Estimation of glomerular filtration rate from serum creatinine and cystatin C in octogenarians and nonagenarians
Estimation of glomerular filtration rate from serum creatinine and cystatin C in octogenarians and nonagenarians
Marcelo B Lopes 0 2
Lara Q Araújo 1
Michelle T Passos 0 2
Sonia K Nishida 0 2
Gianna M Kirsztajn 0 2
Maysa S Cendoroglo 1
Ricardo C Sesso 0 2
0 Nephrology, School of Medicine, Federal University of São Paulo , São Paulo , Brazil
1 Geriatrics Divisions, Paulista School of Medicine, Federal University of São Paulo , Rua Botucatu 740, 04023-900, São Paulo , Brazil
2 Nephrology, School of Medicine, Federal University of São Paulo , São Paulo , Brazil
Background: Equations to estimate GFR have not been well validated in the elderly and may misclassify persons with chronic kidney disease (CKD). We measured GFR and compared the performance of the Modification of Diet in Renal Disease (MDRD), the Chronic Kidney Disease-Epidemiology Collaboration (CKD-Epi) and the Berlin Initiative Study (BIS) equations based on creatinine and/or cystatin C in octogenarians and nonagenarians. Methods: Using cross-sectional analysis we assessed 95 very elderly persons (mean 85 years) living in the community. GFR was measured by iohexol (mGFR) and compared with estimates using six equations: MDRD, CKD-Epi_creatinine, CKD-Epi_cystatin, CKD-Epi_creatinine-cystatin, BIS_creatinine and BIS_creatinine-cystatin. Results: Mean mGFR was 55 (range,19-86) ml/min/1.73 m2. Bias was smaller with the CKD-Epi_creatinine-cystatin 2 and the CKD-Epi_creatinine equations (−4.0 and 1.7 ml/min/1.73 m ). Accuracy (percentage of estimates within 30% of mGFR) was greater with the CKD-Epi_creatinine-cystatin, BIS_creatinine-cystatin and BIS_creatinine equations (85%, 83% and 80%, respectively). Among the creatinine-based equations, the BIS_creatinine had the greatest accuracy at mGFR < 60 ml/min/1.73 m2 and the CKD-Epi_creatinine was superior at higher GFRs (79% and 90%, respectively). The CKD-Epi_creatinine-cystatin, BIS_creatinine-cystatin and CKD-Epi_cystatin equations yielded the greatest areas under the receiver operating characteristic curve at GFR threshold = 60 ml/min/1.73 m2 (0.88, 0.88 and 0.87, respectively). In participants classified based on the BIS_creatinine, CKD-Epi_cystatin, or BIS_creatinine-cystatin equations, the CKD-Epi_creatinine-cystatin equation tended to improve CKD classification (net reclassification index: 12.7%, p = 0.18; 6.7%, p = 0.38; and 15.9%; p = 0.08, respectively). Conclusions: GFR-estimating equations CKD-Epi_creatinine-cystatin and BIS_creatinine-cystatin showed better accuracy than other equations using creatinine or cystatin C alone in very elderly persons. The CKD-Epi_creatinine-cystatin equation appears to be advantageous in CKD classification. If cystatin C is not available, both the BIS_cr equation and the CKD-Epi_cr equation could be used, although at mGFR < 60 ml/min/1.73 m2, the BIS_cr equation seems to be the best alternative.
Chronic kidney disease; Creatinine; Cystatin C; Elderly; Glomerular filtration rate; Iohexol
Chronic kidney disease (CKD) is a major public health
problem and its prevalence is rising, particularly in the
elderly, reflecting mainly the worldwide increased life
expectancy of the population [
]. The measurement of
kidney function remains challenging and CKD is
imprecisely defined in older adults. Most previous studies in
the elderly  have used creatinine-based equations
to estimate GFR (eGFR), but these equations may be
particularly limited due to non-GFR determinants of serum
creatinine such as presence of chronic illnesses, decreased
muscle mass and diet [
]. The relative imprecision of
creatinine-based GFR estimates can potentially result in
misclassification of individuals as having CKD, leading to
unnecessary diagnostic and therapeutic interventions.
