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Cystatin C – Accurate Kidney Function Marker

Cystatin C is a protein produced by the body and used as a highly accurate marker for kidney function. It is considered more reliable than the traditional creatinine test.

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Things worth knowing about "Cystatin C"

Cystatin C is a protein produced by the body and used as a highly accurate marker for kidney function. It is considered more reliable than the traditional creatinine test.

What is Cystatin C?

Cystatin C is a small protein produced by virtually all nucleated cells in the human body. It belongs to the family of cysteine protease inhibitors and plays a protective role against certain enzymes. In clinical practice, cystatin C is primarily used as a biomarker for kidney function: it is almost exclusively filtered from the blood by the kidneys and is therefore well-suited to assess the glomerular filtration rate (GFR) – the filtering capacity of the kidneys.

Biological Function

In the body, cystatin C acts as a natural inhibitor of cysteine proteases, a group of enzymes that break down proteins. This inhibitory function plays a role in inflammatory processes, tissue remodeling, and protection against uncontrolled protein degradation. Because the production rate of cystatin C is largely constant and barely influenced by muscle mass, diet, or sex, it provides particularly reliable measurements.

Clinical Importance as a Kidney Marker

Cystatin C is used in diagnostics especially when kidney function needs to be assessed and the conventional creatinine marker appears unreliable. This is particularly relevant for the following groups:

Very elderly or very young patients
Individuals with significantly reduced muscle mass (e.g., due to muscular diseases or malnutrition)
Pregnant women
Patients with early kidney dysfunction where creatinine levels are still within normal range

An elevated cystatin C level in the blood may indicate impaired kidney function, even when other markers appear normal. Combining cystatin C with creatinine further improves the accuracy of GFR estimation.

Diagnosis and Measurement

Cystatin C is measured through a simple blood test. No special preparation such as fasting is required. The reference range is generally between 0.5 and 1.0 mg/L, though it may vary slightly depending on the laboratory and measurement method. GFR is estimated using established formulas (e.g., the CKD-EPI Cystatin C equation), which also take age and sex into account.

Elevated Cystatin C Levels – Possible Causes

A rise in cystatin C levels may indicate a variety of conditions:

Chronic kidney disease (CKD)
Acute kidney injury
Heart failure with impaired kidney function
Diabetes mellitus with kidney involvement (diabetic nephropathy)
Inflammatory conditions (cystatin C can also serve as an inflammation marker)
Hyperthyroidism (overactive thyroid)

Decreased cystatin C levels may be observed in hypothyroidism (underactive thyroid) or during treatment with corticosteroids.

Advantages Over Creatinine

Compared to creatinine, cystatin C offers several important advantages:

Independence from muscle mass and body composition
Earlier detection of kidney function decline
Less influence from diet or physical activity
Better predictive value for cardiovascular events and mortality

References

Inker LA et al. – New Creatinine- and Cystatin C-Based Equations to Estimate GFR without Race. New England Journal of Medicine, 2021.
KDIGO Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney International Supplements, 2013.
Shlipak MG et al. – Cystatin C versus Creatinine in Determining Risk Based on Kidney Function. New England Journal of Medicine, 2013.

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