Cystatin C is a protein produced by nearly all nucleated cells in the human body. Its measurement has gained increasing recognition as a valuable indicator in various health assessments, offering insights into bodily functions.
What is Cystatin C
Cystatin C is a small protein that inhibits cysteine proteases. It is produced at a relatively constant rate by most cells in the body. After production, Cystatin C is freely filtered by the glomeruli in the kidneys. It is then reabsorbed and metabolized by the renal tubules, rather than being secreted into the urine. This consistent production and specific kidney handling make it a useful marker for kidney function.
Why Cystatin C is Measured
Cystatin C is primarily measured to assess kidney function, particularly for estimating the Glomerular Filtration Rate (eGFR), which reflects how well the kidneys filter waste from the blood. While creatinine has been a long-standing marker for kidney function, Cystatin C offers advantages. Creatinine levels can be influenced by muscle mass, diet, and certain medications, leading to less accurate eGFR estimations in individuals with extremes of body size or specific dietary habits. Cystatin C levels are less affected by these non-renal factors, providing a more precise assessment of kidney filtration capacity. This makes it a preferred option for evaluating kidney health in populations where creatinine may be less reliable, such as the elderly, young children, or individuals with significant muscle wasting.
Understanding Your Cystatin C Levels
Interpreting Cystatin C levels directly relates to kidney health. Normal ranges vary slightly by laboratory and assay used, generally falling between 0.60 and 1.00 mg/L for adults. Elevated levels of Cystatin C usually indicate decreased kidney function, meaning the kidneys are not filtering waste efficiently. For example, a level above 1.00 mg/L often suggests renal impairment, with higher values correlating to more severe reductions in glomerular filtration. Doctors use these values, often integrated into eGFR equations, to stage kidney disease and guide treatment decisions. Very low Cystatin C levels are less common and typically do not signify a health concern related to kidney function, though they may be observed in conditions like liver disease or in individuals with very high muscle mass.
Factors Affecting Cystatin C Levels
Several factors unrelated to kidney function can influence Cystatin C levels, necessitating comprehensive interpretation by a healthcare provider. Age is one factor, as levels can naturally increase slightly with advancing age, even in the absence of kidney disease. Sex also plays a role, with minor differences between males and females.
Inflammatory conditions, such as systemic lupus erythematosus or rheumatoid arthritis, can cause elevated Cystatin C levels due to increased cellular turnover. Thyroid dysfunction, particularly hyperthyroidism, can lead to lower Cystatin C levels, while hypothyroidism may result in higher levels. Obesity has also been associated with elevated Cystatin C, reflecting a potential subclinical inflammatory state or metabolic changes. Certain medications, including corticosteroids like prednisone, can increase Cystatin C levels by affecting its production or metabolism. Therefore, a doctor considers a patient’s full medical history, current medications, and other clinical findings to accurately interpret Cystatin C results and determine their true meaning for kidney health.