Cystatin C is considered to be a potential alternative
to serum creatinine for estimating GFR [
suggests that cystatin C is less dependent upon muscle
mass than creatinine and it is assumed that it should
provide more accurate GFR estimates, particularly in
populations with reduced muscle mass such as the
]. There has been limited evaluation of the
GFR estimating equations in the elderly . Recently the
Berlin Initiative Study (BIS) developed two novel equations
to estimate kidney function in older people [
], but they
have not been validated in other data sets.
The aims of this study were to measure GFR, to compare
the performance of some frequently used or recently
described GFR estimating equations based on creatinine
and cystatin C, alone or combined, and test their
usefulness in the classification of CKD in individuals 80 years of
age or older.
This is a cross sectional study of community-dwelling
independent elderly aged 80 years or older recruited in
the southern region of the city of São Paulo, Brazil. The
elderly were identified through regional surveys among
those individuals living in the neighborhood of the
Federal University of São Paulo campus from November
2010 through December 2011. During this period, 294
elderly were identified and after interview and physical
examination 200 were enrolled in a longitudinal
epidemiologic study at the Department of Geriatrics. All
clinically stable individuals and independent in activities
of daily living were acceptable to participate in the study.
The exclusion criteria were: to be institutionalized,
unable or unwilling to give consent, to have an acute
infectious disease, a moderate or severe cognitive
impairment, heart failure (NYHA class III or IV), known cirrhosis,
previously received dialysis, unstable chronic pulmonary
disease, previous immunosuppressive therapy within 6 mo.,
previous chemotherapy for cancer, known HIV infection,
and previously reported allergic reaction to iodine. From
August 2011 to May 2012, of the 294 initial individuals, 94
were excluded after checking the selection criteria. The
purpose and the procedures for this study were explained
by telephone to the remaining 200 and, of these, 97
returned to the clinics for the renal function studies
scheduled within a week of the date of interview. There were no
sociodemographic or clinical characteristics significantly
different between the participants (n = 97) and the
remaining 103 individuals enrolled in the epidemiologic
Geriatric study that did not undergo renal function tests.
The study protocol was approved by the Research Ethics
Committee of the Federal University of São Paulo and all
participants gave their written informed consent.
Determination of serum creatinine
After the blood sample was collected from each participant,
the material was immediately processed and analyzed in
the same day and always in the same laboratory at the
university Hospital do Rim. Serum creatinine was measured
using a modified kinetic Jaffé colorimetric method on an
autoanalyzer (Beckman Coulter AU 400, CA, USA), which
was calibrated to isotope dilution mass spectrometry
(IDMS) using a standard reference material (914a) traceable
to the National Institutes of Standards and Technology
(NIST). Total analytical variation (CV%) of the colorimetric
method was 1.4-3.0%. The intra-assay coefficient variation
(CV) for creatinine was 1.9%.
Determination of plasma cystatin C
Plasma cystatin C levels were determined by an automated
particle-enhanced immunoturbidimetric method [
a Beckman AU 400 analyzer (Beckman Coulter, Inc. CA,
USA), reagents (code Nos. LX002, s2361, X0973, X0974)
obtained from DakoCytomation (Glostrup, Denmark) and
following the procedures recommended by the reagent
producer. The intra-assay CV was 4.7% and the
interassay CV was 5.2% at a cystatin C level of 1.0 mg/L. All
samples were frozen at −70°C and analyzed within 7 days
in May 2012.
Determination of iohexol clearance 
Five mL of iohexol (Omnipaque 300 mg iodine/mL, GE
Healthcare) were administered intravenously in an
antecubital vein and blood samples were obtained at 2, 3, 4
and 5 hours after infusion. Iohexol clearance (referred to
as ‘measured GFR’ (mGFR)) was calculated based on the
measured decline in plasma concentrations of iohexol,
using the slope intercept method (one-compartment
]. Plasma iohexol concentrations were
determined by a capillary electrophoresis method [
] in an
automated instrument (5010 Beckman Instruments, Palo
Alto, CA). The inter-assay CV was between 5.7% and
7.4%, and the intra-assay CV were between 3.4 and 4.0%.
