Article

Calibration of the Siemens Cystatin C Immunoassay Has Changed over Time

Clinical ChemistryUppsala UniversityUppsala, Sweden.
Clinical Chemistry (Impact Factor: 7.91). 03/2011; 57(5):777-8. DOI: 10.1373/clinchem.2010.159848
Source: PubMed

Full-text

Available from: Anders Larsson, Feb 02, 2015
serum free light chain assay. Clin Chem Lab Med
2010;48:73–9.
6. Davern S, Tang LX, Williams TK, Macy SD, Wall JS,
Weiss DT, Solomon A. Immunodiagnostic capabilities
of anti-free immunoglobulin light chain monoclonal
antibodies. Am J Clin Pathol 2008;130:702–11.
Stanley S. Levinson
2,3*
2
Department of Veterans Affairs
Pathology and Laboratory Medicine Service
Robley Rex Medical Center
Louisville, KY
3
Department of Pathology and
Laboratory Medicine
University of Louisville
Louisville, KY
* Address correspondence to the author at:
Laboratory Service
Department of Veterans Affairs
Medical Center
800 Zorn Ave.
Louisville, KY 40206
Fax 502-287-6265
E-mail levinson@louisville.edu
Previously published online at
DOI: 10.1373/clinchem.2010.158774
Calibration of the Siemens
Cystatin C Immunoassay Has
Changed over Time
To the Editor:
The Siemens cystatin C immuno-
assay has been widely used in clin-
ical research, particularly in the
US. In recent years, however, the
results obtained with the method
appear to have changed.
The glomerular filtration rate
(GFR) is generally accepted as the
best overall indicator of kidney
function and is an important mea-
sure for assessing kidney disease.
Several studies have shown cystatin
C to be superior to creatinine for
estimation of the GFR (1 ), which is
usually expressed as the relative GFR
[in units of mL min
1
(1.73
m
2
)
1
]. This practice has led to the
development of formulas to con-
vert cystatin C measurements in
milligrams per liter to a calcu-
lated GFR in these units, without
the need for demographic coeffi-
cients (2, 3 ). The formulas were
developed from studies that
compared cystatin C concentra-
tions with measured GFRs by
using such exogenous markers
as iohexol, diethylenetriamine
pentaacetic acid or
51
Cr-EDTA
clearance (4 ).
We were concerned that the
calibration of the Siemens cystatin
C method had changed during the
last 5 years, because we noted that
thecystatinCconcentrationsofpar-
ticipants in a longitudinal cohort
improved substantially over time.
The study investigators considered
these results implausible because the
GFR is known to decrease with age.
The aim of the present study was to
determine whether the Siemens
cystatin C method has changed its
calibration during the last 5 years
and, if so, to quantify the magni-
tude of the change.
We used our laboratory’s cur-
rent routine method from Gentian
as a comparative method. To ensure
that the comparative method had
been stable, we verified that each lot
of the Gentian reagent produced re-
sults within 3% of the originally as-
signed values for 10 patient serum
pools prepared in 2005 and stored at
70 °C. Freshly collected and lith-
ium heparin–treated samples ob-
tained from routine cystatin C re-
quests were then used to compare
the 2 cystatin C methods. New sets of
patient samples were used for each
comparison between 2006 and 2010
(Table 1).
Plasma cystatin C was analyzed
on an Architect ci8200
analyzer
(Abbott Diagnostics) with reagents
and calibrator from Gentian and on
a BN ProSpec analyzer (Siemens
Healthcare Diagnostics) with re-
agents and calibrator from Siemens.
A Deming regression analysis
showed no significant difference
between the 2 methods in 2006
(Table 1); however, both the y in-
tercept and the slope differed sig-
nificantly from 2007 onward. In
2005, the Gentian (x) and Siemens
(y) methods showed strong agree-
ment (y 1.0507x 0.0369 mg/L;
r
2
0.994; n 95). In March
2006, the 2 methods also gave com-
parable results (y 1.005x 0.026
mg/L; r
2
0.995; n 92). In De-
cember 2007, the slope of the linear
regression analysis had decreased
by approximately 7% while main-
taining a high r
2
value (y
Table 1. Comparison of the Siemens and Gentian cystatin C methods by
Deming regression analysis.
