GFR Calculator

Estimate glomerular filtration rate to assess kidney function accurately

🩺 GFR Calculator

Estimate glomerular filtration rate to assess kidney function

What is a GFR Calculator?

A GFR (Glomerular Filtration Rate) calculator is a medical tool that estimates how well your kidneys are filtering waste from your blood. GFR is the primary metric used to assess kidney function and is essential for diagnosing, staging, and monitoring chronic kidney disease (CKD).

The calculator uses variables such as age, sex, blood creatinine levels (and sometimes cystatin C), height, and weight to estimate your kidney function. Different equations are available for adults and children, with the CKD-EPI equation being the most widely recommended for adults.

Regular GFR monitoring is crucial for early detection of kidney disease, guiding treatment decisions, and adjusting medications that are cleared by the kidneys. It's an essential tool for both healthcare providers and patients managing kidney health.

How to Use the GFR Calculator

Select whether you're calculating for an adult or child using the tabs above
Enter your serum creatinine level (available from a blood test) and select the unit (mg/dL or µmol/L)
For adults: Enter your age and select your gender and race. For children: Enter height in cm or inches
Click 'Calculate GFR' to see your estimated glomerular filtration rate
Review your results, including the CKD stage interpretation and recommendations. Always consult with a healthcare professional for medical advice

Key Insights About GFR

GFR is the primary metric for assessing kidney function and is used to diagnose, stage, and monitor chronic kidney disease (CKD), as well as to guide treatment decisions such as medication dosing, dialysis, or kidney transplant
Recent guidelines recommend using race-free equations (such as the 2021 CKD-EPI equation) and, when possible, combining serum creatinine and cystatin C for greater accuracy, especially near clinical decision thresholds
GFR calculators provide estimates, not direct measurements, and may be less accurate at higher GFR values or in certain populations (e.g., extremes of age, muscle mass, or ethnicity)
Always interpret GFR in context (age, sex, body size), use the same equation for longitudinal monitoring, and supplement with urine albumin/creatinine ratio for a complete assessment
GFR calculators are essential for early detection of CKD, monitoring disease progression, and adjusting medications that are cleared by the kidneys
A normal GFR is typically 90 mL/min/1.73m² or higher, but values can vary with age. A GFR below 60 mL/min/1.73m² for three months or more indicates chronic kidney disease

Understanding GFR and Kidney Function

What is GFR?

Glomerular Filtration Rate (GFR) is a measure of how much blood passes through the glomeruli (tiny filters in the kidneys) each minute. The glomeruli filter waste and excess fluid from the blood, which then becomes urine. A normal GFR varies by age, sex, and body size, but generally, a GFR of 90 mL/min/1.73m² or higher is considered normal in healthy adults.

GFR Calculation Equations

Several equations are used to estimate GFR, each with its own strengths:

CKD-EPI (2021): The most widely recommended equation for adults. It's race-free and provides accurate estimates across different populations and GFR ranges.
MDRD: An older equation that was commonly used but is now being replaced by CKD-EPI. It may underestimate GFR at higher values.
Mayo Quadratic: Developed for use in potential kidney donors and may be more accurate for people with normal or high kidney function.
Schwartz Formula: Specifically designed for children and uses height and serum creatinine to estimate GFR.

CKD Stages Based on GFR

Stage 1 (GFR ≥90): Normal or high kidney function. If there are no other signs of kidney damage, this is healthy.
Stage 2 (GFR 60-89): Mildly decreased kidney function. May be normal for older adults.
Stage 3a (GFR 45-59): Mild to moderately decreased kidney function. Medical evaluation recommended.
Stage 3b (GFR 30-44): Moderately to severely decreased kidney function. Closer monitoring needed.
Stage 4 (GFR 15-29): Severely decreased kidney function. Preparation for kidney replacement therapy may be needed.
Stage 5 (GFR <15): Kidney failure. Dialysis or kidney transplant is typically required.

Clinical Applications of GFR

GFR is used in various clinical scenarios:

Diagnosing chronic kidney disease and determining its stage
Monitoring kidney function over time in patients with CKD, diabetes, or hypertension
Adjusting medication dosages for drugs that are cleared by the kidneys
Evaluating potential kidney donors
Determining when to initiate dialysis or consider kidney transplantation

Best Practices for GFR Monitoring

Always interpret GFR in the context of age, sex, body size, and clinical history
Use the same equation consistently for longitudinal monitoring to track changes over time
Supplement GFR with urine albumin-to-creatinine ratio (UACR) for a complete kidney health assessment
When possible, use both serum creatinine and cystatin C for greater accuracy, especially near clinical decision thresholds
Repeat abnormal GFR measurements after 3 months to confirm chronic kidney disease diagnosis

Limitations of GFR Calculators

While GFR calculators are valuable tools, they have some limitations:

They provide estimates, not direct measurements of kidney function
May be less accurate at higher GFR values (>60 mL/min/1.73m²)
Can be affected by extremes of muscle mass, body size, or dietary protein intake
May not be accurate in certain populations, such as pregnant women, children with unusual body composition, or people with amputations
Should not be used alone to diagnose kidney disease - clinical context and other tests are essential

Frequently Asked Questions

What is a normal GFR?

A normal GFR is typically 90 mL/min/1.73m² or higher in healthy adults. However, GFR naturally declines with age, and values between 60-89 may be normal for older adults if there are no other signs of kidney damage.

How often should I check my GFR?

The frequency depends on your health status. People with risk factors for kidney disease (diabetes, high blood pressure, family history) should have their GFR checked at least annually. Those with diagnosed CKD may need more frequent monitoring as recommended by their healthcare provider.

Can GFR improve over time?

In some cases, yes. If kidney damage is caught early and the underlying cause is treated (such as controlling blood sugar in diabetes or blood pressure in hypertension), GFR can stabilize or even improve. However, advanced kidney disease is usually progressive.

What's the difference between GFR and eGFR?

GFR is the actual glomerular filtration rate, which can only be measured directly through complex procedures. eGFR (estimated GFR) is calculated using equations based on blood tests and is the standard method used in clinical practice.

Why do different equations give different results?

Different equations use different variables and were developed using different populations. The CKD-EPI equation is currently recommended as the most accurate for most adults. Variations between equations are usually small and don't change clinical management.

Should I use the race-adjusted or race-free equation?

Current guidelines recommend using race-free equations (2021 CKD-EPI) as they provide more equitable care and avoid potential biases. Our calculator uses the most current race-free recommendations.

What should I do if my GFR is low?

If your GFR is below 60 mL/min/1.73m², consult with a healthcare provider. They will evaluate your complete medical history, perform additional tests, and may refer you to a nephrologist (kidney specialist) for further evaluation and management.

References

1. UPMC - Glomerular Filtration Rate (GFR) Calculator and FAQ

2. NIDDK - Estimated Glomerular Filtration Rate Calculators

3. Cleveland Clinic - Estimated Glomerular Filtration Rate (eGFR)

4. National Kidney Foundation - eGFR Calculator - National Kidney Foundation

5. National Kidney Foundation - Estimated Glomerular Filtration Rate (eGFR) - Understanding

6. DaVita - GFR Calculator (Glomerular Filtration Rate)