Abstract Aims/hypothesis: We compared the predictive performance of a GFR based on serum cystatin C levels with commonly used creatinine-based methods in subjects with diabetes. Subjects, materials and methods: In a crosssectional study of 251 consecutive clinic patients, the mean reference (plasma clearance of 99m Tc-diethylene-triaminepenta-acetic acid) GFR (iGFR) was 88±2 ml min . A regression equation describing the relationship between iGFR and 1/cystatin C levels was derived from a test population (n=125) to allow for the estimation of GFR by cystatin C (eGFR-cystatin C). The predictive performance of eGFR-cystatin C, the Modification of Diet in Renal Disease 4 variable formula (MDRD-4) and Cockcroft-Gault (C-G) formulas were then compared in a validation population (n=126). Results: There was no difference in renal function (ml min −1 1.73 m −

A Smith

D A Ratnaike

E Thomas

G Power

Jerums

M A Poon

M C Tsalamandris

R J C Macisaac

S I Jenkins

S Premaratne

T J Panagiotopoulos

CiteSeerX

Diabetologia (2006) 49: 1686–1689
DOI 10.1007/s00125-006-0275-7
SHORT COMMUNICATION
R. J. MacIsaac . C. Tsalamandris . M. C. Thomas .
E. Premaratne . S. Panagiotopoulos . T. J. Smith .
A. Poon . M. A. Jenkins . S. I. Ratnaike . D. A. Power .
G. Jerums
Estimating glomerular filtration rate in diabetes: a comparison
of cystatin-C- and creatinine-based methods
Received: 13 January 2006 / Accepted: 14 March 2006 / Published online: 3 May 2006
# Springer-Verlag 2006
Abstract Aims/hypothesis: We compared the predictive
performance of a GFR based on serum cystatin C levels with
commonly used creatinine-based methods in subjects with
diabetes. Subjects, materials and methods: In a cross-
sectional study of 251 consecutive clinic patients, the mean
reference (plasma clearance of 99mTc-diethylene-triamine-
penta-acetic acid) GFR (iGFR) was 88±2mlmin−1 1.73m−2.
A regression equation describing the relationship between
iGFR and 1/cystatin C levels was derived from a test
population (n=125) to allow for the estimation of GFR by
cystatin C (eGFR-cystatin C). The predictive performance of
eGFR-cystatin C, the Modification of Diet in Renal Disease
4 variable formula (MDRD-4) and Cockcroft–Gault (C–G)
formulas were then compared in a validation population
(n=126). Results: There was no difference in renal function
(ml min−1 1.73 m−2) as measured by iGFR (89.2±3.0),
eGFR-cystatin C (86.8±2.5), MDRD-4 (87.0±2.8) or C–G
(92.3±3.5). All three estimates of renal function had similar
precision and accuracy. Conclusions/interpretation: Esti-
mates of GFR based solely on serum cystatin C levels had
the same predictive potential when compared with the
MDRD-4 and C–G formulas.
Keywords Cockcroft–Gault . Cystatin C . Diabetic
nephropathy . Glomerular filtration rate . MDRD .
Microalbuminuria . Renal insufficiency
Abbreviations C–G formula: Cockcroft–Gault formula .
eGFR-cystatin C: estimation of GFR by cystatin C . iGFR:
reference GFR (plasma clearance of 99mTc-diethylene-
triamine-penta-acetic acid) . MDRD-4 formula:
modification of diet in renal disease 4 variable formula
Introduction
The measurement of serum cystatin C, a low molecular
weight protein (13 kDa) that is freely filtered through the
glomerulus and almost completely reabsorbed and catabo-
lised by tubular cells, has been proposed as a simple,
reliable and accurate marker of GFR [1]. It has recently
been shown that GFR derived from a simple regression
equation based on the relationship between the reciprocal
of serum cystatin C levels and GFR measured by 125I