Bias 95% CI SE P
2006
y
Intercept, mg/L 0.03 0.01 to 0.07 0.021 0.1341
Slope 0.99 0.97–1.02 0.014 0.5623
2007
y
Intercept, mg/L 0.06 0.04–0.08 0.009 0.001
Slope 1.08 1.06–1.09 0.007 0.001
2008
y
Intercept, mg/L 0.12 0.09–0.15 0.016 0.001
Slope 1.17 1.15–1.19 0.011 0.001
2010
y
Intercept, mg/L 0.13 0.11–0.15 0.010 0.001
Slope 1.14 1.13–1.16 0.007 0.001
Letters to the Editor
Clinical Chemistry 57:5 (2011) 777
Page 1
0.928x 0.054 mg/L; r
2
0.997;
n 180). By October 2008, the
bias was larger, and the slope was
substantially different, despite a
high correlation coefficient (y
0.855x 0.099 mg/L; r
2
0.997;
n 88). These results indicated a
15% decrease in the results ob-
tained with the Siemens method. A
retest of the same Siemens re-
agent lot in 2010 with the patient
pools showed a similar slope
(0.864). A final comparison in
May 2010 showed a persistent
bias and a lower slope compared
with our comparative method
(y 0.8725x 0.1133 mg/L;
r
2
0.998; n 112).
The observed change in cali-
bration of the Siemens method in-
creased GFR values. For instance, a
cystatin C concentration corre-
sponding to a GFR of 60
mL min
1
(1.73 m
2
)
1
in 2005
increased to approximately 80
mL min
1
(1.73 m
2
)
1
in 2010.
Owing to the nonlinear rela-
tionship between cystatin C and
the estimated GFR, the bias in cys-
tatin C concentrations in patients
with a normal kidney function and
those with a moderately decreased
kidney function had the greatest ef-
fect on the estimated GFR. There-
fore, it is essential that the calibra-
tion of the cystatin C method be
consistent, because small changes in
cystatin C will introduce a large bias
in GFR estimates. The Siemens di-
rect carbohydrate-deficient transfer-
rin method was reported to have a
downward shift in calibration be-
tween 2006 and 2007 (5 ). Both Sie-
mens methods are based on
particle-enhanced nephelometry,
and the downward shifts in cali-
bration for the 2 methods coin-
cided in time and approximate
magnitude.
The results of this study indi-
cate that a downward shift in cali-
bration for the Siemens cystatin C
method occurred between March
2006 and December 2008. Cystatin
C– based equations to estimate
GFR that were derived from results
obtained from older lots of reagent
and calibrator cannot be used with
the current Siemens method. The
results emphasize the need for an in-
ternational cystatin C reference ma-
terial, which has recently been devel-
oped by the Institute for Reference
Materials and Measurements.
Editor’s Note: Siemens, when
contacted, declined to comment
on this letter.
Author Contributions: All authors con-
firmed they have contributed to the intellec-
tual content of this paper and have met the
following 3 requirements: (a) significant con-
tributions to the conception and design, ac-
quisition of data, or analysis and interpreta-
tion of data; (b) drafting or revising the article
for intellectual content; and (c) final approval
of the published article.
Authors’ Disclosures or Potential Con-
flicts of Interest: Upon manuscript submis-
sion, all authors completed the Disclosures of
Potential Conflict of Interest form. Potential
conflicts of interest:
Employment or Leadership: None
declared.
Consultant or Advisory Role: None
declared.
Stock Ownership: None declared.
Honoraria: None declared.
Research Funding: Uppsala University
Hospital Research Fund.
Expert Testimony: None declared.
Role of Sponsor: The funding organiza-
tions played no role in the design of study,
choice of enrolled patients, review and in-
terpretation of data, or preparation or ap-
proval of manuscript.
References
1. Dharnidharka VR, Kwon C, Stevens G. Serum
cystatin C is superior to serum creatinine as a
marker of kidney function: a meta-analysis. Am J
Kidney Dis 2002;40:221– 6.
2. Flodin M, Jonsson AS, Hansson LO, Danielsson
LA, Larsson A. Evaluation of Gentian cystatin C
reagent on Abbott Ci8200 and calculation of
glomerular filtration rate expressed in mL/min/
1.73 m
2
from the cystatin C values in mg/L.
Scand J Clin Lab Invest 2007;67:560–7.
3. Larsson A, Malm J, Grubb A, Hansson LO. Cal-
culation of glomerular filtration rate expressed in
mL/min from plasma cystatin C values in mg/L.
Scand J Clin Lab Invest 2004;64:25–30.
