*Introduction*

To be able to correctly evaluate the kidney function of a patient,
and to correctly dose a medicine or a contrast agent mainly excreted via the kidneys,
knowledge of the glomerular filtration rate (GFR) of the patient is required.
GFR might be determined using invasive procedures,
*e.g.*, by measuring the renal clearance of inulin, ^{51}Cr-EDTA
or iothalamate, or the plasma clearance of iohexol or
^{51}Cr-EDTA^{1,2}.
Such procedures are expensive, slow and not
completely free of risks for the patient.
Therefore, cystatin C- or creatinine-based estimating equations for GFR have been suggested
with the result being designated as eGFR_{cystatin C} or eGFR_{creatinine},
respectively.
This site suggests a simple strategy to obtain the best estimation of GFR by use of
equations based upon cystatin C- or creatinine-assays
adjusted to international calibrators^{2-8}.
The strategy also allows diagnosis of shrunken pore syndrome with its high morbidity
and mortality^{9-13} due to that the filtration of 5-40 kDa molecules
at this condition is reduced compared to that of small molecules, like water and
creatinine, and the concomitant accumulation of atherosclerosis-promoting
proteins^{13}.
Specifically, the use of the cystatin C-based CAPA-^{6} and the
creatinine-based LM-Rev^{7,8}-equations are recommended.
The mean of the two estimated GFR-values is generally the best estimate for adults
and the reliability of this estimate can be tested by comparison of the two estimates^{3-5}.
For children, the best estimate is obtained from the cystatin C-based estimating
equation alone^{6}.

*Calculating robust estimates of relative GFR*

The level of cystatin C in plasma/serum is relatively independent of
body composition and simple cystatin C-based equations for GFR, containing
only the cystatin C-concentration and the age of the patient as parameters,
are therefore useful for both children and adults^{6,9}.

The creatinine level in plasma/serum is, in addition to GFR, strongly
influenced by a ^{3,7,8}.

For most adult patient populations, the mean of the two GFR estimates,
(eGFR_{cystatin C} + eGFR_{creatinine})/2,
is the best estimate and its reliability can
be tested by comparison of the two separate estimates^{3-5}. For children, the
best estimate is obtained from the cystatin C-based estimating equation^{6}.