Cystatin C is a protein produced consistently by nearly all nucleated cells throughout the human body. Once produced, it circulates in the blood and is freely filtered by the kidneys. The levels of this protein in the bloodstream can therefore provide an indication of how effectively the kidneys are performing their filtration duties.
What is Cystatin C
Cystatin C is a small protein belonging to the cystatin superfamily, functioning as a cysteine protease inhibitor. Its production rate is constant across most cell types, making its circulating levels a stable indicator. It is freely filtered from the blood by the glomeruli, the tiny filtering units within the kidneys.
Following filtration, cystatin C undergoes complete reabsorption and metabolism by the cells of the renal tubules; it is not secreted back into the bloodstream. Due to this unique processing, the concentration of cystatin C in the blood is almost entirely dependent on the glomerular filtration rate (GFR). A higher blood concentration of Cystatin C signifies a lower GFR, indicating reduced kidney filtration capacity.
Why Measure Cystatin C Levels
Measuring cystatin C levels primarily estimates the Glomerular Filtration Rate (GFR), a direct measure of kidney function. GFR indicates how many milliliters of blood the kidneys filter per minute, assessing their ability to remove waste products. Monitoring GFR is standard practice for assessing kidney health and tracking kidney condition progression.
Cystatin C offers advantages over traditional markers because its levels are less influenced by factors like muscle mass, diet, age, sex, or ethnicity. This independence makes it useful in populations where creatinine, another common kidney marker, might be unreliable. For instance, it provides a more accurate GFR estimate in individuals with very low or high muscle mass, or those with rapidly changing nutritional states. Its utility also extends to detecting early declines in kidney function or monitoring treatment effectiveness for kidney health.
Interpreting Your Cystatin C Results
Normal cystatin C ranges can vary between laboratories, but fall within a specific range, such as 0.6 to 1.0 mg/L for adults. An elevated cystatin C level in the blood indicates a reduced glomerular filtration rate (GFR). This can be an early sign of kidney impairment.
Lower levels of cystatin C do not carry significant clinical implications for kidney function. A single cystatin C measurement provides a snapshot of kidney function. Healthcare professionals integrate these results with a patient’s medical history, physical examination findings, and other laboratory tests to form a comprehensive assessment of kidney health.
What Affects Cystatin C Levels
While cystatin C levels primarily reflect kidney function, several factors unrelated to kidney health can also influence its concentration in the blood, which is important for accurate interpretation. Thyroid dysfunction is one such factor; individuals with hyperthyroidism may exhibit lower cystatin C levels, while those with hypothyroidism might show elevated levels. These changes relate to alterations in the protein’s production or metabolism.
Inflammation can also lead to increased cystatin C levels, as it is considered an acute phase reactant. Certain medications, such as corticosteroids, have been observed to elevate cystatin C levels independently of kidney function. Additionally, severe obesity may influence cystatin C levels, potentially due to altered metabolic processes.
Cystatin C Compared to Other Kidney Tests
Cystatin C offers distinct advantages compared to serum creatinine, the most commonly used marker for kidney function. Unlike creatinine, a byproduct of muscle metabolism that varies significantly with muscle mass, diet, age, and sex, cystatin C production is more constant across individuals. This independence makes Cystatin C a more reliable indicator of glomerular filtration rate (GFR) in certain populations.
Cystatin C may be preferred for assessing kidney function in elderly individuals, young children, or those with unusual muscle mass, such as amputees or malnourished patients. It is also useful in individuals with liver disease, where creatinine levels can be misleading. While creatinine remains a valuable test, combining both cystatin C and creatinine measurements often provides a more comprehensive and accurate assessment of kidney function, especially in complex clinical situations.