Measured GFR values were adjusted to 1.73 m2 body
surface area [
GFR estimating equations used (Table 1)
1. The Modification Diet in Renal Disease (MDRD)
2. The Chronic Kidney Disease Epidemiology
creatinine (CKD-Epi_cr) equation [
3. The Chronic Kidney Disease Epidemiology cystatin
C (CKD-Epi_cys) equation [
4. The Chronic Kidney Disease Epidemiology
creatinine-cystatin C (CKD-Epi_cr-cys) equation [
5. The Berlin Initiative Study creatinine (BIS_cr)
6. The Berlin Initiative Study creatinine-cystatin C
(BIS_cr-cys) equation [
Values are expressed as mean ± SD or median and
interquartile range. Bias, precision and accuracy were
measured to determine the performance of each equation
]. Bias was assessed as the mean of the difference
between estimated GFR and measured GFR. The width of
the SD of the mean difference is an estimation of
precision. Accuracy was assessed as the percentage of results
that did not deviate more than 30% from the measured
GFR and also as the combined root mean square error
(CRMSE). These analyses were performed in the whole
2. The Chronic Kidney Disease Epidemiology creatinine (CKD-Epi_cr)
]: 141 × min(Scr/κ, 1)a × max(Scr/κ, 1)‐ 1.209 × 0.993age ×
1.018(if female) × 1.159(if black); where Scr is serum creatinine,
k is 0.7 for females and 0.9 for males, α is −0.329 for females
and −0.411 for males, min is the minimum of Scr/ k or 1, and
max is the maximum of Scr/k or 1.
3. The Chronic Kidney Disease Epidemiology cystatin C (CKD-Epi_cys)
]: 133 × min(Scys/0.8, 1)‐ 0.499 × max(Scys/0.8, 1)‐ 1.328 ×
0.996age(×0.932 if female); where Scys is serum cystatin C, min
indicates the minimum of Scr/κ or 1, and max indicates the
maximum of Scys/κ or 1.
4. The Chronic Kidney Disease Epidemiology creatinine-cystatin C
(CKD-Epi_cr-cys) equation [
]: 135 × min(Scr/κ, 1)α × max(Scr/κ, 1)‐ 0.601 ×
min(Scys/0.8, 1)‐ 0.375 × max(Scys/0.8, 1)‐ 0.711 × 0.995age(×0.969 if female)
(×1.08 if black); where Scr is serum creatinine, Scys is serum cystatin C,
κ is 0.7 for females and 0.9 for males, α is −0.248 for females and −0.207
for males, min indicates the minimum of Scr/κ or 1, and max indicates
the maximum of Scr/κ or 1.
5. The Berlin Initiative Study creatinine (BIS_cr) equation [
]: 3736 ×
creatinine‐ 0.87 × age‐ 0.95 × 0.82(if female).
6. The Berlin Initiative Study creatinine-cystatin C (BIS_cr-cys) equation
]: 767 × cystatin C‐ 0.61 × creatinine‐ 0.40 × age‐ 0.57 × 0.87(if female).
group and in subgroups according to mGFR cutoff of
60 ml/min/1.73 m2. The significance of the differences
among equations was determined by the paired Student
t test, and McNemar’s test for differences in proportions.
The diagnostic accuracy of eGFR equations was evaluated
using receiver operating characteristics (ROC) plots. The
GFR determined with iohexol clearance was used as the
gold standard and the cutoff value was set at 60 ml/min/
1.73 m2 for CKD [
]. Analyses were also performed for
the cutoff value of 45 ml/min/1.73 m2 [
We assessed the use of the equations with best
performances for the classification of CKD by means of the
net reclassification index (NRI) statistic. We compared
the proportion of participants who were reclassified as
having a measured GFR that was less than 60 (or 45)
ml/min/1.73 m2 on the basis of one equation versus the
other. Analyses were performed with the use of SPSS
version 17 (SPSS, Chicago, IL, USA) and MatLab version
5 (The MathWorks Inc. Natick, MA, USA), software.
Of the 97 elderly who underwent the renal function
studies, two were excluded due to incorrect technical
procedures for the iohexol determination, resulting in a
sample of 95 participants for the analyses. The
characteristics of the participants are shown in Table 2. The
average age of the sample was 85 years, ranging from 80
to 97 years, 70% were women, 93% Caucasians.
Mean(±SD) iohexol clearance (mGFR) was 55 ± 15 ml/
min/1.73 m2. Fifty-nine percent had mGFR <60 ml/min/
1.73 m2 (Table 2). A higher percentage of participants
had eGFR < 60 ml/min/1.73 m2 with the BIS_cr-cys
equation and a lower percentage with the MDRD
equation. Performance of the equations is summarized in
Table 2, and in the boxplots shown in Figure 1. In the
overall group, the MDRD and the CKD-Epi_cr equations
overestimated and all the others underestimated GFR.
