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25
| 1 | 2006
Klinisk Biokemi i Norden
of patients already diagnosed S-Creatinine may
be sufficient. If Creatinine is to be used for the
detection of a slight reduction in the renal function
(GFR 70 – 90 mL/min) the sensitivity and specifi-
city of this analysis is insufficient. It can be seen
that around the critical limits between normal and
reduced GFR, Cystatin C scores better in all aspects
e.g. true positives, true negatives, false positives,
false negatives, sensitivity, specificity and predicted
values of positive and negative tests. Though these
performance characteristics are based on low num-
bers of observations, the average figures on these
data are probably fairly correct (Table 1).
Summarising, we can agree that Cystatin C is a
better marker for renal function than S-Creatinine,
has a better analytical quality, higher sensitivity for
mild to moderately reduced renal function and is
useful especially for children, patients with paraple-
gia or others where collection of urine is difficult.
So, the discussion on use of Cystatin C should focus
on whether the Cystatin C is good
enough
from a
clinical point of view.
It is claimed that Cystatin C based equation for
GFR may replace the simplified MDRD prediction
equation for adults (21). It is also claimed that
Cystatin C is able to measure a reduction in renal
function earlier than S-Creatinine does (22), but
the question is whether this is needed in clinical
practice and what impact this earlier knowledge
will have on treatment of the patients. Is Cystatin C
sensitivity and specificity sufficient if the analysis is
going to be used for detection of a mild reduction in
GFR? The results show considerable number of false
negatives and false positives.
The economical aspect should be considered as
well. The approximate price for a single analysis of
S-Creatinin with the Jaffé method is 0.02 Euro and
with an enzymatic method 0.40 Euro. The price for
Cystatin C is 2 Euro. For a laboratory management
it is always important to evaluate new tests care-
fully. This evaluation should not only focus on the
technical and medical criteria but also consider the
cost, totally and in relation to the benefit.
As long as it has not been defined, what the
purpose of the GFR is and how good the analytical
performance should be for that purpose, we can
not recommend the use of Cystatin C as a general
substitute for creatinine concentration or creatinine
clearance measurement. It should still be taken into
account that even though the Cystatin C is better for
detecting renal insufficiency and evaluating GFR, it
is not as good as a GFR measurement in itself.
The question whether the marginal improvement
in GFR estimation by Cystatin C measurements is
clinically important and worth the money, is still
open.
Acknowledgement
We thank Dr A. Larsson for supplying us with his
research results.
Fig. 2.
Comparison between GFR
calculated from S-Creatinine
and S-Cystatin as deviation
from GFR measured and SD
in different ranges of GFR:
GFR calculated minus measured
GFR measured, mL/min *1.73m
2
(Fortsætter side 26)
