eGFR and cystatin C: a closer look at kidney function
Most labs report eGFR from creatinine. Cystatin C is a second marker that cross-checks that number — and sometimes changes the picture.
TL;DR
- Cystatin C is a protein every cell produces at a steady rate. When kidney filtration slows, it builds up in the blood.
- eGFR — estimated glomerular filtration rate — is the standard kidney function measure. It is usually calculated from creatinine.
- The CKD-EPI 2021 equation combines creatinine and cystatin C. The NIDDK recommends it when higher accuracy is needed.
What cystatin C is
Cystatin C is a small protein made by every cell in the body at a steady rate. When the kidneys slow down, cystatin C builds up in the blood. That buildup is what lab tests detect.
How cystatin C works
eGFR stands for estimated glomerular filtration rate — an estimate of how fast the kidneys filter blood. It is the standard number doctors use to track kidney health. The traditional calculation uses creatinine. A newer approach uses cystatin C, or combines both. The combined calculation is now considered the most accurate option.
Think of it like two gauges cross-checking one reading. A car dashboard might show both fuel level and driving range. If they agree, you have confidence in the number. If they diverge, you pay attention to the gap.
Who asks about cystatin C
People who come up with borderline eGFR numbers often encounter this question. A creatinine-based eGFR near 60 can look different when cystatin C is added. This matters most near clinical decision points — like adjusting medication doses or flagging early kidney disease.
What the research says
In 2021, the CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration) published updated equations. They removed race as an input variable. A 2024 review on PubMed Central confirmed the race-free CKD-EPI 2021 equations perform well across diverse populations. The NIDDK (National Institute of Diabetes and Digestive and Kidney Diseases) now recommends the combined equation for higher accuracy.
What to know
Some people are most likely to benefit from the cystatin C calculation. These include older adults, women, and people with lower muscle mass. In those cases, creatinine alone may overestimate how well the kidneys are working.
The Halftime POV
eGFR from creatinine alone is a solid first screen. Adding cystatin C gives a sharper picture, especially when the result is near a threshold that changes clinical decisions. If your standard eGFR comes back borderline, asking your physician about cystatin C is a reasonable next step. It is a low-cost add-on to a standard metabolic panel.
We include cystatin C in our lab panel because it adds meaningful signal without much added cost. A single number rarely tells the whole story. Two markers pointing in the same direction tell you something worth acting on.
Related reading:
- What are biomarkers and how do they fit peptide protocols?
- hs-CRP: the inflammation marker explained
- GGT: liver stress and metabolic marker
- ApoB vs LDL: cardiovascular risk markers
- Fasting insulin: the metabolic marker most labs miss
FAQ
What is cystatin C?
Cystatin C is a small protein produced by every cell in the body at a steady rate. Blood levels rise when the kidneys filter more slowly, making it a useful marker of kidney function — often more accurate than creatinine alone, especially in people with lower muscle mass.
What is eGFR?
eGFR stands for estimated glomerular filtration rate — an estimate of how fast the kidneys filter blood. It is the standard measure of kidney function. The traditional calculation uses creatinine. The CKD-EPI 2021 equation can use cystatin C alone or combined with creatinine for better accuracy.
Cystatin C vs creatinine: which is better?
Neither is universally better. Creatinine is the standard first-line test and works well for most people. Cystatin C is more accurate when creatinine may be misleading — such as in older adults, women, or people with lower muscle mass. The combined equation is now considered the most accurate option.
Disclaimer
This article is educational and is not medical advice. Compounded medications are not FDA-approved. Clinical outcomes depend on individual factors and require physician evaluation. Results vary. Halftime Health is launching soon — join the waitlist to get updates.
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Frequently asked questions
What is cystatin C?
Cystatin C is a small protein produced by every cell in the body at a steady rate. Blood levels rise when the kidneys filter more slowly, making it a useful marker of kidney function — often more accurate than creatinine alone, especially in people with lower muscle mass.
What is eGFR?
eGFR stands for estimated glomerular filtration rate — an estimate of how fast the kidneys filter blood. It is the standard measure of kidney function. The traditional calculation uses creatinine. The CKD-EPI 2021 equation can use cystatin C alone or combined with creatinine for better accuracy.
Cystatin C vs creatinine: which is better?
Neither is universally better. Creatinine is the standard first-line test and works well for most people. Cystatin C is more accurate when creatinine may be misleading — such as in older adults, women, or people with lower muscle mass. The combined equation is now considered the most accurate option.
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