The least biased equation was the CKD-Epi_cr, whereas
the BIS_cr-cys and the CKD-Epi_cys were the most
biased. The most precise equations were the BIS_cr-cys,
the BIS_cr and the CKD-EPI_cr-cys. In the analysis of
accuracy (CRMSE) the equations with best performances
were the CKD-Epi_cr-cys, the BIS_cr and the
CKDEpi_cr. The most accurate equations (greater percentage
of estimates within 30% of mGFR) were the CKD_cr-cys
and the BIS_cr-cys (85% and 83%, respectively); and
significantly inferior results were observed with the
CKDEpi_cys, the MDRD and the CKD-Epi_cr equations.
Similar trends were in general observed when these
analyses were repeated according to mGFR < or ≥60 ml/
min/1.73 m2 (Table 3). However, the BIS equations and
the CKD-Epi_cr-cys had less bias, and the MDRD was
more biased when mGFR was <60 ml/min/1.73 m2. The
opposite was observed with mGFR ≥60 ml/min/1.73 m .
The best performances combining bias and precision
(CRMSE) were obtained with both BIS equations and
the CKD_cr-cys when GFR was <60 ml/min/1.73 m ;
and with the CKD-Epi_cr-cys and the CKD-Epi_cr in
the higher GFR subgroup. As for the accuracy within
30% the best performances were obtained with the
BIS_cr-cys and the CKD-Epi_cr-cys equations at lower
GFR levels; and the CKD-Epi_cr-cys and the
CKDEpi_cr at when GFR was ≥60 ml/min/1.73 m2 (92% and
90%, respectively). The BIS_cr equation had a more
stable performance in the two investigated GFR
subgroups than the other creatinine-based equations and
showed better accuracy at decreased mGFR levels.
The performance of the equations in relation to the
classification of the thresholds of mGFR was assessed by
the area under the ROC curve (Table 4). The equations
with the best sensitivity (correct identification of individuals
with mGFR <60 ml/min/1.73 m ) were the BIS_cr-cys,
BIS_cr, and the CKD-Epi_cys; and the best specificity was
achieved with the MDRD and the CKD-Epi_cr equations.
The greatest area under the ROC curve was observed with
the CKD-Epi_cr-cys and the BIS_cr-cys equations (0.88)
followed by the CKD-Epi_cys (0.87) equation. Although the
area under the curve of the CKD-Epi_cr-cys was greater
compared with the MDRD and the CKD-Epi_cr equations,
the differences were not significant (p = 0.06 and p = 0.09,
respectively). Considering the cutoff point of 45 ml/min/
1.73 m2 the greatest area under the curve was again
obtained with the CKD-Epi_cr-cys (0.81) equation.
Values are expressed in ml/min/1.73 m2, except for accuracy where values are % of subjects. In parenthesis are 95% confidence intervals.
mGFR: glomerular filtration rate measured by iohexol clearance.
CRMSE: combined root mean square error = the square root of [(mean difference between estimated and measured GFR)2 + (SD of the difference)2].
*p < 0.01 compared with mGFR.
ap < 0.01, bp < 0.05 compared with the CKD-Epi creatinine-cystatin C equation.
cp < 0.01, dp < 0.05 compared with the BIS creatinine-cystatin C equation.
ep < 0.01, fp < 0.05 compared with the BIS creatinine equation.
We then assessed the use of the CKD-Epi_cr-cys
combined equation for the reclassification of participants
as having a measured GFR < or ≥60 ml/min/1.73 m2 on
the basis of the other equations (Additional file 1: Table S1).
