Creatinine & GFR Calculator
Calculate your estimated glomerular filtration rate (eGFR) to assess kidney function
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Comprehensive Guide to Creatinine and Glomerular Filtration Rate (GFR)
The creatinine and calculated glomerular filtration rate (GFR) test is a fundamental tool in nephrology for assessing kidney function. This comprehensive guide explains what these tests measure, how they’re interpreted, and what the results mean for your health.
What is Creatinine?
Creatinine is a waste product produced by muscles from the breakdown of creatine phosphate during energy production. It’s filtered out of the blood by the kidneys and excreted in urine. Because creatinine production is relatively constant and proportional to muscle mass, its blood levels provide a reliable indicator of kidney function.
- Normal creatinine levels vary by age, sex, and body size:
- Men: 0.7 to 1.3 mg/dL
- Women: 0.6 to 1.1 mg/dL
- Elevated creatinine levels suggest impaired kidney function
- Low creatinine levels may indicate reduced muscle mass
What is Glomerular Filtration Rate (GFR)?
GFR measures how much blood passes through the glomeruli (tiny filters in the kidneys) each minute. It’s considered the best overall indicator of kidney function. The National Kidney Foundation recommends using GFR to:
- Screen for kidney disease
- Monitor known kidney disease progression
- Evaluate kidney function before prescribing certain medications
- Determine dosing for drugs cleared by the kidneys
How GFR is Calculated
The most common method for estimating GFR uses the CKD-EPI equation (Chronic Kidney Disease Epidemiology Collaboration), which our calculator implements. This equation considers:
- Serum creatinine level
- Age
- Sex
- Race (African American vs. other)
The formula differs slightly for males and females, and includes a correction factor for African Americans due to observed differences in creatinine generation.
GFR Stages and What They Mean
GFR results are categorized into stages that indicate the severity of kidney disease:
| GFR Stage | GFR Range (mL/min/1.73m²) | Description | Medical Interpretation |
|---|---|---|---|
| 1 | >90 | Normal kidney function | No kidney damage with normal or increased GFR |
| 2 | 60-89 | Mildly decreased function | Mild kidney damage with mildly decreased GFR |
| 3a | 45-59 | Mild to moderate decrease | Moderate kidney damage with moderate decrease in GFR |
| 3b | 30-44 | Moderate to severe decrease | Moderate to severe kidney damage |
| 4 | 15-29 | Severe decrease | Severe kidney damage (preparing for kidney failure) |
| 5 | <15 | Kidney failure | Established kidney failure (dialysis or transplant needed) |
Factors Affecting Creatinine and GFR
Several factors can influence your test results:
| Factor | Effect on Creatinine | Effect on GFR |
|---|---|---|
| Muscle mass | Higher muscle mass increases creatinine | No direct effect |
| Age | Decreases with age (less muscle mass) | Decreases with age (normal aging process) |
| Pregnancy | May decrease slightly | Increases by 40-50% during pregnancy |
| Diet (high protein) | May temporarily increase | No significant effect |
| Dehydration | May increase | May appear falsely low |
| Medications | Some may increase or decrease | Some may affect (e.g., NSAIDs decrease) |
When to Get Tested
The National Kidney Foundation recommends GFR testing if you have any of these risk factors:
- Diabetes (the leading cause of kidney disease)
- High blood pressure (second leading cause)
- Family history of kidney disease
- Age 60 or older
- Obese (BMI ≥ 30)
- African American, Hispanic, Native American, or Asian American ethnicity
- Previous kidney problems
- Taking medications that can affect the kidneys
You should also get tested if you experience symptoms of kidney disease:
- Fatigue or weakness
- Difficulty concentrating
- Poor appetite
- Trouble sleeping
- Muscle cramping at night
- Swollen feet and ankles
- Puffiness around eyes
- Dry, itchy skin
- Frequent urination (especially at night)
Improving Kidney Function
While you can’t reverse kidney damage, you can take steps to preserve remaining function:
- Control blood sugar if you have diabetes
- Manage blood pressure (target: <130/80 mmHg for most people with kidney disease)
- Follow a kidney-friendly diet:
- Limit sodium to <2,300 mg/day
- Reduce protein intake (0.6-0.8 g/kg body weight)
- Limit phosphorus (800-1,000 mg/day)
- Control potassium if levels are high
- Exercise regularly (30 minutes most days)
- Maintain healthy weight
- Avoid NSAIDs (ibuprofen, naproxen) if possible
- Don’t smoke
- Limit alcohol to ≤1 drink/day for women, ≤2 for men
- Stay hydrated but avoid excessive fluid intake
Limitations of GFR Estimation
While eGFR is extremely useful, it has some limitations:
- Less accurate in people with very high or very low muscle mass
- May overestimate GFR in obese individuals
- Less precise at GFR >60 mL/min/1.73m²
- Race correction factor is controversial (some labs have removed it)
- Doesn’t account for rapid changes in kidney function
- Can be affected by certain medications
For these reasons, eGFR should be interpreted along with other tests like:
- Urinalysis (checking for protein or blood in urine)
- Kidney ultrasound or CT scan
- Cystatin C test (alternative GFR marker)
- 24-hour urine collection for creatinine clearance
When to See a Nephrologist
You should consider seeing a kidney specialist (nephrologist) if:
- Your eGFR is <30 mL/min/1.73m²
- You have eGFR <60 with significant protein in urine
- Your eGFR is declining rapidly (>5 mL/min/year)
- You have difficult-to-control blood pressure
- You’re considering pregnancy with kidney disease
- You have genetic kidney disease (like polycystic kidney disease)
Frequently Asked Questions
Can GFR fluctuate?
Yes, GFR can vary slightly day-to-day due to:
- Hydration status
- Diet (especially protein intake)
- Exercise
- Illness or infection
- Menstrual cycle in women
Doctors look at trends over time rather than single measurements.
Is there a difference between GFR and eGFR?
Yes:
- GFR is the actual filtration rate measured by complex tests
- eGFR is an estimate calculated from creatinine using equations
For most clinical purposes, eGFR is sufficiently accurate and much more convenient.
Can you have normal creatinine but low GFR?
Yes, this can happen in:
- Elderly individuals (who have less muscle mass)
- People with malnutrition or muscle-wasting diseases
- Those with very low body weight
This is why eGFR equations include age, sex, and sometimes weight – to account for these variations.
What’s the connection between GFR and kidney disease?
Chronic Kidney Disease (CKD) is defined as:
- GFR <60 mL/min/1.73m² for ≥3 months, OR
- Markers of kidney damage (like protein in urine) for ≥3 months
The lower your GFR, the more severe your kidney disease. CKD is classified into stages based on GFR values as shown in the table above.
Authoritative Resources
For more information about creatinine and GFR testing: