Cystatin C is a small protein produced by all nucleated cells and released into the bloodstream at a relatively constant rate. The kidneys, specifically the filtering units called glomeruli, freely filter this protein out of the blood. Measuring the concentration of Cystatin C in a blood sample provides a direct way to assess how effectively the kidneys are clearing waste. The test is increasingly used because its blood concentration is tightly linked to the rate at which the kidneys filter the blood.
Cystatin C: A Superior Marker for Kidney Health
Cystatin C is often preferred over the traditional serum creatinine test. Unlike creatinine, which is a waste product of muscle metabolism, Cystatin C is produced by every nucleated cell, leading to a more stable production rate. This stability means the blood level of Cystatin C is less influenced by a person’s muscle mass, age, gender, or diet.
The production of creatinine is highly dependent on muscle mass, which can make test results misleading in people with low muscle mass, such as the elderly, those with severe malnutrition, or individuals with certain chronic diseases. Because Cystatin C is not affected by these factors, it offers a more reliable and sensitive marker for detecting subtle declines in kidney function, especially in the early stages of kidney disease.
Interpreting Raw Cystatin C Values
A Cystatin C test result is reported as a concentration, usually in milligrams per liter (mg/L) or nanomoles per liter (nmol/L). The typical healthy range for adults generally falls between 0.60 and 1.15 mg/L, though this can vary slightly between laboratories and age groups. This raw number provides a direct measure of the amount of the protein circulating in the blood.
The interpretation of the raw value is straightforward: higher Cystatin C levels indicate poorer kidney filtration, while lower levels suggest better filtration. An elevated concentration signals that the kidneys are not clearing the protein from the blood efficiently, causing it to build up. This is similar to a clogged drain; if the drain (the kidney) is slow, the water (the Cystatin C) backs up in the sink (the bloodstream).
For example, a result above 1.15 mg/L in an adult would raise concern about reduced kidney function. Conversely, a value near the lower end of the reference range suggests efficient filtration activity. The raw concentration is the foundation for calculating the more clinically useful measure of kidney function.
Converting Cystatin C to Estimated GFR (eGFR)
Doctors convert the Cystatin C value into a standardized metric called the Estimated Glomerular Filtration Rate (eGFR). This conversion uses a specific mathematical formula, such as the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) Cystatin C equation, which incorporates the raw Cystatin C level, age, and sex. The eGFR result is reported in milliliters per minute per 1.73 square meters (mL/min/1.73m²).
The eGFR is the primary clinical metric used to stage Chronic Kidney Disease (CKD). A result of 90 mL/min/1.73m² or higher is generally considered normal. Results below this threshold are categorized into stages of CKD. A value below 60 mL/min/1.73m² for three months or more is the definitive diagnostic point for CKD.
The stages of CKD are defined by eGFR:
- eGFR between 60 and 89 mL/min/1.73m² indicates mild damage (Stage 2).
- eGFR between 30 and 59 mL/min/1.73m² signifies moderate loss of function (Stage 3).
- eGFR between 15 and 29 mL/min/1.73m² indicates severe loss (Stage 4).
- eGFR below 15 mL/min/1.73m² is Stage 5, meaning less than 15% of kidney function remains.
This staging system allows healthcare providers to monitor disease progression and plan appropriate management.
Situations That Can Affect Test Results
Certain physiological conditions and medications can artificially alter Cystatin C blood concentration, complicating test interpretation. These factors can cause the Cystatin C level to be higher or lower than expected, even if the actual kidney function has not changed. Clinicians must consider these confounding variables to ensure a correct diagnosis.
High doses of corticosteroids, commonly known as steroids, increase Cystatin C levels independent of the glomerular filtration rate. Similarly, severe thyroid disease, particularly hyperthyroidism (overactive thyroid) or hypothyroidism (underactive thyroid), can cause non-renal fluctuations in the protein’s concentration. Untreated thyroid conditions can therefore lead to misleading eGFR calculations.
Conditions that cause significant inflammation in the body, such as systemic lupus or high levels of C-reactive protein (CRP), can also elevate Cystatin C. Diabetes has also been associated with higher Cystatin C levels even after adjusting for filtration rate. These non-renal influences mean that a high Cystatin C result must be interpreted cautiously, often requiring follow-up testing or the use of both Cystatin C and creatinine values to gain a clearer picture of true kidney health.