In general the CKD-Epi_cr-cys equation more often
correctly reclassified those with mGFR ≥60 ml/min/
1.73 m2, but the net reclassification index was not
statistically significant with the CKD-Epi_cr-cys equation as
compared with the CKD-Epi_cys (NRI = 6.7, p = 0.38),
BIS_cr (NRI = 12.7, p = 0.18) or the BIS_cr-cys (NRI =
15.9, p = 0.08) equations. Compared with the CKD-Epi_cr,
reclassification with the combined CKD-Epi_cr-cys
equation was not favorable and was more often incorrect in
those with a mGFR ≥60 ml/min/1.73 m2 (NRI = −11.4,
p = 0.30). In a similar analyses for reclassification based on a
mGFR threshold of 45 ml/min/1.73 m2 the CKD-Epi_cr-cys
equation tended to improve classification compared with
all other equations; the NRI was statistically significant
compared with the BIS_cr (NRI = 15.8, p = 0.04), marginally
significant compared with the CKD-EPI_cr (NRI = 20.6,
Values for the area under the curve are ± SE.
tph-veacluuet-soffofrvathlueecoofm6p0amrislo/mnsino/1f.t7h3emar2e: a under the curve for the equations at
*p < 0.06; †p = 0.09 compared with CKD-Epi creatinine-cystatin C.
p-values for the comparisons of the area under the curve for the equations at
the cut-off value of 45 ml/min/1.73 m2: §p = 0.05; ‡p = 0.06, ¶p = 0.08 compared
with CKD-Epi creatinine-cystatin C.
p = 0.08), and not significant versus the CKD-Epi_cys
(NRI = 9.7, p = 0.20), and the BIS_cr-cys equation (NRI =
2.9, p = 0.16) (Additional file 2: Table S2).
Accurate assessment of GFR is essential for the
interpretation of clinical and laboratory abnormalities that
may indicate CKD, for drug dosing, for diagnosis,
management of CKD as well as establishing prognosis.
The definition of CKD in the elderly has been a matter
of debate. It has been questioned the appropriateness of
the arbitrary cutoff value of eGFR < 60 ml/min/1.73 m2
for the definition of CKD, without some adjustment for
the normal gender-specific decline in eGFR with aging
]. Although a reduction in GFR to <60 ml/min/
1.73 m2 has been associated with worse outcomes, a
lower cutoff of 45 ml/min/1.73 m2 has been proposed
for the elderly [
In the present study conducted in a sample of
community-dwelling octogenarians and nonagenarians,
we found a mean mGFR of 55 ml/min/1.73 m2, 59% of
whom with values <60 ml/min/1.73 m2. The percentage
of patients with eGFR <60 ml/min/1.73 m2 varied widely
from 53% to 86% depending on the estimating equation
used, suggesting caution in the interpretation of the
Most studies estimating GFR in the elderly have used
equations based on serum creatinine; nevertheless, it is
well known that the decreased muscle mass frequently
occurring in the elderly affects creatinine generation and
influences estimates [
]. Inflammation, malnutrition
and loss of muscle bulk associated with chronic diseases
can further accentuate muscle metabolic abnormalities and
influence the value of creatinine-based equations [
Among the creatinine based equations assessed, the
MDRD equation overestimates mGFR and its use will
decrease the diagnosis of CKD. It tended to be less
accurate than the CKD-Epi_cr and the BIS_cr equations
as previously reported [
], especially at eGFR
>60 ml/min/1.73 m2 compared with the CKD-Epi_cr
and at lower GFRs compared with the BIS_cr. It should
be noted that the MDRD study population is
considerably younger and does not include persons older than
70 years, but the CKD-Epi Collaboration study does
include older adults; and the CKD-Epi_cr equation was
developed in a cohort with better kidney function
(68 ml/min/1.73 m2). Some studies have reported that
both the MDRD and, to a lesser extent, the CKD-Epi_cr
equations overestimate GFR in individuals older than
70 years with CKD [
]. The BIS_cr equation
underestimated mGFR and more often than the others
incorrectly classified participants as having CKD. Overall
the accuracy of the BIS_cr was slightly superior to the
CKD-Epi_cr equation, but the difference was not as
favorable to the former as described in the BIS original
validation cohort . Our results suggest that the BIS_cr
equation outperforms the MDRD and the CKD-Epi_cr
equations in terms of precision. Notably, the BIS_cr
equation had a superior performance compared with the other
two at decreased GFR levels, mainly due to increased
precision; but yielded substantial underestimation in participants
with mGFR ≥60 ml/min/1.73 m . The marker used for
measuring clearance and the methodology of measuring
creatinine were the same in our study and in the BIS and
do not seem to explain these differences. One could
speculate that residual differences in creatinine standardization
(eventually more influential at low creatinine levels) or
differences in ethnicity, since the BIS participants were
white Europeans, while our population is less homogeneous
in terms of ethnicity, might play a role in the distinct
findings. In fact, ethnicity influences creatinine-based GFR
estimating equations. Flamant et al. [
] have suggested that
eGFR (CKD-Epi_cr equation) with the African American
race-ethnicity correction factor overestimates GFR in
African Europeans. Recently Koppe et al. [
] reported that
the BIS_cr equation in comparison the MDRD and the
CKD-Epi_cr was the most reliable to assess renal function
in elderly patients, especially in those with CKD stages 1–3;
however, no superior results were observed in those aged
>80 years. The participants in that study had several
differences compared with ours: there was a smaller number
of very old individuals, a higher proportion of men, all
subjects were white and had suspected or established renal
Serum cystatin C appears to be less dependent upon
muscle mass, less sensitive to metabolic and extra-renal
factors than creatinine in the elderly and equations
based on this marker seemed to be promising [
contrast to early reports, there are non-GFR
determinants of cystatin C serum levels [
]. The advantage of
the recently described cystatin C-based equations over
the creatinine-based equations is that it is less subject to
the effects of age, sex and race [
]. Bevc et al.
