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, 51Cr-EDTA or iothalamate, or the plasma clearance of iohexol or
51Cr-EDTA1,2. 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.
This site suggests a simple strategy to obtain the best estimation of GFR by use of equations based upon cystatin C- and creatinine-assays
adjusted to international calibrators3-7. 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 estimates3-5. For children, the best estimate is obtained from the cystatin C-based estimating equation alone6.
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 adults6.
The creatinine level in plasma/serum is, in addition to GFR, strongly influenced by a person´s muscle mass. Knowledge of the age
and sex of a person allows calculation of the mean muscle mass of a person of that age and sex and is therefore used in addition to
the creatinine level to generate creatinine-based estimating equations for GFR7.
For most adult patient populations, the mean of the two GFR estimates, relative GFRCC+creat, is the best estimate and its reliability can
be tested by comparison of the two separate estimates3-5. For children, the best estimate is obtained from the cystatin C-based estimating equation6.
Optimal evaluation of results3
If, for an adult, the GFR estimated by the cystatin C-based equation agrees with that estimated by the creatinine-based equation, no invasive determination of GFR is
required and the GFR-estimate representing the mean of the two estimates, relative GFRCC+creat, should be used.
If the GFRs estimated by the cystatin C- and creatinine-based equations do not agree, a clinical evaluation of the patient has to be performed.
If the muscle mass of the patient deviates considerably from that of his/her age and sex category (e.g.,
because of paralysis, immobility, anorexia or excessive bodybuilding) or if the patient recently ingested boiled meat or a
medicine affecting the tubular excretion of creatinine, a GFR-estimate based solely upon cystatin C should be used8-12.
If the patient is treated with glucocorticoids orally or intramuscularly his/her synthesis of cystatin C is significantly increased and in this case a GFR-estimate
based solely upon creatinine (+age and sex) should be used13.
For adult patients who do not belong to any of the above-mentioned categories, an invasive determination of GFR might be required.
In hyperthyroidism the cystatin C level will increase and the creatinin level
decrease without corresponding changes in GFR14.
The tools of this site for calculation of GFR are based upon the cystatin C-based equation of reference 6
and the creatinine-based equation of reference 7. Although both equations work for both adults and children6,7,15 the
best estimate for children is that obtained by using only the cystatin C-based equation6.
Calculating absolute GFR from relative GFR
To study the kidney function of a person, the "relative glomerular filtration rate" (relative GFR),
which has the unit "mL·min-1·(1.73m2)-1", sometimes written as "mL/min/1.73 sqm", is often used.
The relative GFR of a person is thus normalized to a certain body surface area, which allows the use of virtually the same
reference values for males and females, adults and children. The relative GFR of a person is accordingly also independent
of his/her actual body surface area. The relative GFR is suitable for assessing and monitoring the kidney function of a patient.
But if you want to correctly dose a medicine or a contrast agent mainly excreted via the kidneys, knowledge of the absolute GFR (mL/min) of
the patient is required. The estimating equations accounted for above provide estimates of the relative GFR. The tool below
can be used to easily calculate the absolute GFR (mL/min) of a person from his relative GFR (mL · min-1·(1.73 m2)-1), weight and height.
The DuBois and DuBois formula is thereby used to estimate body surface area16.