Formal Name of the Test:
Cystatin C with Glomerular Filtration Rate, Estimated (eGFR)
Cystatin C is a non-glycosylated cysteine proteinase inhibitor with a low molecular weight (13,250 kD) that is produced by all nucleated cells and found in bodily fluids such as serum. Because it is produced at a constant rate and is freely filtered by the kidneys, its serum concentration is inversely proportional to the glomerular filtration rate (GFR), with higher values indicating lower GFRs and lower values indicating higher GFRs, similar to creatinine. Cystatin C is handled differently in the kidney than creatinine. While both are freely filtered by glomeruli, cystatin C, unlike creatinine, is reabsorbed and metabolized by the proximal renal tubules once filtered. As a result, cystatin C does not enter the final excreted urine in any significant amount under normal conditions. The serum concentration of cystatin C remains constant in the presence of infections, inflammatory or neoplastic states, and is unaffected by body mass, diet, or drugs. As a result, cystatin C may be a more accurate indicator of renal function (GFR) than creatinine. GFR (eGFR) can be calculated from serum cystatin C using an equation that includes the patient’s age and gender. Inker et al developed the CKD-EPI cystatin C equation, which demonstrated a good correlation with measured iothalamate clearance in patients with all common causes of kidney disease, including kidney transplant recipients. Cystatin C eGFR may be superior to creatinine eGFR in certain patient groups with abnormally high or low muscle mass (for example quadriplegics, very elderly, or malnourished individuals). Because blood levels of cystatin C equilibrate faster than creatinine, serum cystatin C may be more accurate than serum creatinine when kidney function is rapidly changing (for example amongst hospitalized individuals).
There is no need for any prior preparation.
Test Performed at:
Quest Diagnostics, Test Code 94588
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