Residual renal function calculated from serum cystatin C measurements and knowledge of the weekly standard Kt/V urea.

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Peritoneal Dialysis International, Vol. 32, pp. 102–108
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Copyright © 2012 International Society for Peritoneal Dialysis
102
Residual Renal Function Calculated from
Serum Cystatin C Measurements and
Knowledge of the Weekly Standard
Kt/V Urea
Residual renal clearance was found to be a predictor
of survival in dialysis patients (1,2). Monitoring and
preserving residual renal function (RRF) in dialysis pa-
tients is therefore important (3,4). Cystatin C (CysC) is
a protein of low molecular weight. Studies by Delaney et
al. and Hoek et al. showed strong correlations between
serum levels of CysC and RRF in dialysis patients (5,6).
The Hoek study developed an estimated equation:
Estimated RRF (mL/min/1.73 m2) =
–0.70 + [22 / CysC (mg/L)].
A recent study by Al Malki et al. (7) showed a signifi-
cant inverse relationship between serum levels of CysC
and values of “weekly standardized” Kt/V (Std Kt/V) in
functionally anephric patients:
Std Kt/V = –0.703 × CysC + 7.254.
In the present study, we aimed to assess the role of
serum levels of CysC and dialytic clearance in measuring
RRF. We hypothesized that the difference between mea-
sured CysC and an estimate from the Al Malki equation
would correlate with RRF as measured by the average of
urinary creatinine and urea clearance. We also postulated
that the hypothesized correlation might be stronger than
Hoek’s RRF, which uses 1 / CysC alone.
METHODS
In this cross-sectional, single-center pilot study, blood
and urine samples were prospectively collected from
patients (n = 15) with end-stage renal disease receiving
peritoneal dialysis (n = 7) and conventional thrice-weekly
high-flux hemodialysis therapy (3 – 4 hours thrice weekly;
n = 8). All patients provided written informed consent.
Patients were excluded if their dialysis prescription had
been changed within the preceding 3 months. The study
was approved by the Health Sciences Research Ethics Board
at the University of Western Ontario (HSREB #16598E).
We measured serum levels of CysC, urea, and creatinine
in the study patients. For the hemodialysis patients, pre-
dialysis blood samples were used to measure serum CysC
before the mid-week hemodialysis session, although our
recent study demonstrated that pre-dialysis serum CysC
does not vary between hemodialysis sessions (8). Serum
levels of CysC were determined by immunonephelometry
using an N Latex Cystatin C kit (Siemens Healthcare Di-
agnostics, Deerfield, IL, USA) on a BN ProSpec analyzer
(Dade Behring, Marburg, Germany) at the reference
laboratory at the Children’s Hospital of Eastern On-
tario in Ottawa, Canada, with an established coefficient
of variation.
We also obtained 24- to 45-hour urinary collections
from the patients. The RRF was determined as the average
of urinary creatinine and urea clearances, further ad-
justed for body surface area using the DuBois formula (9).
From the peritoneal dialysis patients, we also obtained
a 24-hour peritoneal effluent collection for measure-
ment of total urea loss. Using those samples, daily urea
clearances were calculated, and values of Std Kt/V were
derived (7 × daily K in liters) using the Watson equation
for V (4,10). To derive the Std Kt/V for the hemodialysis
patients, the efficacy of a single hemodialysis treatment
was taken as the single-pool Kt/V, calculated using urea
kinetic modeling (3,11,12).
For all 15 patients, RRF was estimated using the Hoek
equation (5). By rearranging the Al Malki equation, Std
Kt/V could be used to predict the pre-dialysis levels of
CysC (“expected pre-dialysis CysC”). The expected levels
of CysC did not consider RRF (7). The difference between
the expected pre-dialysis CysC and the measured CysC was
defined as ΔCysC. The equation for ΔCysC-estimated RRF
was derived using the ΔCysC value.
Statistical analysis was performed using the GraphPad
Prism software for Windows (version 4.03: GraphPad
Software, San Diego, CA, USA). Means and standard
deviations are reported for normally distributed data;
otherwise, medians, with 25th and 75th percentiles
(interquartile range) are given. A linear regression equa-
tion was derived from the values of ΔCysC. The Pearson
correlation coefficient was used to assess the strength of
the relation between measured RRF and values of ΔCysC,
and between the measured RRF and the estimated RRF
ShoRt RepoRtS
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103
PDI january 2012 – Vol. 32, no. 1 SHORT REPORTS
using both the Hoek equation and the ΔCysC equation.
The Bland–Altman test was used to calculate bias and the
standard deviation of the bias between the estimated
RRF and the measured RRF. A value of p < 0.05 was con-
sideredsignificant.
RESULTS
For the 15 patients with completed measurements,
mean age was 63 ± 15 years. The three most common
causes of end-stage renal disease were hypertensive
nephropathy (33%), diabetic nephropathy (27%),
and glomerulonephritis (14%). The mean measured
pre-dialysis CysC was 4.57 ± 1.02 mg/L. The mean Std
Kt/V was 2.61 ± 0.67 and 1.65 ± 0.59 with and without
consideration of RRF respectively. The mean measured
RRF was 1.73 ± 0.67 mL/min/1.73 m2.
We observed a statistically significant correlation
between measured RRF and ΔCysC (r = 0.90, p < 0.0001).