reported that in adult patients aged >65 years, a simple
cystatin C formula (100/serum cystatin C) was reliable
and comparable to creatinine-based formulas . We
observed that GFR estimates based on equations that
use cystatin C as the sole filtration marker have not been
more accurate than creatinine based estimates, as also
reported in younger adult and elderly populations
]. In fact, our data show that the CKD-Epi_cys
performance tended to be inferior to the CKD-EPI_cr
and this trend was more evident when compared with
the BIS_cr equation. Equivalent performance of the
CKD-Epi_cys and CKD-Epi_cr equations [
], and slightly
better accuracy of the BIS_cr compared with the
CKDEpi_cys in the elderly have been reported [
suggests that unmeasured and largely unknown non-GFR
determinants of cystatin C serum levels are similar in
magnitude to those of creatinine.
The combined creatinine-cystatin C equations
(CKDEpi_cr-cys and BIS_cr-cys) had the greatest accuracy
(values within 30%), although both (more markedly the
BIS_cr-cys) underestimated GFR. As for the
classification of CKD, the combined equations performed better
than the others, followed by the CKD-Epi_cys. The ROC
analyses revealed that the combined equations and the
cystatin alone equation had the greatest areas under the
curve at GFR =60 ml/min/1.73 m2 threshold. There was
no significant difference in CKD classification comparing
the equations using creatinine alone (CKD-Epi_cr and
BIS_cr). The CKD-Epi_cr-cys combined equation tended
to improve the reclassification of patients as having
CKD compared with most of the others.
To explain the tendency of best performance with the
equations that combine creatinine and cystatin C it has
been proposed that errors due to the non-GFR
determinants of creatinine and cystatin C are independent and
smaller in an equation that uses both markers than in an
equation that uses only one marker. Possible reasons for
the continued imprecision are the residual contribution
of non-GFR determinants of each marker, as well as
physiologic variation in GFR and error in measurement
Our study suggests that cystatin C should not replace
creatinine in clinical practice; however, the combination
of creatinine and cystatin C appears to provide more
accurate GFR estimates in the very elderly. In adults,
Inker et al. [
] showed that the combined
creatininecystatin C equation could be used as a confirmatory test
for the diagnosis of CKD. Kilbride et al. [
] have shown
that the CKD-Epi equations (based on creatinine, cystatin
C or both) have equivalent and satisfactory performance
in older people; and worked as well as compared with
younger populations. In fact, the accuracy that we
obtained with the CKD-Epi_cr-cys equation (85%) was
identical to that reported by those authors in individuals
older than 80 years [
]. Schaeffner et al. [
] have recently
suggested that the BIS_cr-cys combined equation should
be used to estimate GFR in the elderly. However, their
data need to be validated in external validation samples.
Typically, an equation performs best in the data set from
which it was derived. Our findings did not confirm the
overall superiority of the BIS equations performance in
relation to the CKD-Epi equations. It is possible that some
characteristics of our study sample, the greater
participation of women, the greater ethnic miscegenation, the
higher mean age, the lower mean body surface area, the
lower mean mGFR and the smaller sample size may have
influenced these discrepant findings. In addition, the
comparisons reported in the BIS study [
] were not with the
most recently developed CKD-Epi_cys and CKD-Epi_cr-cys
] as we did.
