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
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 eGFRcystatin C or eGFRcreatinine,
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 calibrators2-8.
The strategy also allows diagnosis of shrunken pore syndrome with its high morbidity
and mortality9-13 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
Specifically, the use of the cystatin C-based CAPA-6 and the
creatinine-based LM-Rev7,8-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 estimates3-5.
For children, the best estimate is obtained from the cystatin C-based estimating
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 adults6,9.
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 most adult patient populations, the mean of the two GFR estimates,
(eGFRcystatin C + eGFRcreatinine)/2,
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 results
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,
(eGFRcystatin C + eGFRcreatinine)/2, should be used.
The closer the agreement between eGFRcystatin C and eGFRcreatinine,
the more reliable is the mean value as a GFR estimate and if the
eGFRcystatin C/eGFRcreatinine>-ratio is between 0.8 and 1.20
the mean value is generally reliable.
If the GFR estimates obtained by the cystatin C- and creatinine-based equations
do not agree, e.g., if the eGFRcystatin C/eGFRcreatinine-ratio
is outside the interval 0.8 - 1.20,
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 used14-18.
If the patient is treated with large amounts of glucocorticoids
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 used19.
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 creatinine level
decrease without corresponding changes in GFR20.
The tools of this site for estimation of GFR are based upon the
cystatin C-based equation of reference 6 (the CAPA-equation)
and the creatinine-based equation of references 7 and 8 (the LM-Rev-equation).
Diagnosing shrunken pore syndrome
If eGFRcystatin C is less than 70% of eGFRcreatinine, i.e.
the eGFRcystatin C/eGFRcreatinine-ratio is < 0.70,
and no non-renal factors influence the estimates, the patient suffers from shrunken
pore syndrome with strong increase in morbidity and mortality, inter alia
due to cardiovascular manifestations9-13. The increase in morbidity and mortality
is inversly proportional to the eGFRcystatin C/eGFRcreatinine-ratio
starting at a ratio of 0.8021. It should be emphasized that it has not yet
been established that the shrunken pore syndrome exists in children younger than
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
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
(tab ”Relative GFR”).
The tab ”Absolute GFR”
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 area22.
- Soveri I, Berg U, Björk J, Elinder CG, Grubb A, Mejare I, Sterner G, Bäck SE on
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- Grubb A. Non-invasive estimation of glomerular filtration rate (GFR). The Lund model:
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+ unpublished observations
- Björk J, Grubb A, Sterner G, Nyman U: Revised equations for estimating glomerular
filtration rate based on the Lund-Malmö study cohort.
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- Nyman U, Grubb A, Larsson A, Hansson L-O, Flodin M, Nordin G, Lindström V, Björk J:
The revised Lund-Malmö GFR estimating equation outperforms MDRD and CKD-EPI across GFR,
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- Grubb A. Cystatin C is indispensable for evaluation of kidney disease.
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- Grubb A, Lindström V, Jonsson M, Bäck SE, Åhlund T, Rippe B, Christensson A.
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- Dardashti A, Nozohoor S, Grubb A, Bjursten H. Shrunken Pore Syndrome is associated
with a sharp rise in mortality in patients undergoing elective coronary artery bypass grafting.
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- Preiss DJ, Godber IM, Lamb EJ, Dalton RN, Gunn IR. The influence of a cooked-meat meal
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- Tangri N, Stevens LA, Schmid CH, Zhang YL, Beck GJ, Greene T, Coresh J, Levey AS.
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- Karawajczyk M, Ramklint M, Larsson A. Reduced cystatin C-estimated GFR and increased
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- Herou E, Dardashti A, Nozohoor S, Zindovic I, Ederoth P, Grubb A, Bjursten H. The
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