4. Shlipak MG, Katz R, Kestenbaum B, Fried LF,
Newman AB, Siscovick DS, et al. Rate of kidney
function decline in older adults: a comparison
using creatinine and cystatin C. Am J Nephrol
2009;30:171–8.
5. Helander A, Nordin G. Insufficient standardiza-
tion of a direct carbohydrate-deficient transferrin
immunoassay. Clin Chem 2008;54:1090 –2.
Anders Larsson
1*
Lars-Olof Hansson
1
Mats Flodin
1
Ronit Katz
2
Michael G. Shlipak
3,4
1
Clinical Chemistry
Uppsala University
Uppsala, Sweden
2
University of Washington
Seattle, WA
3
San Francisco VA Medical Center
San Francisco, CA
4
University of California, San Francisco
San Francisco, CA
* Address correspondence to this author at:
Department of Medical Sciences
University Hospital
S-751 85 Uppsala, Sweden
Fax 46-18-552562
E-mail: anders.larsson@akademiska.se
Previously published online at
DOI: 10.1373/clinchem.2010.159848
No Effect of Anticoagulant on
Hb A
1c
Analysis by the IFCC
Reference Procedure
To the Editor:
The use of glycohemoglobin (Hb
A
1c
)
1
as a diagnostic marker has
been limited because of the lack of
a standardized method. Hb A
1c
es-
timation with the commercially
available instruments is based on
different analytical and immuno-
1
Nonstandard abbreviations: Hb A
1c
, glycohemo-
globin; ICSH, International Council for Standard-
ization in Haematology; R
conc
, calibration curve of
glycated hexapeptide percentages in a mixture of
glycated and nonglycated hexapeptide calibrants;
Sx, Hb A
1c
signal; Sy, Hb A
0
signal; R
signal
, ratio of
Hb A
1c
signal to the Hb A
0
signal.
Letters to the Editor
778 Clinical Chemistry 57:5 (2011)
Page 2
  • Source
    • "CysC standardization is required to improve large discrepancies between the nephelometric and turbidimetric methods, but also differences observed between different assays using the same turbidimetric methods [9,11,13,14,[20][21][22]32] . This will also prevent that, within a single method, calibrations change over time, as it has been described with the PENIA method from Siemens [33] . Just as for creatinine , the standardization of cysC measurement was a prerequisite before proposing cysC-based equations to estimate GFR in clinical practice [3,19]. "
    [Show abstract] [Hide abstract] ABSTRACT: Background: Cystatin C is increasingly used in glomerular filtration rate (GFR) estimation equations. The dependence of cystatin C results upon the analytical method has been a major source of controversy. Methods: Cystatin C was measured with non-standardized turbidimetric Roche Generation 1 and standardized nephelometric Siemens assays in 3666 and additionally with standardized Roche Generation 2 and Siemens in 567 blood samples of the Berlin Initiative Study. Cystatin C-based GFR was assessed with CKD-EPIcys (Chronic Kidney Disease Epidemiology) and CAPA (Caucasian, Asian, Pediatric, Adult) equations and the impact of the assays on GFR estimation was determined. Equation performance compared to measured GFR was evaluated. Results: Concordance of Roche Gen2 and Siemens was high with median difference of 0.003±0.13mg/L (limits of agreement: -0.12 to 0.12) and Passing Bablok correlation was essentially perfect. Roche Gen1 assay showed worse concordance with Siemens: median difference was 0.08±0.13mg/L (limits of agreement: -0.18 to 0.34) and correlation was inferior. Mean difference (±SD) of estimated GFRCKD-EPIcys was 0±4mL/min/1.73m² for Gen2 and Siemens compared to -5±8 with Gen1. Performance of GFR estimating equations was not influenced by the choice of Siemens or Gen2 assays. Conclusions: Standardization of Roche Gen2 assay improved accuracy of cystatin C measurement compared to Siemens. It suggests only negligible method bias and results in equal performance of both assays when estimating GFR indicating that successful calibration has led to major progress in cystatin C analysis.