The strengths of this study are the evaluation of mGFR
using a precise method in a sample of very elderly
persons, and the comparison and validation of several
recently proposed equations to estimate GFR based on
creatinine and cystatin C. None of the previous studies
have specifically assessed the comparison of these new
equations in this age group. The limitations of the study
are: the relatively small sample of octogenarians and
nonagenarians, owing to the difficulties in recruiting
individuals in this age group for a lengthy procedure
such as the GFR determination, resulted in a lower
power to detect statistically significant differences; the
number of participants and the recruitment methods
used restrict the representativeness of the population as
a whole; errors in the measured GFR may account for
some imprecision and limiting interpretation of the
analyses. The cystatin C assay was not calibrated against the
international certified reference material ERM-DA471/
IFCC prior the use in GFR estimating equations; this
lack of standardization may have caused imprecision in
the estimates. Therefore, these results should be
interpreted with caution. Although the most reliable
method to analyze plasma iohexol is with high
performance liquid chromatography, previous studies have
shown that measurements by capillary electrophoresis had
an excellent correlation with those by high performance
liquid chromatography [
]. For reasons of feasibility and
precision in an elderly cohort with a mean age of
85.3 years, we measured only plasma clearance and not
urine clearance. Early studies suggest that tubular
secretion of iohexol is negligible [
] and there is no convincing
evidence that iohexol clearance is different than other
reference GFR procedures compared to urinary inulin
The combined CKD-Epi_cr-cys and BIS_cr-cys equations,
although underestimating GFR, showed better accuracy
than other equations using creatinine or cystatin C alone
in very elderly persons. The CKD-Epi_cr-cys equation
appears to be advantageous in CKD classification. If
cystatin C is not available, both the BIS_cr equation and
the CKD-Epi_cr equation could be used, although at
mGFR < 60 ml/min/1.73 m2, the BIS_cr equation seems to
be the best alternative.
Additional file 1: Table S1. Reclassification of the participants with the use
of the CKD-Epi creatinine-cystatin C equation versus: the CKD-Epi creatinine,
the CKD-Epi cystatin C, the BIS creatinine or the BIS creatinine-cystatin C for
estimated GFR, according to the cut-off value of mGFR = 60 ml/min/1.73 m .
Additional file 2: Table S2. Reclassification of the participants with the use
of the CKD-Epi creatinine-cystatin C equation versus: the CKD-Epi creatinine,
the CKD-Epi cystatin C, the BIS creatinine or the BIS creatinine-cystatin C for
estimated GFR, according to the cut-off value of mGFR = 45 ml/min/1.73 m .
CKD: Chronic kidney disease; GFR: Glomerular filtration rate; mGFR: Measured
glomerular filtration rate; eGFR: Estimated glomerular filtration rate; MDRD: The
modification diet in renal disease; CKD-Epi_cr: The Chronic kidney disease
epidemiology creatinine; CKD-Epi_cys: The chronic kidney disease epidemiology
cystatin C; CKD-Epi_cr-cys: The chronic kidney disease epidemiology
creatininecystatin C; BIS_cr: The berlin initiative study creatinine; BIS_cr-cys: The berlin
initiative study creatinine-cystatin C.
The authors declare that they have no competing interest.
MBL participated in the conception, design of the study, data collection, data
analysis, data interpretation, and approved the manuscript. LQA participated
in the conception, design of the study, data collection, data interpretation,
and approved the manuscript. MTP participated in the data analysis,
laboratory parameters analysis, data interpretation, and approved the
manuscript. SKN participated in the data analysis, laboratory parameters
analysis, data interpretation, and approved the manuscript. GMK participated
in the conception of the study, data analysis, data interpretation, provided
important intellectual content, critically revised the manuscript, and
approved the manuscript. MSC participated in the conception, design of the
study, data collection, data interpretation, provided important intellectual
content, critically revised the manuscript, and approved the manuscript. RCS
participated in the conception, design of the study, data analysis, data
interpretation, provided important intellectual content, and has written the
manuscript. All authors read and approved the final manuscript.
This research was supported by the Brazilian Research Council Grant
No. 472115/2010-3. Ricardo Sesso received a research grant from the Brazilian
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