Cystatin C is a protein produced by most cells in the body. It is released into the bloodstream and filtered by the kidneys. Healthy kidneys maintain a balanced level of cystatin C in the blood. Elevated levels suggest kidneys are not functioning effectively.
The Role of Cystatin C and Its Measurement
Cystatin C serves as a marker for assessing kidney function, particularly its ability to filter waste from the blood. It is considered a sensitive indicator of the glomerular filtration rate (GFR), which measures how well the tiny filters in your kidneys, called glomeruli, are working. The test for cystatin C involves a simple blood draw, similar to other routine blood tests. This protein is produced at a relatively constant rate and is primarily filtered by the renal glomeruli and then broken down, not reabsorbed or secreted.
Cystatin C is a more reliable marker of kidney function than creatinine. Creatinine, a waste product from muscle metabolism, can be influenced by factors such as muscle mass, diet, age, and sex, leading to variations in its levels. Because cystatin C levels are less affected by these variables, it can provide a more accurate assessment of kidney health, especially in individuals with unusual muscle mass, such as bodybuilders or the elderly, or those with unclear creatinine results.
Interpreting Elevated Cystatin C Levels
A high level of cystatin C in the blood generally indicates reduced kidney function or a decreased glomerular filtration rate (GFR), meaning the kidneys are not filtering waste products efficiently. The higher the concentration, the less effective the kidneys are at this task.
Laboratories use cystatin C values to calculate an estimated GFR (eGFR), which provides a numerical measure of kidney health. A normal eGFR for adults is typically around 100 mL/min/1.73 m², but values can vary with age. A healthcare provider may diagnose chronic kidney disease (CKD) if your eGFR remains below 60 mL/min/1.73 m² for three months or longer. Elevated cystatin C levels may also suggest an increased risk of heart disease or heart failure.
Common Causes of High Cystatin C
Elevated cystatin C levels primarily point to reduced kidney function, such as in chronic kidney disease (CKD) or acute kidney injury (AKI). In AKI, cystatin C levels can rise even before creatinine levels show a significant change, making it a valuable early indicator. Conditions like diabetic nephropathy, a kidney complication of diabetes, and hypertensive nephropathy, caused by high blood pressure, can also lead to increased cystatin C.
Beyond kidney-specific issues, several non-kidney related factors can also influence cystatin C levels. These include:
Thyroid dysfunction, particularly hyperthyroidism.
Inflammation and certain inflammatory conditions, such as those reflected by C-reactive protein (CRP).
Some medications, like corticosteroids, which can increase cystatin C production without kidney function decline.
Advanced age, which is associated with naturally increasing concentrations even without clinical kidney disease.
Certain cancers.
Conditions like HIV.
What to Do After a High Result
Receiving a high cystatin C result warrants a consultation with a healthcare professional for proper diagnosis and management. Your doctor will consider your medical history, symptoms, and other test results to interpret the finding. Further diagnostic tests may be recommended, such as repeat blood tests to confirm the trend, urine tests to check for protein or other markers of kidney damage, or imaging studies of the kidneys.
Lifestyle adjustments are often advised to support kidney health. These include:
Adopting a kidney-friendly diet, focusing on fresh fruits, vegetables, whole grains, and limiting sodium, phosphorus, and potassium intake as appropriate.
Maintaining a healthy weight through regular physical activity and managing underlying conditions like diabetes and high blood pressure.
Reviewing current medications with your healthcare provider, as some drugs can influence cystatin C levels or kidney function.
Considering referral to a kidney specialist (nephrologist), particularly if your estimated GFR falls below 30 mL/min/1.73 m² or if there’s evidence of rapid kidney function decline.
Ongoing monitoring of kidney function as part of your management plan.