Serum creatinine has a drawback in the measurement of glomerular filtration rate (GFR) in that it may vary according to muscle mass. Cystatin C is a 13 kilodalton protein that is filtered by the glomerulus and reabsorbed and metabolized by tubular cells. The amount that is excreted into the urine is negligible. Its production is very steady, and not dependent on muscle mass. It has been proposed as an alterate marker Ifor estimating GFR. (coresh)

An elevated serum cystatin C level may indicate a worse cardiovascular risk in patients with the metabolic syndrome. (18456039) . The literature is emerging, and showing that it has benefit as a marker. Here are two formulae that might be useful in demonstrating the relationships between serum creatinine and serum cystatin C.

CYSTATIN FORMULA

GFR = 77.24 X cys^-1.2623
GFR =99.43 X cys^-1.5837
cys = serum cystatin c

cystatin c in mg/L
GFR (Dade Behring Calibration):___
GFR:(Dako Calibration)___

Touchcalc automatically converts to metric from feet, inches or pounds

CYSTATIN REFERENCES

• Dade-Behring Assay from Siemens
• Dako Control set
• Stevens LA, Coresh J, Schmid CH, Feldman HI, Froissart M, Kusek J, Rossert J, Van Lente F, Bruce RD 3rd, Zhang YL, Greene T, Levey AS. Estimating GFR using serum cystatin C alone and in combination with serum creatinine: a pooled analysis of 3,418 individuals with CKD. Am J Kidney Dis. 2008 Mar;51(3):395-406
• Larsson A, Malm J, Grubb A, Hansson LO. Calculation of glomerular filtration rate expressed in mL/min from plasma cystatin C values in mg/L. Scand J Clin Lab Invest. 2004;64(1):25-30.
• Menon V, Shlipak MG, Wang X, Coresh J, Greene T, Stevens L, Kusek JW, Beck GJ, Collins AJ, Levey AS, Sarnak MJ Cystatin C as a risk factor for outcomes in chronic kidney disease.Ann Intern Med. 2007 Jul 3;147(1):19-27.