The Cystatin C blood test assesses how effectively the kidneys filter waste from the bloodstream. Doctors use this test primarily to calculate the estimated Glomerular Filtration Rate (eGFR), which provides a numerical measure of kidney health. The test is particularly valuable when standard markers may give misleading results, offering a more precise look at the organ’s performance.
Understanding Cystatin C
Cystatin C is a small protein produced at a constant rate by nearly all nucleated cells throughout the body. Once released into the bloodstream, its concentration is directly related to the kidneys’ filtration capacity. Due to its small molecular weight, Cystatin C is freely filtered out of the blood by the glomeruli, the primary filtering units of the kidneys.
After filtration, the protein is almost completely reabsorbed and broken down by the renal tubules. If the kidneys are not filtering efficiently, less Cystatin C is cleared from the blood, causing its concentration to rise. This steady production and clearance process makes its serum level a reliable reflection of the glomerular filtration rate.
Primary Clinical Purpose of the Test
The main reason a doctor orders a Cystatin C test is to accurately determine the Glomerular Filtration Rate (GFR). GFR is the single best indicator of overall kidney function and is used to detect and stage Chronic Kidney Disease (CKD). The test is often requested when kidney function results are unclear or when existing markers are suspected to be unreliable.
A precise GFR measurement is needed to identify early stages of CKD, allowing for timely interventions to slow the progression of kidney damage. The test also monitors the effectiveness of treatments for established kidney disease and helps adjust the dosing of certain medications cleared by the kidneys.
Comparing Cystatin C to Creatinine
For decades, the standard marker for estimating GFR has been serum creatinine, a waste product of muscle metabolism. Creatinine levels are significantly influenced by a person’s muscle mass, age, sex, and diet, which can lead to inaccuracies in GFR estimation.
Cystatin C offers an advantage because its production is largely independent of these factors, providing a more stable and accurate measure of kidney filtration. The test is considered superior for populations with highly variable muscle mass, such as young children, older adults, and individuals with severe muscle wasting or high muscle bulk. Using Cystatin C provides a more sensitive detection of early kidney disease, especially when creatinine-based estimates appear borderline or misleading.
Interpreting Test Results
The Cystatin C concentration measured in the blood is used in a mathematical formula to calculate the estimated Glomerular Filtration Rate (eGFR). This calculation converts the protein level into a rate, typically measured in milliliters per minute per 1.73 square meters of body surface area. A higher concentration of Cystatin C indicates that the kidneys are filtering poorly, resulting in a lower calculated eGFR.
An eGFR value of 90 or higher is considered normal. Values below 60 for three months or more signify Chronic Kidney Disease. A low eGFR indicates the kidneys are struggling to remove waste products, requiring further investigation and management. In some cases, a combined eGFR calculation using both Cystatin C and creatinine may be used to achieve the highest level of accuracy.