What Is eGFR, and Why Do Creatinine and Cystatin C Give Different Results?

Estimated glomerular filtration rate (eGFR) is how much blood your kidneys filter each minute, estimated from a blood test rather than measured, in mL/min per 1.73 m² of body surface. In healthy adults it’s usually 90 or more, and it declines slowly with age. An eGFR under 60 for more than three months is one way chronic kidney disease (CKD) is defined. Because it’s an estimate, the result depends on which substance in your blood it’s based on: creatinine, cystatin C, or both.

This article is general information, not medical advice.

How It’s Estimated

The kidneys clear waste substances from the blood, so a substance’s blood level rises as filtering falls. The CKD Epidemiology Collaboration (CKD-EPI) equations turn a level into eGFR using your age and sex:

• Creatinine (CKD-EPI 2021). Creatinine comes from muscle, so people with a lot of muscle read lower eGFRs, and people with little muscle (older or frail adults, amputees, people on a vegetarian diet) can read falsely high ones. A large meat meal or creatine supplements can raise it.
• Cystatin C (CKD-EPI 2012). Made by all cells, it depends much less on muscle, but can be affected by thyroid disease, high-dose steroids, inflammation, and obesity.
• Creatinine and cystatin C together (CKD-EPI 2021). In the study that developed the 2021 equations, Inker and colleagues found the combined equation the most accurate, and it no longer adjusts for race, which earlier equations did.

The KDIGO 2024 guideline recommends confirming a creatinine-based eGFR with cystatin C when accuracy matters, such as when the result is near a treatment threshold or muscle mass is unusual.

The Stages

KDIGO classifies CKD by eGFR (G stages) together with albumin in the urine (A stages):

eGFR (mL/min/1.73 m²)	Stage	Description
90 or more	G1	Normal or high
60 to 89	G2	Mildly decreased
45 to 59	G3a	Mildly to moderately decreased
30 to 44	G3b	Moderately to severely decreased
15 to 29	G4	Severely decreased
Under 15	G5	Kidney failure

G1 and G2 count as CKD only with other signs of kidney damage, most often albumin in the urine: a urine albumin-to-creatinine ratio (UACR) of 30 mg/g or more.

Reading Your Results

• One low result isn’t a diagnosis: CKD needs abnormal results for more than three months.
• Dehydration, a recent heavy meal of meat, and intense exercise can lower a creatinine-based eGFR temporarily.
• Compare like with like: creatinine eGFR with creatinine eGFR, from the same lab where you can.

Following It in Kaizora

Kaizora keeps each equation as its own metric on Labs: Estimated Glomerular Filtration Rate (CKD-EPI 2021, Creatinine), (CKD-EPI 2012, Cystatin C), and (CKD-EPI 2021, Creatinine–Cystatin C), beside Serum Creatinine, Serum Cystatin C, and Urine Albumin-to-Creatinine Ratio. Log each as your lab reported it, with the range it printed, so a change of equation never looks like a change in your kidneys.

The Short Version

• eGFR estimates kidney filtering from a blood test; under 60 for three months is CKD.
• Creatinine depends on muscle; cystatin C much less; the combined equation is most accurate.
• KDIGO stages CKD by eGFR and urine albumin together.

Sources

• Inker LA et al., “New creatinine- and cystatin C–based equations to estimate GFR without race,” New England Journal of Medicine 385, no. 19 (2021): 1737–1749. doi:10.1056/NEJMoa2102953
• Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group, “KDIGO 2024 clinical practice guideline for the evaluation and management of chronic kidney disease,” Kidney International 105, no. 4S (2024): S117–S314. doi:10.1016/j.kint.2023.10.018