The estimated Glomerular Filtration Rate (eGFR) assesses how well the kidneys are functioning. Laboratories report this value as a single number reflecting the kidneys’ filtering capacity. Derived from a simple blood test, eGFR is often the first indication of potential kidney impairment. Understanding the eGFR allows for the timely identification of chronic kidney disease (CKD) and guides necessary healthcare decisions.
Understanding the Estimated Glomerular Filtration Rate Calculation
The term “estimated” in eGFR is used because the result is mathematically derived rather than directly measured. The actual Glomerular Filtration Rate (GFR) is the total volume of blood filtered by the kidneys per minute. Since measuring the true GFR directly is complex and time-consuming, laboratories rely on equations for estimation.
The eGFR calculation is based on measuring serum creatinine, a waste product from normal muscle breakdown. Creatinine is filtered by the kidneys; thus, as kidney function declines, creatinine levels in the blood rise. The eGFR equation uses this serum creatinine level along with demographic factors to produce the estimated rate.
Modern calculations rely on the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation. This formula integrates a person’s age and sex alongside the measured creatinine value. Recent updates to the CKD-EPI equation removed the race variable to ensure a standardized assessment of kidney function.
The final eGFR value is standardized and expressed in milliliters per minute per 1.73 square meters (\(\text{mL/min/1.73m}^2\)). This standardization accounts for typical body surface area, making the result comparable across individuals. The equation may also incorporate cystatin C, another filtration marker, to improve accuracy, especially in individuals with unusual muscle mass.
Interpreting the EGFR Chart: Stages of Chronic Kidney Disease
The estimated Glomerular Filtration Rate classifies the severity of chronic kidney disease (CKD) using the Kidney Disease: Improving Global Outcomes (KDIGO) system. This classification uses five stages, labeled G1 through G5, based on the numerical eGFR range.
Stage G1 represents a normal filtration rate (90 \(\text{mL/min/1.73m}^2\) or greater). A CKD diagnosis at this stage requires other signs of kidney damage, such as protein in the urine. Stage G2 indicates mildly decreased function (60 to 89 \(\text{mL/min/1.73m}^2\)), also requiring evidence of damage for a CKD diagnosis.
Moderately decreased function is Stage G3, subdivided into G3a (45 to 59 \(\text{mL/min/1.73m}^2\)) and G3b (30 to 44 \(\text{mL/min/1.73m}^2\)). Most individuals diagnosed with CKD fall into this range, where complications like anemia or bone disease may begin to appear.
Stage G4 signifies severely decreased kidney function (15 to 29 \(\text{mL/min/1.73m}^2\)). At this point, planning for advanced care is initiated, as the kidneys struggle to filter waste products. The final stage, G5, is defined by an eGFR of less than 15 \(\text{mL/min/1.73m}^2\). This severe impairment indicates established kidney failure, often requiring dialysis or a kidney transplant.
Variables That Affect EGFR Results
While eGFR reliably indicates long-term kidney function, non-disease factors can cause temporary fluctuations or inaccurate readings. Because the calculation relies on serum creatinine, anything that acutely influences creatinine levels can skew the eGFR estimate.
A high-protein diet or creatine supplements before a blood test can transiently elevate serum creatinine. Since higher creatinine is interpreted as lower kidney function, the eGFR value may be artificially low. Conversely, individuals with very low muscle mass, such as those who are frail, may have creatinine levels lower than expected for their actual kidney function.
Hydration status is another factor, as dehydration can temporarily concentrate the blood, leading to a false increase in creatinine. Intense physical exercise within 24 hours of the blood draw can also cause a temporary rise due to muscle breakdown. These temporary changes do not reflect chronic kidney damage.
Certain medications can interfere with the eGFR result by affecting creatinine excretion or the laboratory measurement. Nonsteroidal anti-inflammatory drugs (NSAIDs), for example, can reduce blood flow to the kidneys, causing a reversible dip in the eGFR. Therefore, a single eGFR result should be interpreted alongside a person’s overall medical history.
Clinical Management and Monitoring of Kidney Function
A low or declining eGFR requires structured clinical management and monitoring to slow kidney damage progression. Following a concerning result, confirmatory testing is performed to ensure the finding is chronic, not temporary. This usually involves a repeat eGFR measurement and assessment of the urine albumin-to-creatinine ratio (uACR).
The uACR measures the amount of albumin leaking into the urine. Albuminuria is an early marker of kidney damage, even when the eGFR is near the normal range. Combining eGFR and uACR provides a complete picture of kidney health and the risk of disease progression.
Management focuses on controlling underlying conditions that drive kidney deterioration, primarily high blood pressure and diabetes. Maintaining blood pressure at an individualized target, often below 130/80 \(\text{mmHg}\), is a goal to protect the kidney filters. Medications like Angiotensin-Converting Enzyme (ACE) inhibitors or Angiotensin II Receptor Blockers (ARBs) are prescribed, offering protection against kidney damage, especially with albuminuria.
Monitoring frequency depends on the CKD stage and uACR result, with higher-risk categories requiring more frequent tests. Referral to a nephrologist is recommended when eGFR falls below 30 \(\text{mL/min/1.73m}^2\) (Stage G4) or when function declines rapidly. Lifestyle adjustments, including dietary changes to limit sodium and protein, are part of the strategy to preserve remaining kidney function.