The Estimated Glomerular Filtration Rate (eGFR) is a measure of how well the kidneys are working. It is the most commonly used test to screen for, diagnose, and monitor Chronic Kidney Disease (CKD). It estimates the amount of blood passing through the glomeruli each minute.
How is eGFR Calculated?
The CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration) equation is the current standard recommended by the National Kidney Foundation (NKF) and the American Society of Nephrology (ASN). In 2021, the equation was updated to remove the race variable to ensure equity in kidney disease diagnosis.
Where: Scr is serum creatinine (mg/dL); κ is 0.7 for females and 0.9 for males; α is -0.241 for females and -0.302 for males.
Standard CKD Stages
The KDIGO (Kidney Disease: Improving Global Outcomes) guidelines classify CKD into stages based on eGFR, which help guide clinical management and therapy.
| Stage | eGFR (mL/min/1.73m²) | Description |
|---|---|---|
| Stage 1 | ≥ 90 | Kidney damage with normal or high GFR |
| Stage 2 | 60 – 89 | Kidney damage with mildly decreased GFR |
| Stage 3a | 45 – 59 | Mild to moderately decreased GFR |
| Stage 3b | 30 – 44 | Moderate to severely decreased GFR |
| Stage 4 | 15 – 29 | Severely decreased GFR |
| Stage 5 | < 15 | Kidney failure (ESRD) |
Clinical Examples and Scenarios
Let's look at practical examples of how eGFR is utilized in clinical decision making.
Example 1: Routine Screening
Profile: Mark, a 45-year-old male with hypertension.
- Creatinine: 0.9 mg/dL
- Age: 45
- Calculation (CKD-EPI): eGFR = 106 mL/min/1.73m²
- Interpretation: Mark has a normal eGFR (>90). Assuming no other markers of kidney damage (like proteinuria), his kidney function is considered normal.
Example 2: Medication Adjustments
Profile: Sarah, a 65-year-old female requiring a new medication cleared by the kidneys.
- Creatinine: 1.5 mg/dL
- Age: 65
- Calculation (CKD-EPI): eGFR = 39 mL/min/1.73m²
- Interpretation: Sarah is in Stage 3b CKD. The physician must check the medication's monograph and adjust the dose according to her eGFR to avoid toxicity.
Limitations of eGFR
While the CKD-EPI equation is robust for the general adult population, it relies on serum creatinine, which is a breakdown product of muscle metabolism. Therefore, it has limitations in certain populations:
- Extremes of Muscle Mass: Bodybuilders, amputees, or individuals with severe muscle wasting (e.g., malnutrition, paraplegia) may have inaccurate eGFR readings.
- Dietary Intake: High intake of cooked meat or creatine supplements can temporarily increase serum creatinine.
- Rapidly Changing Kidney Function: In Acute Kidney Injury (AKI), serum creatinine is not at a steady state, making eGFR equations inaccurate.
- Pregnancy: Glomerular filtration rate increases significantly during pregnancy, and standard eGFR equations are not validated for this population.
When to Consult a Nephrologist
A referral to a nephrologist is typically recommended when eGFR falls below 30 mL/min/1.73m² (Stage 4 CKD), when there is a rapid decline in eGFR (a drop of >5 mL/min/1.73m² in one year), or in the presence of significant proteinuria or hematuria.
References
- Inker LA, Eneanya ND, Coresh J, et al. New Creatinine- and Cystatin C-Based Equations to Estimate GFR without Race. N Engl J Med. 2021.
- Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. KDIGO 2012 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney inter. Suppl. 2013;3:1-150.
- National Kidney Foundation. Estimated Glomerular Filtration Rate (eGFR).