    Full-text · Article · Mar 2016 · Clinica chimica acta; international journal of clinical chemistry
  • Source
    • "However, these equations have been developed by the use of a limited sample of test subjects and have been poorly validated [13,15]. There is also concern because of the lack of standardized calibration for the measurement of the plasma level of cystatin C1718192021. Now, a standardized calibration is available [22] and the CKD-EPI consortium has recently proposed equations (based on cystatin C alone or combined with creatinine) to better estimate GFR [23]. "
    [Show abstract] [Hide abstract] ABSTRACT: Chronic kidney disease (CKD) is a major issue in public health. Its prevalence has been calculated using estimation of glomerular filtration rate (GFR) by the creatinine-based equations developed in the Modified Diet in Renal Disease (MDRD) and Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) study. Recently, new equations based either on cystatin C (CKD-EPI Cys) or both cystatin and creatinine (CKD-EPI mix) have been proposed by the CKD-EPI consortium. The aim of this study was to measure the difference in the prevalence of stage 3 CKD, defined as an estimated GFR less than 60 mL/min/1.73 m2, in a population using these four equations. CKD screening was performed in the Province of Liège, Belgium. On a voluntary basis, people aged over 50 years have been screened. GFR was estimated by the four equations. Stage 3 CKD was defined as a GFR less than 60 mL/min/1.73 m2. The population screened consisted of 4189 people (47% were men, mean age 63 ± 7y). Their mean serum creatinine and plasma cystatin C levels were 0.88 ± 0.21 mg/dL and 0.85 ± 0.17 mg/L, respectively. The prevalence of CKD in this population using the MDRD, the CKD-EPI, the CKD-EPI Cys and the CKD-EPI mix equations was 13%, 9.8%, 4.7% and 5%, respectively. The prevalence of CKD was significantly higher with the creatinine-based (MDRD and the CKD-EPI) equations compared to the new cystatin C-based equations. Prevalence of CKD varies strongly depending on the method used to estimate GFR. Such discrepancies are of importance and must be confirmed and explained by additional studies, notably by studies using GFR measured with a reference method. Trial registration B70720071509
    Full-text · Article · Mar 2013 · BMC Nephrology
  • Source
    • "Larsson ve ark. da longitudinal kohort araştırmasına katılan kronik böbrek yetmezliği hastalarının GFR değerlerinin artan yaş ile azalması beklenirken, son beş yılda artması nedeniyle, Sc'nin kalibrasyonunun değiştiğinden şüphelenmiş, Siemens analizöründe Siemens marka kit ile Architect ci8200 (Abbott Diagnostics ) analizöründe Gentian marka kiti ve her iki analizörü 6 yıl önceden değerleri bilinerek muhafaza edilmiş hasta havuzları ile karşılaştırmış ve 2006-2008 yılları arasında Siemens Sc'nin kalibrasyonunun değiştiğini, bu nedenle Sc ölçümündeki az farkların GFR değerinde büyük hataya sebep olduğunu belirtmiş, Sc'nin stabilizasyonunu değerlendirmek için uluslararası bir referans materyalinin gerekliliğine vurgu yapmıştır [10]. Sonuç olarak tıbbi laboratuvara yeni kurulan bir yöntemin rastgele ve sistematik hata oranlarını ortaya çıkaracak deneylerin yapılarak performansının değerlendirilmesi ve güvenilir, doğru sonuçların elde edilebilmesi "
    [Show abstract] [Hide abstract] ABSTRACT: Objective: In reliable clinical laboratory practice, to use test results for the maximum benefit of patients, the new method in a laboratory should be validated. In this study our purpose is to evaluate performance characteristics of cystatin C immunoturbidimetric method in Roche Cobas Integra 800 analyser, and compare this method with immunonephelometric method in Dade Behring BNII analyser. Methods: Precision, linearity, recovery, interference experiments in Roche Cobas Integra 800 analyser and method comparison experiments were performed for cystatin C. Results: For Roche Cobas Integra 800 instrument, low and high concentrations for withinrun and day to day CV values were 3.97%, 1.32% and 7.24%, 4.16% respectively. In linearity experiment, regression graph equation was found to be y = 0.9817x - 0.0149 and r(2) = 0.99. In recovery experiment, % recovery was found to be 81. In hemolysis interference experiment, interference was detected for the hemoglobin concetrations above 200 mg/dL, Method comparison experiment was performed by analyzing 100 patients serum in both Dade Behring BNII nephelometry and Roche Cobas Integra 800 analyser. Correlation factor was r(2) = 0.95, Deming regression equation was y = 0,98x + 0,22. Conclusion: It was found for turbidimetric method, CV values were higher, % recovery was lower and hemolysis interference was detected at lower concentrations than what was stated in package insert of cystatin C kits. However, Deming regression results indicated turbidimetric and nephelometric methods were compatible.
    Preview · Article · Jan 2013 · Türk Biyokimya Dergisi / Turkish Journal of Biochemistry
Show more