As we know, creatinine (Cr) and blood urea nitrogen (BUN) are commonly used to evaluate kidney function. When calculating glomerular filtration rate (GFR), creatinine is typically the marker used. However, the problem with creatinine is that its levels can be influenced by various factors such as age, sex, race, muscle mass, diet, medications, exercise, and specific diseases.
So, is there a more accurate test available? Yes, there is another important marker that is unaffected by these factors: Cystatin C.
What is Cystatin C?
Cystatin C is a low-molecular-weight protein produced by all nucleated cells in the body. It is filtered through the glomeruli (tiny filters in the kidneys) and then reabsorbed by tubular cells where it is broken down. As a result, the concentration of Cystatin C in the blood is solely dependent on the glomerular filtration rate (GFR).
This is in contrast to creatinine, which can be influenced by many factors unrelated to kidney function, such as muscle mass and diet. Cystatin C levels are not affected by factors like age, sex, or race, making it a more reliable indicator of kidney function.
Why Use Cystatin C?
Unlike creatinine, Cystatin C levels are exclusively dependent on kidney function and are not influenced by other variables such as age, gender, muscle mass, or diet.
When kidney function is normal, Cystatin C levels remain stable. However, when kidney function declines, Cystatin C levels increase. The normal range for serum Cystatin C is typically between 0.61 – 0.95 mg/dL. It’s important to note that the reference range may vary slightly depending on the laboratory conducting the test.
How to Calculate Glomerular Filtration Rate (GFR) Using Cystatin C
Cystatin C can be used to calculate the glomerular filtration rate (eGFR), which is a key measure of kidney function. A lower GFR indicates that kidney function is impaired.
- Increased Cystatin C: Associated with cardiac disease, heart failure, and stroke.
- Decreased Cystatin C: Often seen in acute or chronic kidney disease, congestive heart failure, or renal obstruction.
Advantages of Using Cystatin C
- Not Affected by Age, Gender, Muscle Mass, or Diet:
- Cystatin C provides a more accurate reflection of kidney function compared to creatinine, as it is not influenced by these factors.
- Independent of External Factors:
- Cystatin C’s concentration is determined only by kidney function, making it a more reliable indicator.
- Early Detection of Decreased GFR:
- Cystatin C can detect early decreases in kidney function, even before creatinine levels begin to show a significant change.
- Useful in Diabetic Patients:
- Cystatin C is particularly valuable as an early marker for kidney function in diabetic patients, who are at higher risk for kidney disease.
- Post-Kidney Transplant Monitoring:
- It is an excellent marker for monitoring kidney function after transplantation or during chemotherapy.
- Pediatric Evaluation:
- Cystatin C is also an effective measure of kidney function in children and adolescents.
Disadvantages of Cystatin C
- Effect of Steroid Use:
- Steroid medications can lower Cystatin C production, potentially overestimating kidney function in patients taking these drugs.
- Effect of Hyperthyroidism:
- Hyperthyroidism can cause an increase in Cystatin C levels, potentially leading to misinterpretation of kidney function.
Given these variables, proper interpretation of Cystatin C results should be conducted by a healthcare professional.
Conclusion
As a kidney specialist, I always evaluate kidney function using both creatinine and Cystatin C. The use of Cystatin C allows for a more accurate and reliable assessment of kidney function, especially in cases where creatinine levels may be influenced by external factors.
If you are having a blood test that includes this marker, it is likely being used to evaluate your kidney function.
Take care of your kidney health and have a wonderful day!