Guide to the Estimated Glomerular Filtration Rate (eGFR) with Cystatin C

WRITTEN BY

The Levels Team

Updated: 05/22/2025|12 min read

Summary

Estimated Glomerular Filtration Rate (eGFR) is a calculated value that estimates how well your kidneys filter blood. It uses an equation that takes into account cystatin C values along with other factors such as age and sex.

Why It Matters

The estimated glomerular filtration rate (eGFR) tells you how fast your kidneys are filtering your blood. Your kidneys filter 120 to 150 quarts of blood every day through roughly a million tiny filtering units called glomeruli. The eGFR estimates how many milliliters of blood your kidneys can filter per minute.

When these filters are healthy, they:

• Remove waste products and excess water efficiently
• Keep important proteins in your blood
• Maintain proper balance of electrolytes
• Filter at a steady, consistent rate

When kidney filters become damaged:

• The filtering rate drops, meaning less blood gets cleaned
• Waste products build up in your blood
• Important proteins may leak out through damaged filters
• The ability to maintain proper fluid and mineral balance declines

What makes eGFR (estimated glomerular filtration rate) valuable is that kidney function can decline by up to 90% before you feel any symptoms. By measuring how well your kidneys are filtering - either through creatinine, cystatin C, or ideally both - you can detect kidney problems early, when treatment is most effective.

The filtering rate typically declines gradually with age, but a significant drop or consistent downward trend can signal kidney damage from conditions like:

• High blood pressure
• Diabetes
• Inflammatory diseases
• Certain medications
• Inherited kidney disorders

Estimated GFR is used because a measured GFR(mGFR) would require patient hospitalization and complex test procedures and analyses that are not practical for routine care.

Associated Symptoms

Estimated glomerular filtration rate (eGFR) levels themselves are laboratory findings rather than medical conditions. However, a low eGFR may be associated with kidney dysfunction, which can present with various symptoms.

Common symptoms that may indicate conditions associated with low eGFR:

• Fatigue: Persistent tiredness and low energy, may be related to waste product buildup and reduced kidney function
• Changes in urination: Alterations in frequency, volume, or appearance of urine, may reflect changes in kidney filtration
• Swelling: Fluid retention in feet, ankles, or face, could result from the kidneys' decreased ability to remove excess fluid
• Shortness of breath: Difficulty breathing, particularly with exertion, may be associated with fluid buildup in the lungs
• Nausea: Feeling of sickness or discomfort in the stomach, may be related to waste product accumulation
• Poor appetite: Decreased desire to eat, may reflect metabolic changes and waste product effects
• Muscle cramps: Painful muscle contractions, can be related to electrolyte imbalances
• Sleep problems: Difficulty sleeping or staying asleep, potentially due to metabolic disturbances
• Difficulty concentrating: Mental fogginess or trouble focusing, may be associated with waste product effects on brain function
• High blood pressure: Elevated blood pressure readings, could result from fluid retention and hormonal changes
• Itchy skin: Persistent itching, may be related to mineral imbalances and waste product buildup
• Metallic taste in mouth: Altered taste sensation, may be due to the accumulation of waste products

It's important to understand that kidney function can decline by up to 90% before symptoms become noticeable. This makes laboratory testing particularly valuable for early detection of kidney problems, when treatment is most effective. Many factors can cause these symptoms, and eGFR is just one piece of diagnostic information that should be interpreted alongside other clinical findings.

Clinical Ranges

Lab Reference Range: ≥90 mL/min/1.73 m²

Lifestyle Factors That Can Impact It

Your daily habits can have a significant effect on your eGFR. Ones that may benefit it include:

• Regular exercise improves blood flow to the kidneys and helps maintain healthy blood pressure
• Healthy weight maintenance reduces stress on kidneys and prevents obesity-related inflammation
• A low-salt diet helps control blood pressure and reduces kidney workload
• Adequate hydration ensures proper blood flow and helps kidneys filter waste efficiently
• Blood pressure control prevents damage to kidney blood vessels and maintains proper filtration
• Blood sugar control prevents diabetic damage to kidney filters and blood vessels
• Smoking cessation improves blood flow to the kidneys and reduces inflammatory damage
• Limited alcohol use prevents dehydration and reduces stress on kidney filtration

Habits that can harm eGFR:

