Cystatin C is a small protein produced by all nucleated cells and is freely filtered by the glomeruli in the kidneys. The cystatin C test is a blood test used to assess kidney function, particularly the glomerular filtration rate (GFR). It is considered an alternative or adjunct to serum creatinine-based GFR estimation, especially in patients where creatinine may be less reliable (e.g., elderly, malnourished, or those with fluctuating muscle mass).
Advantages of Cystatin C Over Creatinine
- Less affected by muscle mass, age, and sex.
- More sensitive in detecting early kidney disease, particularly in the elderly.
- It is useful in diagnosing chronic kidney disease (CKD) in borderline cases where creatinine-based estimates may be misleading.
- May predict cardiovascular risk independently of kidney function.
Clinical Indications
- Assessing kidney function when creatinine-based eGFR is unreliable.
- Confirming CKD in patients with borderline eGFR results.
- Evaluating kidney function in critically ill patients with acute kidney injury.
- Monitoring kidney function in those at high risk of CKD, such as diabetics or hypertensive patients.
Limitations
- Higher cost ($50 private pay) compared to creatinine testing (generally bulk billed)
- Limited availability in some laboratories.
- It may be affected by thyroid dysfunction, corticosteroid use, and inflammatory conditions.
Conclusion
The cystatin C test provides a more accurate estimate of kidney function in certain populations, particularly those where creatinine-based eGFR is unreliable. It is increasingly used in clinical practice to improve CKD diagnosis and risk stratification, particularly in patients with normal or borderline serum creatinine levels.