Test ID: CSTC    
Cystatin C, Serum

As an index of glomerular filtration rate, especially in patients where serum creatinine may be misleading (eg, very obese, elderly, and malnourished patients)

Assessing renal function in patients suspected of having kidney disease

Monitoring treatment response in patients with kidney disease

Cystatin C is a low molecular weight (13,250 kD) cysteine Proteinase inhibitor that is produced by all nucleated cells and found in body fluids, including serum. Since it is formed at a constant rate and freely filtered by the kidneys, its serum concentration is inversely correlated with the glomerular filtration rate (GFR); that is, high values indicate low GFRs while lower values indicate higher GFRs, similar to creatinine.

The renal handling of cystatin C differs from creatinine. While both are freely filtered by glomeruli, once it is filtered, cystatin C, unlike creatinine, is reabsorbed and metabolized by proximal renal tubules. Thus, under normal conditions, cystatin C does not enter the final excreted urine to any significant degree.

The serum concentration of cystatin C remains unchanged with infections, inflammatory or neoplastic states, and is not affected by body mass, diet, or drugs. Thus, cystatin C may be a more reliable marker of renal function (GFR) than creatinine.

> or =23 years: 0.57-1.29 mg/L

Reference values have not been established for patients that are <23 years of age.

Elevated levels of cystatin C indicate decreased glomerular filtration rate

Due to immaturity of renal function in neonates, cystatin C levels are higher in those <3 months of age.(1)

Lipemic or frozen specimens, which become turbid after thawing, may interfere with the assay.

In a study to evaluate cystatin C as a measure of renal function in comparison to serum creatinine, 500 patients had cystatin C measured by nephelometry and glomerular filtration rate (GFR) measured by nonradiolabeled iothalamate clearance.(2) In addition, serum creatinine was measured and the patients' medical records reviewed. The correlation of 1/cystatin C with GFR (r=0.90) was significantly superior than 1/creatinine (r=0.82, p<0.05) with GFR. The superior correlation of 1/cystatin C with GFR was observed in the various clinical subgroups of patients studied (ie, subjects with no suspected renal disease, renal transplant patients, recipients of some other transplant, patients with glomerular disease, and patients with non-glomerular renal disease). Using the lower 5th percentile age-adjusted normal values for iothalamate clearance, receiver operator characteristic curves were generated. The area under the curve for 1/cystatin C was significantly greater than for 1/creatinine. These results indicate that cystatin C may be superior to serum creatinine for the assessment of GFR in a wide spectrum of patients-both those with renal disease and those without renal disease. Others have similarly found that cystatin C correlates better than serum creatinine for assessment of GFR.(3)

1. Finney H, Newman DJ, Thakkar H, et al: Reference ranges for the plasma cystatin C and creatinine measurements in premature infants, neonates, and older children. Arch Dis Child 2000 Jan;82(1):71-75

2. Buehrig CK, Larson TS, Bergert JH, et al: Cystatin C is superior to serum creatinine for the assessment of renal function. J Am Soc Nephrol 2001;12:194A

3. Grubb AO: Cystatin C - properties and use as a diagnostic marker. Adv Clin Chem 2000;35:63-99

4. Coll E, Botey A, Alvarez L, et al: Serum cystatin C as a new marker for noninvasive estimation of glomerular filtration rate and as a marker for early renal impairment. Am J Kidney Dis 2000 Jul;36(1):29-34