• High salt intake increases blood pressure and forces kidneys to work harder to maintain fluid balance
• Dehydration reduces blood flow to kidneys and impairs their ability to filter waste
• Obesity increases inflammation and puts extra stress on kidney function
• Smoking damages blood vessels in the kidneys and reduces blood flow to filtering units
• Excessive alcohol causes dehydration and can directly damage kidney tissue
• A sedentary lifestyle reduces blood flow to the kidneys and contributes to obesity and high blood pressure
• Poor diabetes control and high blood sugar damage kidney filters and blood vessels over time
• High-protein diets can increase kidney workload and strain filtering capacity in people with existing kidney damage

Other Factors That Can Impact It

Medications

• Nonsteroidal anti-inflammatory drugs (NSAIDs): can lower eGFR by affecting kidney blood flow
• ACE inhibitors and ARBs: may lower eGFR while on the meds
• Certain antibiotics: can affect kidney function
• Contrast dye (used for some imaging tests): can temporarily decrease eGFR
• Diuretics: may affect readings through dehydration
• Metformin: may not be recommended with low eGFR

Medical Conditions

• Dehydration: can temporarily lower eGFR
• Cirrhosis: may affect proper estimation of kidney function
• Sepsis: can severely impact kidney function

Physical Factors

• Age: scores may naturally decline as you get older
• Muscle mass: can affect creatinine-based calculations
• Body fat %: can impact normal ranges
• Pregnancy: may increase eGFR
• Exercise: can cause temporary effects on creatinine readings

Testing Accuracy and Stability

The eGFR using cystatin C is typically accurate under proper testing conditions, but results can be impacted by hydration (dehydration can lower your eGFR).

How it Relates to Other Markers

Results from other tests can be combined with your eGFR to build a more complete picture of your kidney function.

• Creatinine: This is the primary component in eGFR calculation and directly affects results.
• Blood urea nitrogen (BUN): BUN provides additional information about kidney function and your hydration status.
• Cystatin C: It is an alternative kidney function marker less affected by muscle mass and creatine supplement intake, which can impact creatinine levels.
• Albumin/creatinine ratio: This test may indicate kidney damage through protein leakage.
• Electrolytes: An electrolyte test can show the kidney's ability to maintain the proper balance of minerals in the blood.
• Hemoglobin: A protein found in red blood cells, hemoglobin may decrease when there's kidney dysfunction.
• Vitamin D: Kidney dysfunction can affect the body's ability to convert vitamin D.
• Parathyroid hormone: This hormone may increase with declining kidney function.

What Results May Mean in the Context of Other Markers

• Low eGFR with high cystatin C: May suggest true kidney dysfunction rather than muscle effects.
• Low eGFR with normal cystatin C: May indicate effects of low muscle mass rather than kidney problems.
• Normal eGFR with high cystatin C: May indicate early kidney dysfunction requiring monitoring.
• Discordant results between equations: Consider repeating tests and investigating potential interferents.

Follow-up Considerations

You should always talk to your doctor if you have medical concerns or questions.

When Re-Testing May be Appropriate

• Mildly low (60 - < 90): Check to get a baseline and then consider annual testing if stable, more frequent if declining
• Moderately low (30-59): Consider testing every 3-4 months, or more often if changes in symptoms
• Severely low (< 30): May require weekly to monthly blood tests depending on stability and rate of decline

Additional Testing or Monitoring Your Doctor May Consider

Mildly low (60 - < 90):

• Basic metabolic panel
• Urinalysis
• Blood pressure monitoring
• Blood sugar testing, if diabetic

Moderately low (30-59):

• Kidney ultrasound or other imaging
• Spot urine protein/creatinine ratio
• Comprehensive metabolic panel
• Anemia workup
• Bone health markers
• Protein/microalbumin studies
• PTH and vitamin D levels
• Cardiovascular risk assessment

Severely low (< 30):

• All of the above plus:
• Regular cardiac evaluation
• Bone density scans
• Nutritional assessments
• Vascular access studies
• Regular PTH/calcium/phosphate monitoring
• Assessment for dialysis readiness

When Additional Care May be Warranted

Mildly low (60 - < 90):

• If eGFR drops more than 5 points in a year
• Development of new symptoms
• Persistent protein in urine
• Uncontrolled blood pressure

Moderately low (30-59):

• Immediate nephrology referral
• Any rapid decline in function
• New or worsening symptoms
• Development of complications
• Significant changes in urinary output

Severely low (< 30):

• Regular nephrology care required
• Emergency care if:
  • Severe symptoms develop
  • Significant fluid overload
  • Very high potassium
  • Mental status changes
  • Severe anemia
  • Uncontrolled blood pressure

Further Reading:

The 2024 Levels Guide to kidneys and metabolic health

Bibliography