The Estimated Glomerular Filtration Rate, or eGFR, is a crucial metric that provides the best available estimate of how effectively your kidneys are working to clean your blood. This calculation is derived from a simple blood test and is universally recognized as the primary tool for screening, diagnosing, and monitoring kidney disease. It essentially estimates the volume of fluid filtered by the kidneys per minute. The eGFR offers healthcare providers a single, standardized number that quickly reflects the functional status of this organ system.
The Role of Glomerular Filtration
The kidneys are complex organs responsible for maintaining the balance of fluid and electrolytes in the body, while also filtering waste products from the bloodstream. Within each kidney are millions of tiny structures called nephrons, each containing a cluster of specialized capillaries known as the glomerulus. The glomerulus acts as the primary filtering unit, where blood pressure forces water and small solutes out of the blood and into the kidney tubules. The Glomerular Filtration Rate (GFR) is the actual volume of blood plasma filtered by these glomeruli over a specific time. Directly measuring the GFR is complex and impractical for routine clinical use, so the Estimated GFR (eGFR) is used as a highly reliable substitute that approximates this filtration capability.
How eGFR is Determined
The eGFR is not measured directly, but is calculated using a mathematical formula based on the concentration of creatinine in your blood. Creatinine is a waste product of normal muscle metabolism produced at a relatively constant rate. Healthy kidneys efficiently remove creatinine from the blood and excrete it in the urine. If kidney function declines, creatinine clearance slows down, causing its level to build up in the bloodstream.
The calculation combines the measured serum creatinine level with demographic factors like age and sex to improve accuracy, since muscle mass varies between these groups. Common formulas, such as the CKD-EPI equation, are used by laboratory computers to automatically generate the eGFR number. This result is reported in milliliters per minute per 1.73 square meters (\(\text{mL}/\text{min}/1.73\text{m}^2\)), where the inclusion of body surface area standardizes the result for comparison across individuals.
Interpreting Your eGFR Score
The eGFR score is used to classify the stage of Chronic Kidney Disease (CKD), providing a framework for management and prognosis. A normal eGFR for a young, healthy adult is typically 90 \(\text{mL}/\text{min}/1.73\text{m}^2\) or higher, though the rate naturally decreases with age. The most significant threshold is 60 \(\text{mL}/\text{min}/1.73\text{m}^2\); a value below this level sustained for three months or more indicates the presence of CKD. This sustained low eGFR distinguishes CKD from temporary conditions like acute kidney injury.
CKD Stages by eGFR
Stage 1 (90 or above) is only diagnosed if other signs of kidney damage, like protein in the urine, are present. Stage 2 (60–89) also requires evidence of damage for a CKD diagnosis. Stage 3 represents a moderate to severe loss of function and is further divided into Stage 3a (45–59) and Stage 3b (30–44), which is often when symptoms may first appear. Stage 4 (15–29) indicates severely reduced kidney function. Stage 5 (Below 15) represents established kidney failure and often necessitates a decision regarding dialysis or transplant.
Factors Influencing eGFR Accuracy and Rate
Because eGFR is an estimate based on a formula, its accuracy can be affected by factors that influence serum creatinine levels. People with significantly higher-than-average muscle mass, such as bodybuilders, produce more creatinine, which can lead to a falsely low eGFR score. Conversely, individuals with very low muscle mass, like frail older adults or amputees, may have a falsely high eGFR, masking actual kidney impairment.
Dietary habits can also create temporary fluctuations; eating a large amount of cooked meat before a blood test can briefly elevate creatinine levels. Short-term dehydration concentrates the blood and can cause a transient drop in the eGFR, which typically resolves once fluid balance is restored. Additionally, certain medications, including some non-steroidal anti-inflammatory drugs (NSAIDs), can interfere with creatinine secretion or temporarily reduce the GFR. These variables emphasize why healthcare providers look for consistent trends in eGFR over time, rather than reacting to a single result.
Next Steps Following a Low eGFR
Receiving a low eGFR result is an indication for further investigation and monitoring, not necessarily an immediate cause for alarm. The first step is often to repeat the blood test after a few weeks to confirm the low number is persistent and not due to a temporary factor like dehydration or diet. If confirmed, additional tests, such as a urine albumin-to-creatinine ratio (uACR), are used to check for protein in the urine, a direct marker of kidney damage. Management focuses on controlling underlying conditions like high blood pressure and diabetes. Lifestyle adjustments, including maintaining a healthy weight, increasing physical activity, and adopting a low-salt diet, can help slow the rate of decline. Referral to a nephrologist is typically advised when the eGFR falls below 30 \(\text{mL}/\text{min}/1.73\text{m}^2\) (Stage 4), or if there is a rapid, sustained decline.