iothalamate clearance was a more accurate estimate of renal
function than the Cockcroft–Gault (C–G) formulas in
subjects with and without diabetes [2]. However, the bias,
precision (standard deviation of the difference from a
reference method) and accuracy (within 15, 30 and 50% of
reference method values) for a GFR estimated from serum
cystatin C levels have not been compared with the
abbreviated Modification of Diet in Renal Disease 4
variable formula (MDRD-4) or C–G formula in patients
with diabetes and a wide range of renal function.
R. J. MacIsaac (*) . C. Tsalamandris . E. Premaratne .
S. Panagiotopoulos . T. J. Smith . G. Jerums
Endocrine Centre and the Department of Medicine,
Austin Health and University of Melbourne,
Heidelberg Repatriation Hospital,
Level 2, Centaur Building,
Heidelberg West, VIC, 3081, Australia
e-mail: r.macisaac@unimelb.edu.au
Tel.: +61-3-94965489
Fax: +61-3-94963365
M. C. Thomas
Danielle Alberti Memorial Centre for Diabetes Complications,
Baker Medical Research Institute,
Melbourne, VIC, Australia
A. Poon
Division of Nuclear Medicine and Centre for PET,
Austin Health,
Heidelberg, VIC, Australia
M. A. Jenkins . S. I. Ratnaike
Division of Laboratory Medicine, Austin Health,
Heidelberg, VIC, Australia
D. A. Power
Department of Nephrology, Austin Health,
Heidelberg, VIC, Australia
Subjects, materials and methods
Patient population and study design
This study was a cross-sectional survey of patients
attending the diabetes clinic at Austin Health, a tertiary
referral centre and teaching hospital of the University of
Melbourne, Victoria, Australia. The study population
consisted of 251 consecutive patients who had a mean
reference GFR (plasma clearance of 99mTc-diethylene-
triamine-penta-acetic acid) (iGFR) measurement and
sufficient clinical and biochemical data for inclusion. The
majority of patients (92%) were Caucasian; there were no
patients of African-American origin. Patients with known
non-diabetic renal disease were excluded. Creatinine and
cystatin C concentrations were measured in a fasting blood
sample collected on the morning of the iGFR estimation.
We randomly selected 125 subjects (the test population),
from the entire study population and a regression equation
describing the relationship between iGFR and the recipro-
cal of serum cystatin C levels was derived to allow the
estimation of GFR by cystatin C (eGFR-cystatin C). In the
remaining 126 subjects (the validation population), the
bias, precision and accuracy of eGFR-cystatin C and both
the MDRD-4 and C–G formulas were compared. Informed
consent was obtained from patients, as approved by the
Austin Health Human Research Ethics Committee, for the
unrestricted use of clinical data for non-interventional
research studies.
Laboratory methods
The reference value iGFR was obtained by measuring the
plasma disappearance of 99mTc-diethylene-triamine-penta-
acetic acid employing the Brochner–Mortensen correction
[3]. Urinary albumin and serum electrolytes were measured
as described previously [3]. Creatinine was measured on an
automatic analyser (Hitachi 911; Roche Diagnostics,
Mannheim, Germany), which produces creatinine values
that fall within less than ±15% of the reference MDRD
method. This method is considered acceptable with respect
to bias and precision by the Australian working group on the
automatic reporting of estimated GFR [4], and hence we did
not adjust serum creatinine concentrations. Cystatin C was
measured using a particle-enhancing immunonephelometric
0
50
100
150
200
250
iG
F
R
 
