Calculation of 6-hour D/P creatinine ratio from the 4-hour peritoneal equilibration test. The effect of dwell duration on the results.
OBJECTIVE
Since the introduction of the peritoneal equilibration test (PET), the 4-hour dialysate/plasma creatinine (D/P Cr) has been used by several authors for determining continuous ambulatory peritoneal dialysis (CAPD) prescriptions. However, the results have been unsatisfactory because the 4-hr D/P Cr does not accurately reflect the D/P Cr in 24-hr collections. The PET and the 24-hr dialysate collections differ in the duration of dwell and the tonicity and volume of dialysate, all of which influence the equilibrated D/P Cr. It can be assumed that the D/P Cr in 24-hr collections in these patients is closer to a 6-hr D/P Cr. Because a 6-hr PET is inconvenient, we developed a mathematical model to calculate the 5- and 6-hr D/P using the results of a standard PET.


METHODS
In a retrospective analysis, D/P Cr ratios in 24-hr collections and D/P Cr ratios calculated from a mathematical formula were correlated. Using a mathematical model, the data collected fit an exponential relation of the type D/P = a(1-e-t/tau). The values of a and tau are unique for a given patient and were determined using a nonlinear regression technique. The formula performed well on our published data-the true and predicted 6-hr D/P Cr being 0.696 and 0.71, respectively.


METHODS
The University Hospital and Clinics, Dalton Cardiovascular Research Center and Dialysis Clinic, Inc., Columbia, Missouri.


METHODS
All CAPD patients on four 2-L exchanges/day at the time of the 24-hr collections were included.


METHODS
None.


METHODS
Closeness of 4-hr and 6-hr D/P Cr values to those of 24-hr ratios.


RESULTS
The study group comprised 74 patients (age, mean +/- SEM: 56.4 +/- 1.8 yr) with 80 PETs and 145 (24-hr) collections. The interval between the two tests was 8.3 +/- 0.9 months (0-48.7 months). The median 24-hr D/P Cr of 0.760 did not differ significantly from the predicted median 6-hr D/P Cr of 0.755. A subgroup analysis, based on transport type, showed that this relationship was most precise in the high-average transporters. The predicted 6-hr D/P Cr was within 10% of the 24-hr D/P Cr in 48% of patients and within 20% in 77% of patients. The margin of error was greatest in the low transporters.


CONCLUSIONS
To conclude, the 4-hr D/P Cr from a PET cannot be used interchangeably with the D/P Cr in the 24-hr dialysate collections, hence, the clearances calculated thereof will be inaccurate. Using the proposed model, it is feasible to use the 4-hr PET results to obtain 5- and 6-hr D/P Cr values. In our study, using this model, the extrapolated 6-hr D/P Cr is similar to the D/P Cr in 24-hr dialysate collections only in the high-average transporters. Hence, the best way to determine clearances in peritoneal dialysis patients is still by collecting 24-hr dialysates.
Mehrotra
R
R
Department of Internal Medicine, University of Missouri-Columbia 65212, USA.
Khanna
R
R
Yang
T C
TC
Kathuria
P
P
Moore
H L
HL
Prowant
B F
BF
Nolph
K D
KD
Twardowski
Z J
ZJ
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001693:Biological Transport, Active; D003404:Creatinine / Q000097:blood*; D003928:Diabetic Nephropathies / Q000150:complications; D003955:Diagnostic Tests, Routine / Q000379:methods*; D015314:Dialysis Solutions / Q000032:analysis / Q000378:metabolism*; D005260:Female; D006801:Humans; D006973:Hypertension / Q000150:complications; D007676:Kidney Failure, Chronic / Q000209:etiology / Q000503:physiopathology; D008297:Male; D008657:Metabolic Clearance Rate / Q000502:physiology; D008875:Middle Aged; D008954:Models, Biological; D008962:Models, Theoretical; D010531:Peritoneal Dialysis, Continuous Ambulatory / Q000379:methods / Q000592:standards; D010537:Peritoneum / Q000378:metabolism*; D007690:Polycystic Kidney Diseases / Q000150:complications; D015203:Reproducibility of Results; D013997:Time Factors
D015314:Dialysis Solutions; D003404:Creatinine