eGFR Calculator
Estimate your glomerular filtration rate using the CKD-EPI equation
Comprehensive Guide: How eGFR is Calculated
The estimated glomerular filtration rate (eGFR) is the best overall measure of kidney function. It estimates how much blood passes through the glomeruli (tiny filters in the kidneys) each minute. Healthcare providers use eGFR to screen for, detect, and monitor chronic kidney disease (CKD).
What is eGFR?
eGFR stands for estimated glomerular filtration rate. It’s a calculated value that:
- Estimates how well your kidneys are filtering blood
- Is reported in mL/min/1.73m² (milliliters per minute per 1.73 square meters of body surface area)
- Helps determine your stage of kidney disease
- Guides treatment decisions for kidney-related conditions
The CKD-EPI Equation: The Gold Standard
The most accurate formula currently used is the CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration) equation, which was developed in 2009 and updated in 2021. This equation is more accurate than the older MDRD formula, especially for people with normal or near-normal kidney function.
The CKD-EPI equation considers:
- Serum creatinine level (mg/dL)
- Age (years)
- Sex (male or female)
- Race (Black or non-Black)
The 2021 update removed the race coefficient, but our calculator includes both options for clinical context. The standard CKD-EPI equation (for non-Black patients) is:
For females with creatinine ≤ 0.7 mg/dL:
eGFR = 144 × (creatinine/0.7)-0.328 × (0.993)age
For females with creatinine > 0.7 mg/dL:
eGFR = 144 × (creatinine/0.7)-1.209 × (0.993)age
For males with creatinine ≤ 0.9 mg/dL:
eGFR = 141 × (creatinine/0.9)-0.411 × (0.993)age
For males with creatinine > 0.9 mg/dL:
eGFR = 141 × (creatinine/0.9)-1.209 × (0.993)age
Understanding Your eGFR Results
Your eGFR number helps determine your stage of kidney disease:
| Stage | eGFR (mL/min/1.73m²) | Description | Actions |
|---|---|---|---|
| 1 | ≥90 | Normal kidney function | Maintain healthy lifestyle |
| 2 | 60-89 | Mildly reduced function | Monitor, reduce risk factors |
| 3a | 45-59 | Mild to moderate reduction | Evaluate and treat complications |
| 3b | 30-44 | Moderate to severe reduction | Prepare for kidney failure |
| 4 | 15-29 | Severe reduction | Plan for kidney replacement |
| 5 | <15 | Kidney failure | Start kidney replacement |
Factors That Affect eGFR Accuracy
While eGFR is the best available measure of kidney function, several factors can affect its accuracy:
- Muscle mass: Creatinine comes from muscle breakdown, so people with very high or very low muscle mass may have inaccurate eGFR results
- Diet: High protein intake can temporarily increase creatinine levels
- Medications: Some drugs like cimetidine and trimethoprim can affect creatinine levels
- Acute illness: Severe infections or other acute conditions can temporarily alter kidney function
- Pregnancy: eGFR normally increases during pregnancy
- Extreme body sizes: The standard eGFR is normalized to 1.73m² body surface area
Alternative Methods for Measuring GFR
While eGFR is the most common method, there are other ways to measure glomerular filtration rate:
| Method | Description | Accuracy | Use Cases |
|---|---|---|---|
| Measured GFR (mGFR) | Uses injected markers like iohexol or iothalamate that are cleared only by the kidneys | Gold standard | Research, clinical trials, when precise measurement is needed |
| Cystatin C | Blood test that measures a protein filtered by the kidneys | Comparable to creatinine-based eGFR | Confirming eGFR when creatinine may be unreliable |
| 24-hour urine collection | Measures creatinine clearance over 24 hours | Less accurate than mGFR but better than eGFR in some cases | When eGFR may be unreliable (extreme muscle mass, etc.) |
| BUN (Blood Urea Nitrogen) | Measures urea nitrogen in blood | Less accurate for GFR estimation | General kidney function screening |
When to See a Doctor About Your eGFR
You should consult a healthcare provider if:
- Your eGFR is less than 60 for 3 months or more (possible CKD)
- Your eGFR drops by 25% or more in a short period
- You have symptoms like swelling, fatigue, or frequent urination
- You have risk factors like diabetes, high blood pressure, or family history of kidney disease
- Your eGFR is normal but you have protein in your urine
Improving and Maintaining Kidney Function
If your eGFR shows reduced kidney function, these strategies may help:
- Control blood sugar: If you have diabetes, keeping A1C below 7% can protect kidneys
- Manage blood pressure: Target below 120/80 mmHg (or as recommended by your doctor)
- Follow a kidney-friendly diet: Limit sodium, phosphorus, and potassium if needed
- Stay hydrated: Drink enough water but avoid excessive fluid intake
- Exercise regularly: Aim for 150 minutes of moderate activity per week
- Avoid NSAIDs: Over-the-counter pain relievers like ibuprofen can harm kidneys
- Don’t smoke: Smoking damages blood vessels and worsens kidney disease
- Limit alcohol: No more than 1 drink per day for women, 2 for men
- Maintain healthy weight: Obesity increases risk of kidney disease
- Get regular check-ups: Monitor eGFR and other kidney function tests
Common Questions About eGFR
Q: Can eGFR fluctuate?
A: Yes, eGFR can vary slightly due to hydration status, diet, or acute illnesses. However, significant changes over time may indicate kidney problems.
Q: Is a high eGFR good?
A: While high eGFR (above 90) generally indicates good kidney function, extremely high values (above 120) might suggest hyperfiltration, which can be harmful over time.
Q: Can you have kidney disease with normal eGFR?
A: Yes, especially if you have protein in your urine (albuminuria). That’s why doctors often check both eGFR and urine albumin-to-creatinine ratio (UACR).
Q: How often should eGFR be checked?
A: It depends on your risk factors. People with diabetes or hypertension should have it checked annually. Those with known kidney disease may need tests every 3-6 months.
Q: Does eGFR change with age?
A: Yes, eGFR normally declines with age. After age 40, GFR decreases by about 1 mL/min/1.73m² per year in healthy individuals.