0 50 100 150 200 250
eGFR-Cystatin C
(ml min   1.73 m  )−2−1 (ml min   1.73 m  )−2−1 (ml min   1.73 m  )−2−1
(ml min   1.73 m  )−2−1(ml min   1.73 m  )−2−1(ml min   1.73 m  )−2−1
0
50
100
150
200
250
iG
F
R
 
0 50 100 150 200 250
MDRD-4 formula
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250
iG
F
R
 
0 50 100 150 200 250
C-G formula
--  75
--  50
--  25
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75
100
eG
F
R
-C
ys
ta
tin
 C
 −
iG
F
R
 
0 50 100 150 200
(eGFR-Cystatin C + iGFR)/2 
---  75
---  50
 ---  25
0
25
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75
100
M
D
R
D
- 4
 fo
rm
ul
a 
−
iG
F
R
 
0 50 100 150 200
(MDRD-4 formula + iGFR)/2 
--  75
--  50
--  25
0
25
50
75
100
C
-G
 fo
rm
ul
a 
−
iG
F
R
0 50 100 150 200
(C-G formula + iGFR)/2 
(m
l m
in
   
1.
73
 m
  )−2
−1
(m
l m
in
   
1.
73
 m
  )−2
−1
(m
l m
in
   
1.
73
 m
  )−2
−1
(m
l m
in
   
1.
73
 m
  )−2
−1
(m
l m
in
   
1.
73
 m
  )−2
−1
(m
l m
in
   
1.
73
 m
  )−2
−1
a b c
d e f
Fig. 1 Comparison of estimates of renal function with reference
iGFR (plasma clearance of 99mTc-diethylene-triamine-penta-acetic
acid) values for 126 subjects in the validation group. Regression
lines for iGFR and eGFR-cystatin C (a), iGFR and the MDRD-4
variable formula (b) and iGFR and the Cockcroft–Gault (C–G)
formula (c). Dotted line, line of identity. The Bland–Altman plot of
disagreement between eGFR-cystatin C (d), the MDRD-4 formula
(e) and the C–G formula (f) with reference iGFR values
1687
assay (Dade Behring, Marburg, Germany) as described
previously [5].
Statistical analysis
The predictive performance of eGFR-cystatin C, the
MDRD-4 and C–G formulas was assessed using the
methods of Bland–Altman and Manjunath et al. [6].
Accuracy was also estimated within 15, 30 and 50% of
iGFR values. The accuracy of each method for estimating
renal function was compared by means of chi square tests.
For continuous variables, data are means±SEM, except
where presented as medians (interquartile range). A p value
of <0.05 was considered statistically significant, and a 95%
level of confidence was used.
Results
Patient characteristics, correlations with cystatin C
and derivation of eGFR-cystatin C
The study population had a mean age of 60±1 years (range
22–84) and was comprised mainly of patients with type 2
diabetes (82%). The mean measured iGFR was 88± 2 ml
min−1 1.73 m−2 (range 9–181) and 22% of subjects had an
iGFR<60 ml min−1 1.73 m−2. The mean (interquartile
range) of serum cystatin C and creatinine levels was 0.91
(0.78, 1.15), range 0.51–416 mg/l, and 76 (65, 94), range
34–354 μmol/l, respectively. The AER status for the study
population was 61, 25 and 14% for normo-, micro- and
macroalbuminuria respectively. There were no significant
differences in the clinical or biochemical characteristics for
the entire, test and validation study populations. The
regression equation, GFR = (84.6/cystatin C) −3.2 (r=0.82,
p<0.0001) was derived from the relationship of iGFR
values and the reciprocal of cystatin C levels in the test
population. This equation had the same characteristics as
the regression equation derived from the entire study
population, i.e. GFR=(86.7/cystatin C)−4.2 (r=0.86,
p<0.0001).
Comparison of eGFR-cystatin and the MDRD-4
and C–G formulas
The comparison of the correlations between iGFR values
and eGFR-cystatin C, MDRD-4 and the C–G formulas,
together with their Bland–Altman plots for the validation
group are shown in Fig. 1.
The detailed performance of renal function estimated by
eGFR-cystatin C or by the MDRD-4 and C–G formulas for
predicting iGFR levels is shown in Table 1. There was no
significant difference in renal function (ml min−1 1.7 m−2)
as measured by iGFR (89.2±3.0), eGFR-cystatin C (86.8±
2.5) or by the MDRD-4 (87.0±2.8) and C–G (92.3±3.5)
formulas. Overall, eGFR-cystatin C and the MDRD-4 and
C–G formulas had similar precision, combined root mean
squared error values and accuracy (within 15 and 30%)
when compared to iGFR levels. However, eGFR-cystatin C
had a greater accuracy within 50% of iGFR values than the
C–G formula (p<0.04).
Table 1 Predictive performance of eGFR-cystatin C, the MDRD-4 variable (MDRD-4) formula and Cockcroft–Gault (C–G) formula in the
validation population (n=126)
Equation eGFR-cystatin C (n=126) MDRD-4 (n=126) C–G (n=126)
Gradient (95% CI) 1.04 (0.94 to 1.15) 0.90 (0.79 to 1.00) 0.71 (0.63 to 0.79)
Intercept (95% CI) −1.5 (−11.1 to 8.1) 11.3 (1.8 to 20.8) 23.5 (15.3 to 31.7)
R 0.87 0.84 0.84
r2 0.75 0.70 0.70
p <0.0001 <0.0001 <0.0001
eGFR-cystatin/MDRD-4/ C–G (ml min−1 1.73 m−2) 86.8±2.5 87.0±2.8 92.3±3.5
eGFR-cystatin/MDRD-4/ C–G (95% CI) 82.0 to 91.7 81.5 to 92.4 85.4 to 99.2
Bias −2.4 −2.2 3.1
Precision 16.4 18.3 21.2
CRMSE 16.6 18.4 21.4
Accuracy within 15% 59% 60% 63%
Accuracy within 30% 88% 89% 81%
Accuracy within 50% 98% 96% 94%a
The mean iGFR (plasma clearance of 99mTc-diethylene-triamine-penta-acetic acid) level for the validation group was 89.2±3.0
(ml min−1 1.73 m−2)
Bias: the mean difference between each marker of renal function and iGFR; Precision: the standard deviation of the above difference;