The association between measured RRF and ΔCysC could
be expressed as
measured RRF (mL/min/1.73 m2) =
ΔCysC × 0.3601 + 0.5034.
The ΔCysC-estimated RRF was plotted against
measured RRF, with a significant linear correlation
being obtained (r2 = 0.81, p ≤ 0.001, Figure 1). The bias
was 0.001 ± 0.290 mL/min/1.73 m2. The correlation
coefficient between the Hoek RRF and the measured
RRF was r2 = 0.69 (p < 0.0001, Figure 1), with a bias of
2.70± 0.847 mL/min/1.73 m2.
DISCUSSION
Hoek et al. derived an equation to obtain RRF using
serum 1 / CysC in both hemodialysis and peritoneal
dialysis patients. However, those authors ignored the
dialytic clearance of CysC (5). In the present study, we
showed a strong correlation between measured RRF and
ΔCysC (r = 0.90, p < 0.0001), which considers only the
dialytic clearance. The linear relationship between the
two parameters is expressed as
measured RRF (mL/min/1.73 m2) =
ΔCysC × 0.3601 + 0.5034.
The bias was 0.001 ± 0.290 mL/min/1.73 m2 (p =
0.40). Figure 1 shows that the regression lines for the
ΔCysC-estimated RRF is closer to the line of identity than
is Hoek’s estimated RRF.
Why does the Hoek approach overestimate measured
RRF? The overestimation probably reflects a difference
in the Std Kt/V between our study population and the
Hoek study population. In the Hoek study, the mean pre-
dialysis CysC was significantly higher (5.8 – 6.1 mg/L)
that that observed in our study (4.6 ± 1.20 mg/L),
despite the measured RRF in our study being lower than
that in Hoek’s study (1.73 ± 0.67 mL/min/1.73 m2 and
2.7 to 3.3 ± 1.3 to 1.5 mL/min/1.73 m2 respectively).
Those findings indicate that the mean Std Kt/V, without
consideration of renal clearance, was lower in the Hoek
study than in our study. It is therefore not surprising
that the Hoek RRF systemically overestimated RRF in our
study population. Incorporating Std Kt/V can eliminate
this systematic error.
When the glomerular filtration rate is below 30 mL/
min/1.73 m2, the accuracy of the nuclear medicine iso-
topic measurement method for that rate is limited (13).
We therefore used the average of the urinary creatinine
and urea clearances as our reference RRF. We did not find
a correlation between Std Kt/V and CysC, and we did not
expect to because of the small sample size and the nar-
row range of Std Kt/V values obtained from patients on
identical dialysis modalities. Although Sjostrom et al.
(14) suggested that renal clearance of CysC takes the
form of hyperbolic function (1 / X), we were unable to
incorporate Std Kt/V in deriving RRF without using the
Al Malki equation.
The major limitation of our study is its small size.
Moreover, both the Al Malki equation, which was used
to derive ΔCysC, and the estimated RRF equation in
the present study require further validation. Another
limitation is the small and narrow range of RRF values
among the study patients. However, the present study
was designed as a pilot to test our hypothesis. Our work
uncovered the importance of incorporating Std Kt/V into
the estimated RRF equation, because failure to do so can
Figure 1 — Correlation analysis of the Hoek estimated residual
renal function (eRRF) and the measured residual renal func-
tion (RRF), and of the change (Δ) in cystatin C eRRF and the
measured RRF (r2 = 0.69 and 0.81 respectively, p ≤ 0.0001).
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104
SHORT REPORTS january 2012 – Vol. 32, no. 1 PDI
result in systemic bias when Std Kt/V values are different.
We plan to conduct a larger study with wide ranges of
Std Kt/V values to validate the ΔCysC equation.
DISCLOSURES
The authors have no conflicts of interest to disclose.
Shih-Han S. Huang1–3
Guido Filler4
Robert M. Lindsay1,2*
Department of Medicine1
Department of Medical Biophysics2
The University of Western Ontario
London, Ontario, Canada
Nephrology Division3
London Health Sciences Centre
Department of Paediatrics4
The University of Western Ontario
London Health Sciences Centre
London, Ontario, Canada
*email: robert.lindsay@lhsc.on.ca
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doi:10.3747/pdi.2011.00047
patient Able to Stay on peritoneal Dialysis
After Retroperitoneal-Approach
Radical Nephrectomy
KEY WORDS: Radical nephrectomy; retroperitoneal.
Chronic renal failure patients have a higher risk for
urogenic carcinoma (1,2). They need treatment with radi-
cal nephrectomy more frequently than a general popu-
lation does. In addition, some non-dialysis-dependent
patients may become dialysis-dependent after radical
nephrectomy. Facing a choice of modality for renal re-
placement therapy (RRT) after nephrectomy, patients
usually choose or change to hemodialysis.
Compared with the transperitoneal route, radical
nephrectomy using the retroperitoneal approach can
preserve peritoneal function. But the literature con-
tains only a few reports of successful peritoneal dialysis
(PD) after radical nephrectomy. Here, we report a case
series of 5 patients who underwent radical nephrec-
tomy by the retroperitoneal approach to preserve the
peritoneum, allowing PD to be reinstated immediately
after surgery without interruption or to be successfully
started de novo.
This single copy is for your personal, non-commercial use only.
For permission to reprint multiple copies or to order presentation-ready copies
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