Kt/V value adequate for hemo-dialysis cannot be directly applied to CAPD. Lysaghtet at. (8) compared urea Kt/V values in 30 CAPD and35 hemodiabysis patients and found significantlylower Kt/V values in patients on CAPD comparedwith those in patients on hemodialysis (0.59 ± 0.4

CAPD Adequacy

1434 Volume 2 ‘ Number 9 #{149}1992

versus 1 .0 1 ± 0.05, corrected to thrice-weekly di-

abysis). We found the average weekly urea Kt/V inthe G group to be 2.3, which is equivalent to a he-

modiabysis Kt/V of approximately 0.7 for thrice-weekly dialysis. A urea Kt/V of approximately 0.6 to

A 0.7 in hemodiabysis patients is associated with overt

unemic symptomatobogy. Yet, these symptoms were

not seen in the G group on in the patients in the studyof Lysaght et at.

To explain this apparent paradox. Keshaviah et at.

(1 9) has proposed the peak concentration hypothesis.Peak BUN concentration may be associated with

uremic toxicity; therefore, a higher Kt/V for urea isrequired in hemodiabysis to achieve a peak concen-

tration of BUN at on below the steady-state BUN

concentration with CAPD. Invoking this hypothesis,Keshaviah et at. calculated that the weekly Kt/V inCAPD for minimally adequate dialysis is 1 .7, equiv-

B alent to a weekly hemodiabysis Kt/V of 2.6. Our studysupports this hypothesis. Patients in the P group hadweekly urea Kt/V values of less than 1 .7 (average,1 .5 ± 0. 1) whereas those in the I group had valuesapproximately of 1.7.

Compared with urea Kt/V, we found creatinine

kinetics to be a more-sensitive predictor of clinicaloutcome in CAPD patients (univaniate analysis).Twandowski and Nobph’s (9) recommended minimumcreatinine clearance of 40 to 50 L/wk standardizedto 1 .73 m2 body surface area for adequate dialysis issupported by our study. All patients in the P grouphad weekly creatinine clearances of less than 4 1 LI

wk (mean 35 ± 1 L/wk), whereas the I group, consid-C ened to have minimally adequate dialysis. had a mean

weekly creatinine clearance of 44 L.The standardized PET (1 1), to determine penitoneal

transfer rates of creatinine, urea, sodium, and glu-cose during a 4-h dwell exchange is simple to execute

and is becoming common practice among clinicianscaring for CAPD patients. The dialysate-to-plasmaratio of creatinine (D/P) at 4 h correlates roughly withclinical response to CAPD. The glucose equilibration

curves are particularly useful in determining thecharacteristics of the membrane and the choice ofPD modality (e.g. , standard CAPD versus cyclic PD).To strengthen the predictive value of the creatinineD/P ratio, we introduced the EN (10), which stand-

ardized the D/P ratio to the patient’s creatinine pro-duction and takes into consideration the volume of

exchanges per day. It is an estimate of the penitonealclearance of creatinine based on a 4-h dwell, thusavoiding 24-h dialysate and urine collections. Thecreatinine production was chosen because it is aneasily measured value, unlike the volume of creati-nine distribution, which must be estimated clinically,resulting in frequent errors. We found the EN. cab-culated from the PET 4-h exchange, was as good indifferentiating the three outcome groups as was theweekly creatinine clearance (univariate analysis).

Brandes et al

Journal of the American Society of Nephrology 1435

In conclusion, both urea and creatinine kinetics

can predict clinical outcome in CAPD patients. How-ever, because creatinine kinetics appear to be a more-sensitive predictor of CAPD outcome, it may be moresuitable than urea kinetics for determining the ade-

quacy of, and the need for adjusting. CAPD prescrip-tions. Of the creatinine kinetic models, the EN is the

simplest to perform because it can be calculated froma single 4-h dwell exchange and does not require a

24-h urine collection. Because there have now been

several small sample studies addressing the ade-

quacy of CAPD with conflicting results and conclu-sions, a mubticenten prospective study is indicated.

The study should include both urea and creatininekinetics and should monitor patients for at least 2

yr. a time when many transfers from CAPD occur.Further, only new patients commencing CAPDshould be enrolled to avoid any prevalence/incidencebias when a cross-sectional start point is used, aswas done in this and previous studies on CAPD ade-quacy (5,6,7,10).

ACKNOWLEDGMENTS

We acknowledge Dr. Jacob Lemann for his advice during this clinical

study.

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1 3. Randerson DH, Chapman GV, Farrell PC.Amino acid and dietary status in long-termCAPD patients. In: Atkins RC, Farrell PC, Thom-son N, eds. Penitoneal Dialysis. Boston: MartinusNijhoff Publishers; 1981:179-191.

14. Twardowsld ZJ: PET-A simpler approach fordetermining prescriptions for adequate dialysistherapy. Adv Penitoneal Dial 1 990;6: 186-191.

1 5. Mitch WE, Walser M: A proposed mechanism forreduced creatinine excretion in severe chronicrenal failure. Nephnon l978;21 :248-254.

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