What is Estimated Glomerular Filtration Rate?
The glomerulus is a microscopic network of blood vessels within each kidney where filtration begins. eGFR estimates how many millilitres of blood these nephron units filter per minute, standardized to 1.73 m² body surface area. This standardization allows direct comparison across patients regardless of height or weight.
eGFR reflects kidney function more reliably than creatinine alone, as the latter varies with muscle mass, age, and diet. Values are categorized into five stages of kidney disease:
- Stage 1 (≥90 mL/min/1.73 m²): Normal or mildly reduced function with no or minimal kidney damage.
- Stage 2 (60–89): Mildly reduced function; kidney damage present but asymptomatic.
- Stage 3a (45–59): Mild to moderate reduction; patients may first notice fatigue or altered urination patterns.
- Stage 3b (30–44): Moderate to severe reduction; risk of complications increases substantially.
- Stage 4 (15–29): Severe reduction; preparation for renal replacement therapy may begin.
- Stage 5 (<15): Kidney failure; dialysis or transplantation typically required.
Equations for eGFR Estimation
Four validated equations are recommended by the National Institute of Diabetes and Digestive and Kidney Diseases. Each incorporates serum creatinine (SCr) in mg/dL or serum cystatin C (Scys) in mg/L, along with demographic adjustment factors for age, sex, and race.
CKD-EPI Creatinine (2009):
eGFR = 141 × min(SCr/κ, 1)^α × max(SCr/κ, 1)^−1.209 × 0.993^Age × sex_coef × race_coef
CKD-EPI Cystatin C (2012):
eGFR = 133 × min(Scys/0.8, 1)^−0.499 × max(Scys/0.8, 1)^−1.328 × 0.996^Age × sex_coef
CKD-EPI Combined Creatinine-Cystatin C (2012):
eGFR = 135 × min(SCr/κ, 1)^β1 × max(SCr/κ, 1)^−0.601 × min(Scys/0.8, 1)^−0.375 × max(Scys/0.8, 1)^−0.711 × 0.995^Age × sex_coef × race_coef
MDRD (4-variable):
eGFR = 175 × SCr^−1.154 × Age^−0.203 × sex_coef × race_coef
SCr— Serum creatinine concentration in mg/dL; higher values indicate reduced filtration.Scys— Serum cystatin C concentration in mg/L; independent of muscle mass and dietary protein.Age— Patient age in years; eGFR naturally declines approximately 1 mL/min/1.73 m² per year after age 30.κ (kappa)— Sex-specific creatinine standardization constant: 0.7 for females, 0.9 for males.α (alpha)— Sex-specific exponent: −0.329 for females, −0.411 for males.sex_coef— Adjustment coefficient (0.988 for females in CKD-EPI Creatinine; 1.018 for females in CKD-EPI Cystatin).race_coef— African ancestry coefficient: 1.159 multiplier for Black individuals in older equations; 1.0 for other groups.
Interpreting Your eGFR Result
eGFR interpretation requires clinical context. A single measurement below 60 mL/min/1.73 m² warrants confirmation by repeat testing, as acute illness, dehydration, or medication effects can temporarily reduce filtration. Persistent reduction over three months confirms chronic kidney disease.
Age affects interpretation significantly. Healthy older adults (≥65 years) may have eGFR of 45–59 with normal renal ageing, whereas the same value in a 40-year-old suggests pathology. Conversely, young athletic individuals with high muscle mass may have eGFR above 100 despite normal function.
Race-based adjustment coefficients in the MDRD and 2009 CKD-EPI equations have drawn scrutiny; some evidence suggests these multipliers may mask disease burden in Black populations. The 2021 CKD-EPI race-free equation and discussion with your clinician about individual factors provide more nuanced assessment.
Key Considerations When Using eGFR
Several clinical and laboratory factors influence eGFR accuracy and interpretation.
- Muscle mass affects creatinine-based estimates — Athletes, bodybuilders, and very muscular individuals may have falsely low eGFR despite healthy kidneys. Conversely, elderly or severely debilitated patients may have artificially preserved eGFR. Cystatin C-based or combined equations often perform better in these populations.
- Acute changes warrant serial measurement — Illness, dehydration, major surgery, or new medications can temporarily lower eGFR. Do not diagnose chronic kidney disease on a single abnormal result. Most guidelines recommend confirming eGFR reduction on a second test weeks to months later.
- Laboratory variability exists across centres — Different assays for creatinine and cystatin C produce slightly different reference ranges. Always compare your result to the range provided by your testing laboratory rather than generic cutoffs.
- Medication interactions influence interpretation — ACE inhibitors, ARBs, NSAIDs, and contrast agents can acutely reduce eGFR. If eGFR drops sharply after starting a new drug, alert your doctor. This may represent reversible change rather than progressive disease.
Managing Chronic Kidney Disease
When eGFR falls below 60 mL/min/1.73 m², targeted management slows progression. Diabetes and hypertension account for two-thirds of CKD cases; rigorous glycaemic and blood-pressure control (target <130/80 mmHg) reduces decline by 20–30%.
Lifestyle modifications include a kidney-friendly diet low in sodium (under 2,300 mg/day) and phosphorus, adequate protein intake (0.8 g/kg body weight unless Stage 4–5), and avoidance of nephrotoxic drugs (NSAIDs, certain antibiotics). Regular exercise and smoking cessation also improve outcomes.
Medications such as ACE inhibitors or angiotensin II receptor blockers provide renoprotection beyond blood-pressure lowering and are first-line therapy in diabetes-related CKD. SGLT2 inhibitors have recently shown cardiovascular and renal benefits even in non-diabetic CKD. Referral to a nephrologist is recommended once eGFR drops below 30 mL/min/1.73 m² to prepare for renal replacement therapy if needed.