From the Guidelines
The serum cystatin C test plays a crucial role in assessing impaired renal function, particularly in patients where creatinine measurements may be less reliable, and is recommended for confirmatory testing in specific circumstances, such as in individuals with low muscle mass or those at risk for or with chronic kidney disease. This test serves as an alternative or complement to creatinine-based tests for estimating glomerular filtration rate (GFR) 1. Cystatin C is produced at a constant rate by all nucleated cells in the body and is freely filtered by the kidneys, making it an excellent marker for kidney function. The test is especially valuable for patients with reduced muscle mass (elderly, malnourished individuals, or those with muscle-wasting conditions), where creatinine levels might falsely appear normal despite impaired kidney function.
Key Points to Consider
- Cystatin C is not significantly affected by diet, muscle mass, or most medications, providing a more consistent measure of kidney function across diverse patient populations 1.
- Healthcare providers typically order this test when monitoring chronic kidney disease, assessing medication dosing for drugs cleared by the kidneys, or evaluating kidney function in patients with conditions that might affect creatinine reliability.
- Normal cystatin C levels generally range from 0.51 to 0.98 mg/L, with higher values indicating potential kidney impairment.
- The 2021 CKD-EPI (Epidemiology) creatinine equation refit without the race variable is recommended for use in all laboratories in the United States, and cystatin C is recommended for confirmatory testing in specific circumstances 1.
- The use of cystatin C and the urinary albumin-creatinine ratio (UACR) in CKD screening, diagnosis, and management is increasingly important, as highlighted in the new guidelines and commentary 1.
Clinical Application
In clinical practice, the serum cystatin C test should be used in conjunction with other tests, such as creatinine-based tests, to provide a more accurate assessment of kidney function. This is particularly important in patients with chronic kidney disease or those at risk for kidney disease, as early detection and treatment can help prevent progression of the disease. By using cystatin C as a confirmatory test, healthcare providers can make more informed decisions about patient care and treatment.
From the Research
Role of Serum Cystatin C in Assessing Impaired Renal Function
- Serum Cystatin C is a reliable marker of kidney function, especially in detecting mild reductions in glomerular filtration rate (GFR) 2.
- It is produced at a constant rate by all nucleated cells and is freely filtered by the renal glomeruli, making it a useful indicator of renal function 2.
- Serum Cystatin C level is independent of age, sex, and muscle mass, which makes it a more accurate marker of GFR compared to serum creatinine level 2, 3.
Comparison with Other Markers of GFR
- Serum Cystatin C has been shown to have a greater sensitivity (93.4%) than serum creatinine level (86.8%) in detecting impaired renal function 2.
- It also has a higher diagnostic accuracy than serum creatinine and calculated creatinine clearance from the Cockcroft-Gault formula, especially in female patients 4.
- The use of Cystatin C-based estimated GFR (eGFRcys) has been recommended as an alternative and complementary biomarker for kidney function assessment, as it does not differ by race like creatinine and correlates better with adverse outcomes 3, 5.
Clinical Considerations and Recommendations
- The 2012 Kidney Disease Improving Global Outcomes (KDIGO) guidelines recommended using Cystatin C to confirm the diagnosis of chronic kidney disease (CKD) determined by creatinine-based estimated GFR 6.
- The National Kidney Foundation (NKF) and American Society of Nephrology (ASN) Joint Task Force recommended increasing availability and clinical adoption of Cystatin C to assess kidney function 6.
- Recent studies suggest that Cystatin C testing should be routinely available for estimating kidney function, especially in vulnerable individuals such as older adults, females, and those with multiple medical conditions 3.