Abstract :
[en] Serum creatinine (SCr) is the main variable for
estimating glomerular filtration rate (GFR). Due to interassay
differences, the prevalence of chronic kidney disease
(CKD) varies according to the assay used, and calibration
standardization is necessary. For SCr, isotope dilution
mass spectrometry (IDMS) is the gold standard. Systematic
differences are observed between Jaffe and enzymatic
methods. Manufacturers subtract 0.30 mg/dl from Jaffe
results to match enzymatic results (‘compensated Jaffe
method’). The analytical performance of enzymatic
methods is superior to that of Jaffe methods. In the original
Modification of Diet in Renal Disease (MDRD) equation,
SCr was measured by a Jaffe Beckman assay, which was
later recalibrated. A limitation of this equation was an
underestimation of GFR in the high range. The Chronic
Kidney Disease Epidemiology (CKD-EPI) consortium
proposed an equation using calibrated and IDMS traceable
SCr. The gain in performance was due to improving the
bias whereas the precision was comparable. The CKD-EPI
equation performs better at high GFR levels (GFR[60 ml/
min/1.73 m2). Analytical limitations have led to the recommendation
to give a grade ([60 ml/min/1.73 m2) rather
than an absolute value with the MDRD equation. By using
both enzymatic and calibrated methods, this cutoff-grade
could be increased to 90 ml/min/1.73 m2 (with MDRD)
and 120 ml/min/1.73 m2 (with CKD-EPI). The superiority
of the CKD-EPI equation over MDRD is analytical, but
the precision gain is limited. IDMS traceable enzymatic
methods have been used in the development of the Lund–
Malmo¨ (in CKD populations) and Berlin Initiative Study
equations (in the elderly). The analytical errors for cystatin
C are grossly comparable to issues found with SCr.
Standardization is available since 2011. A reference
method for cystatin C is still lacking. Equations based on
standardized cystatin C or cystatin C and creatinine have
been proposed. The better performance of these equations
(especially the combined CKD-EPI equation) has been
demonstrated.
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