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20
| 1 | 2006
Klinisk Biokemi i Norden
Abstract
Glomerular Filtration Rate (GFR) is
usually estimated from creatinine
concentration measurements with
or without use of different algo-
rithms or from creatinine clearan-
ce. These estimates are weakened
by the many factors influencing
creatinine concentration such as
age, sex, body mass, diet etc.
Cystatin C is proposed as a new marker, as it cor-
relates better to GFR than creatinine but criteria for
sensitivity, specificity, and predictive values of posi-
tive and negative tests should be formulated before
Cystatin C is introduced.
The analysis is expensive as compared to creatinine,
so we tried to evaluate the quality of Cystatin based
GFR- estimate and the benefit of introducing it.
We conclude that Cystatin C is a better marker than
creatinine, especially for detection of slightly reduced
kidney function, but the improvement when using it
as a general substitute for creatinine or creatinine
clearance measurement, seems to be marginal.
Introduction
The widely accepted gold standard for measurement
of true Glomerular Filtration Rate (GFR) is the uri-
nary clearance of exogenous substances such as
51Cr-EDTA, ioxehol or inulin. However, because
these tests are expensive and difficult the GFR is
usually estimated from the serum concentration of
creatinine or from creatinine clearance. The main
arguments against GFR estimated from creatinine
clearance are inconvenience and imprecision in col-
lection of urine during 24-h period and the variation
in S-Creatinine due to age, body mass, sex and diet
(1, 2). Other factors that reduce the value of creati-
nine concentrations as a GFR estimate are tubular
creatinine secretion and sensitivity of the analytical
methods to interfering substances in plasma.
Several formulas have been developed to con-
vert S-creatinine concentration to GFR, taking into
account personal demographics such as age, gender
and body mass to achieve closer agreement with
true GFR (3, 4, 5). All these formulas give better cor-
relation to true GFR than creatinine concentration
alone.
Recently an analysis for S-Cystatin C as a new
marker for GFR has been introduced and several aut-
hors (6 – 13) plead for the superiority of Cystatin C
compared to creatinine for estimation of glomerular
filtration rate. Factors such as age, sex, diet, inflam-
mation and body mass do not influence S-Cystatin C
concentration. S-Cystatin C shows better correlation
to GFR (9, 12) than S-Creatinine or creatinine clea-
rance (14 – 17). There is no need for a 24-h urine
collection, which eliminates an important source of
imprecision and simplifies the procedure both for the
patient and for the laboratory.
Recently Larsson et al. (12) have determined the
correlation between true GFR and S-Creatinine or S-
Cystatin C and they concluded that S-Cystatin C
1. Can be used as a new approach to prediction of
GFR as Cystatin C correlation is better than the
traditional creatinine clearance and S-Creatinine
(R2 = 0.91 against R2 = 0.84)
2. Is a better predictor of kidney disease
3. Results in mg/L can be converted to GFR in mh/min
4. Can replace creatinine clearance
5. Reduce number of GFR determinations
6. Is neutral in expenses as compared to creatinine
clearance
S-Cystatin:
A better marker for glomerular filtration rate
than S-Creatinine but is the difference clinically important
and worth the money?
Marta Stahl and Ivan Brandslund
Dept. of Clinical Biochemistry, Vejle County Hospital
E-post: marsta@vgs.vejleamt.dk
