Cystatin C

Cystatin C may be used as an alternative to creatinine and creatinine clearance to screen for and monitor kidney dysfunction in those with known or suspected kidney diseases. It may be especially useful in those cases where creatinine measurement is not appropriate: for instance in those who have liver cirrhosis, are very obese, are malnourished or have a reduced muscle mass. Measuring Cystatin C may also be useful in the early detection of kidney disease when other parameters might still be normal, especially in the elderly.

Researchers are exploring other uses of Cystatin C, and the reasons for doctors ordering it may evolve over time. In addition to kidney dysfunction, it has been associated with an increased risk of cardiovascular disease and heart failure in older adults.

Cystatin C is gaining acceptance as studies confirm and define its usefulness, especially as an early, sensitive marker for chronic kidney disease. It may be ordered when a patient has a known or suspected disease that affects or potentially affects kidney function and so reduces GFR. Early loss of kidney function (often called pre-clinical disease) may not cause elevated creatinine levels or other signs of impairment. When a doctor is not satisfied with the results of other tests such as creatinine, eGFR, or creatinine clearance, he may order a Cystatin C. A doctor may order Cystatin C when he wants to check for early kidney dysfunction, particularly in the elderly, and/or wants to monitor known impairment over time.

Researchers are hoping to learn more about Cystatin C as an indicator of risk of heart failure and death, especially in older patients. In this case, the test may prove useful at times when a patient is at an increased risk of heart problems. For example, the test may be ordered as part of preoperative screening, before prescribing certain medications, or before performing scans that use intravenous dyes.

An increased serum Cystatin C corresponds to a decreased GFR and hence to kidney dysfunction. Since Cystatin C is produced throughout the body at a constant rate and removed by glomerular filtration (and subsequently reabsorbed and broken down in another part of the kidney), it should remain at a steady level in the blood if the kidneys are working efficiently and the GFR is normal. Concentrations of Cystatin C are not affected by gender, age or race. Neither are they generally affected by most drugs, infections, diet or inflammation. Cystatin C levels may be affected by some drugs: corticosteroids can increase levels while cyclosporine can decrease them. In the absence of kidney disease, Cystatin C levels may be elevated in rheumatic diseases and in malignant diseases, although they are not affected by tumor burden (the amount of cancer that someone has).

Recent studies suggest that increased levels of Cystatin C may also indicate an increased risk of heart disease, heart failure, stroke and mortality.

Cystatin C has been associated with hyperhomocysteinemia (often found in renal transplant patients), and it has been shown to increase with the progression of liver disease. At least one study has looked at comparing Cystatin C levels in serum with that found in pleural effusion to help determine the cause of the effusion. These associations may or may not prove clinically useful.