*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.
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 calibrators^{3-8}.
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}.

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 GFR^{3,7,8}.

For most adult patient populations, the mean of the two GFR estimates, relative GFR_{CC+creat}, 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}.

*Optimal evaluation of results*^{3}

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 GFR_{CC+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 used^{9-13}.

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 used^{14}.

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 GFR^{15}.

The tools of this site for calculation of GFR are based upon the cystatin C-based equation of reference 6 (CAPA-equation)
and the creatinine-based equation of references 7 and 8 (LM-Rev-equation).

*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}·(1.73m^{2})^{-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
(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 m^{2})^{-1}), weight and height.
The DuBois and DuBois formula is thereby used to estimate body surface area^{16}.

- Soveri I, Berg U, Björk J, Elinder CG, Grubb A, Mejare I, Sterner G, Bäck SE on behalf of the SBU review group. Measuring GFR: A Systematic Review.
Am J Kidney Dis 2014. In press.
DOI: http://dx.doi.org/10.1053/j.ajkd.2014.04.010
- Methods to estimate and measure renal function. SBU 2013. A systematic review.
- Grubb A. Non-invasive estimation of glomerular filtration rate (GFR). The Lund model: Simultaneous use of cystatin C- and creatinine-based GFR-prediction equations,
clinical data and an internal quality check. Scand J Clin Lab Invest, 2010; 70: 65 - 70
- Nyman U, Grubb A, Sterner G, Björk J: Different equations to combine creatinine and cystatin C to predict GFR. Arithmetic mean of existing equations
performs as well as complex combinations. Scand J Clin Lab Invest 2009; 69: 619-627.
- Grubb A, Nyman U, Björk, J: Improved estimation of glomerular filtration rate (GFR) by comparison of eGFR
_{cystatin C} and eGFR_{creatinine}.
Scand J Clin Lab Invest 2012; 72: 73-77.
- Grubb A, Horio M, Hansson LO, Björk J, Nyman U, Flodin M, Larssson A, Bökenkamp A, Yasuda Y, Blufpand H, Lindström V, Zegers I, Althaus H, Blirup-Jensen S,
Itoh Y, Sjöström P, Nordin G, Christensson A, Klima H, Sunde K, Hjort-Christensen P, Armbruster D, Ferrero C: Generation of a new cystatin C-based estimating
equation for glomerular filtration rate using seven assays standardized to the international calibrator.
Clin Chem 2014; 60: 974 - 986 + unpublished observations
- Björk J, Grubb A, Sterner G, Nyman U: Revised equations for estimating glomerular filtration rate based on the Lund-Malmö study cohort. Scand J Clin Lab Invest 2011; 71: 232-239.
- 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, age and BMI intervals in a large Swedish population. Clin Chem Lab Med 2014; 52: 815-824. DOI: 10.1515/cclm-2013-0741
- Thomassen SA, Johannesen IL, Erlandsen EJ, Abrahamsen J, Randers E. Serum cystatin C as a marker of the renal function in patients with spinal cord injury.
Spinal Cord 2002; 40:524-8.
- Viollet L, Gailey S, Thornton DJ, Friedman NR, Flanigan KM, Mahan JD, Mendell JR. Utility of cystatin C to monitor renal function in Duchenne muscular dystrophy.
Muscle Nerve 2009;40:438–42
- Preiss DJ, Godber IM, Lamb EJ, Dalton RN, Gunn IR. The influence of a cooked-meat meal on estimated glomerular filtration rate. Ann Clin Biochem 2007;44:35– 42.
- Tangri N, Stevens LA, Schmid CH, Zhang YL, Beck GJ, Greene T, Coresh J, Levey AS. Changes in dietary protein intake has no effect on serum cystatin C levels
independent of the glomerular filtration rate. Kidney Int 2011;79:471–7.
- Blackwood WS, Maudgal DP, Pickard RG, Lawrence D, Northfield TC. Cimetidine in duodenal ulcer. Controlled trial. Lancet 1976;2:174–6.
- Risch L, Herklotz R, Blumberg A, Huber AR. Effects of glucocorticoid immunosuppression on serum cystatin C concentrations in renal transplant
patients. Clin Chem 2001; 47:2055-2059.
- Karawajczyk M, Ramklint M, Larsson A. Reduced cystatin C-estimated GFR and increased creatinine-estimated GFR in comparison with iohexol-estimated
GFR in a hyperthyroid patient: A case report. J Med Case Reports 2008; 2:66.
- DuBois D, DuBois EF. A formula to estimate the approximate surface area if height and weight be known. Arch Intern Medicine. 1916; 17:863-871.