The kidneys filter waste products from the blood, balance fluids, and regulate electrolytes. They also produce hormones that control blood pressure, red blood cell production, and activate vitamin D. Assessing kidney function is important for identifying health issues and guiding medical interventions.
The Role of GFR in Kidney Health
Glomerular Filtration Rate (GFR) represents the flow rate of fluid filtered by the glomeruli in the kidneys. This measurement indicates how effectively the kidneys remove waste from the blood. GFR is considered the most reliable indicator of overall kidney function.
Healthcare providers typically estimate GFR (eGFR) rather than directly measuring it, as direct measurement is complex and time-consuming. The eGFR is calculated using a blood test that measures serum creatinine levels. This calculation also incorporates individual factors such as age and sex to provide a more accurate estimation. The eGFR helps diagnose kidney problems, including chronic kidney disease, and determines its progression.
Creatinine Clearance and Its Measurement
Creatinine is a waste product formed from the breakdown of muscle tissue. Healthy kidneys filter creatinine from the blood, which is then excreted in urine. A creatinine clearance test measures the volume of blood plasma cleared of creatinine per unit of time.
Creatinine clearance is traditionally measured through a 24-hour urine collection, alongside a blood sample. This approach is often inconvenient for patients and prone to inaccuracies if the urine collection is incomplete. Due to these challenges, this method is less frequently used for routine assessment compared to estimated GFR.
Comparing GFR and Creatinine Clearance
Although both GFR and creatinine clearance assess kidney function, they are not identical. GFR measures the actual rate at which blood is filtered by the glomeruli, serving as the gold standard for kidney function. Creatinine clearance, while a useful estimate, tends to slightly overestimate the true GFR.
This overestimation occurs because creatinine is not only filtered by the glomeruli but also actively secreted into the urine by the renal tubules. For this reason, and because eGFR calculations are more convenient and less prone to collection errors, eGFR has largely replaced creatinine clearance as the preferred initial assessment. However, creatinine clearance may still be used in specific clinical situations or for precise drug dosing adjustments, especially when serum creatinine levels are unusual.
Interpreting Results for Kidney Function
Both GFR and creatinine clearance values provide insights into kidney health, helping to diagnose and stage kidney disease. Normal GFR values range from 90 to 120 mL/min/1.73 m², though GFR naturally declines with age. Values below 60 mL/min/1.73 m² for three months or more indicate chronic kidney disease.
Kidney disease is categorized into five stages based on GFR levels, with lower values indicating more significant impairment. A GFR below 15 mL/min/1.73 m² signifies kidney failure, potentially requiring dialysis or a transplant. Doctors use these test results to monitor kidney function, detect early signs of disease, and guide treatment decisions. Factors such as muscle mass, diet, hydration levels, and certain medications can influence creatinine levels and the accuracy of both tests.