CRMSE: combined root mean squared error, calculated as the square root of the [(mean difference in estimate—observed)2+
(standard deviation of the difference)2]; eGFR: estimated GFR based on unadjusted serum cystatin C levels; MDRD-4: the simplified four
variable MDRD formula; C–G: Cockcroft–Gault formula
ap<0.04 vs eGFR-cystatin C
1688
Discussion
Using established methods to develop and evaluate predic-
tion equations, a GFR estimated solely from serum cystatin
C levels had a predictive performance equal to commonly
used creatinine-based estimates of renal function in a
population of diabetic subjects with a wide range of renal
function. In particular, bias, precision and accuracy of the
different prediction equations were similar, although eGFR-
cystatin C had a slightly greater accuracy within 50% of
reference iGFR values than the C–G formula. It is doubtful
whether this difference has any clinical significance.
In agreement with previous studies, we found that the
MDRD-4 equations slightly underestimated (−2.2 ml
min−1 1.73 mm−2), whereas the C–G formula slightly
overestimated (+3.1 ml min−1 1.73m−2) reference iGFR
measurements. Also, like others, we found that a calculated
GFR largely lacked accuracy and precision for estimating a
reference isotopic GFR method [7]. Only two-thirds of
values estimated by the three methods used fell within 15%
of the reference iGFR measurements.
Many studies have demonstrated the superiority of
serum cystatin C compared with serum creatinine measure-
ments as a marker of renal function in subjects with
diabetes, although this has not been a universal finding.
Fewer studies have compared serum cystatin C levels and
the C–G formula and, to our knowledge, this study is the
first to incorporate a comparison of eGFR-cystatin C with
both the MDRD-4 and C–G formulas. Recently, trends in
the reciprocal of cystatin C serum levels over a 4-year
period were demonstrated to correlate more closely with
iothalamate clearance than the MDRD-4 and C–G formulas
in subjects with type 2 diabetes and an initial GFR >120 ml
min−1 1.73 m−2 [8]. The possible superiority of serum
cystatin C compared with serum creatinine for predicting
GFR may be explained by the greater influence of factors,
apart from renal clearance, on creatinine production and
secretion such as renal tubular secretion, age, sex and
muscle mass.
Although multiple factors apart from renal function may
influence serum cystatin C levels [9], we found that GFR
calculated solely from serum cystatin C levels and not
adjusted for clinical or biochemical variables compared
favourably with creatinine-based methods that account for
some of the factors known to influence creatinine
production. In healthy subjects, cystatin C levels have
been reported to increase with age, and age-specific
reference ranges have been proposed [9]. Possibly, an
adjustment for age would lead to an even better predictive
performance of an eGFR-cystatin formula, as happens
when effect of age is incorporated in creatinine-based
formulas used to estimate renal function.
The measurement of serum cystatin C levels in subjects
with diabetes may have practical implications apart from
providing an estimation of renal function. Thus there is also
evidence to suggest that serum cystatin C levels predict
cardiovascular risk better than serum creatinine levels or
GFR levels estimated by the MDRD formula [10]. A
rigorous cost-effectiveness analysis for measuring cystatin
C levels as opposed to measuring creatinine levels and
subsequently estimating a GFR using established formulas
has not been performed to date. We estimate that a single
cystatin C measurement, using the method employed in
this study, would cost approximately A$6—(Australian
dollars). In comparison, measurement of a single creatinine
level on the auto analyser used in this study (Hitachi 911) is
estimated to cost only A$1.75. The current lack of an
automated method for measuring cystatin C levels
undoubtedly accounts for a significant proportion of the
increased cost associated with measuring cystatin C levels.
It is not our intention that the formula used in this study
should be applied in clinical practice as a means of
estimating GFR from serum cystatin C levels. However, we
do hope that our findings, which also support the literature
describing the potential usefulness of cystatin C as a
marker of GFR, will trigger large population studies
resulting in a further validation and possible improvement
of the predictive performance of an estimated GFR based
on measurement of serum cystatin C levels for subjects
with and without diabetes.
Acknowledgement This work was supported by the National
Health and Medical Research Council of Australia (Grant number
266505).
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Estimating glomerular filtration rate in diabetes: a comparison of cystatin-C-and creatinine-based methods

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Estimating glomerular filtration rate in diabetes: a comparison of cystatin-C-and creatinine-based methods

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