What is the difference between Glomerular Filtration Rate (GFR) and creatinine clearance?

From the Guidelines

Glomerular filtration rate (GFR) and creatinine clearance are both measures of kidney function, but they differ in important ways, with GFR being the actual volume of fluid filtered by the kidneys per unit time and creatinine clearance being an estimate of GFR based on how efficiently the kidneys remove creatinine from the blood. The key differences between GFR and creatinine clearance are:

• GFR is the gold standard for assessing kidney function, while creatinine clearance is an estimate of GFR
• GFR is typically estimated using formulas like the CKD-EPI or MDRD equations, while creatinine clearance can be directly measured through a 24-hour urine collection or estimated using the Cockcroft-Gault formula
• Creatinine clearance tends to overestimate true GFR by about 10-20% because creatinine is not only filtered by the glomeruli but also secreted by the renal tubules Some important points to consider when using GFR and creatinine clearance in clinical practice are:
• The MDRD study equation is more accurate and precise than the Cockcroft-Gault equation for persons with a GFR less than approximately 90 mL/min per 1.73 m2 1
• The use of GFR (rather than serum creatinine measurement) to characterize kidney function is a critical element in the strategy for improving care of patients with chronic kidney disease 1
• Measurement of creatinine clearance by using timed (for example, 24-hour) urine collections does not provide more accurate estimates of GFR than do prediction equations 1 In summary, GFR is the preferred measure of kidney function, and it should be estimated using prediction equations like the MDRD study equation, rather than relying on serum creatinine concentration alone or measuring creatinine clearance.

From the Research

Difference between Glomerular Filtration Rate (GFR) and Creatinine Clearance

• GFR is the rate at which the glomerulus filters plasma to produce an ultrafiltrate, and it is used as an overall assessment of kidney functions 2.
• Creatinine clearance (CrCl) is a method for measuring GFR, which is calculated from the formula: CrCl (mL/min/1.73m2)=(urine volume×urine creatinine×1.73)/(serum creatinine×30 min×body surface area) 3.
• The main difference between GFR and CrCl is that GFR is a direct measure of kidney function, while CrCl is an indirect measure that estimates GFR based on creatinine levels in the urine and serum 4.
• CrCl has several limitations, including the need for a 24-hour urine collection, which can be inconvenient and prone to errors, and the fact that creatinine secretion can vary between individuals 5.

Comparison of GFR and Creatinine Clearance

• Studies have shown that estimated GFR (eGFR) using equations such as the Cockcroft-Gault (CG), Modification of Diet in Renal Disease (MDRD), and Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equations can provide a more accurate assessment of GFR than CrCl 4, 6.
• The CG equation has been shown to be positively biased, while the MDRD equation has been shown to be negatively biased 4.
• The use of CrCl remains questionable for clinical practice, and many clinical labs have switched to ID-GC/MS-standardized assays for the determination of serum and urine creatinine to improve analytical accuracy 5.

Clinical Implications

• The choice between GFR and CrCl should be based on the individual patient's needs and the clinical context, taking into account the limitations and biases of each method 6, 3.
• In critically ill patients with acute kidney injury, the use of CrCl as a reference method for assessing kidney function is not recommended due to its poor precision and high error rate 3.
• The use of eGFR equations, such as the CKD-EPI equation, may provide a more accurate assessment of GFR in these patients, but individual-level inaccuracies can still occur 6.