What is the creatinine (Cr) clearance?

From the Guidelines

The most accurate method to estimate creatinine clearance (CrCl) is the Cockcroft-Gault formula, which is calculated as Estimated CrCl (ml/min) = [(140 - age) · weight]/[72 · SCr (mg/dl)] (· 0.85 if female) 1. This formula is widely used in clinical practice due to its simplicity and relatively accurate estimation of renal function. The Cockcroft-Gault formula requires serum creatinine, age, weight, and gender to estimate CrCl. Other formulas, such as Jelliffe, Wright, and MDRD, can also be used to estimate CrCl, but the Cockcroft-Gault formula is generally considered the most practical and reliable method for clinical use 1. Some key points to consider when estimating CrCl include:

• Serum creatinine (SCr) should be measured in mg/dl, and the formula should be adjusted accordingly.
• The formula should be adjusted for female patients by multiplying the result by 0.85.
• The Cockcroft-Gault formula provides a relatively accurate estimate of CrCl, but it may not be suitable for patients with severe renal failure or decreased muscle mass 1. It is essential to note that CrCl is only a crude measure of renal function, and it may not accurately reflect the glomerular filtration rate (GFR) in patients with declining renal function 1. However, the Cockcroft-Gault formula remains the most widely used and accepted method for estimating CrCl in clinical practice. Some of the advantages of using the Cockcroft-Gault formula include:
• It is relatively simple to calculate and requires minimal laboratory measurements.
• It provides a relatively accurate estimate of CrCl, which is essential for assessing kidney function and guiding medication dosing.
• It is widely used and accepted in clinical practice, making it a useful tool for healthcare professionals.

From the Research

Creatinine Clearance Estimates

• The Cockcroft-Gault (C-G) and MDRD study equations are commonly used to estimate glomerular filtration rate (GFR) as a function of serum creatinine 2.
• However, these equations require complex calculations and several parameters, leading to the development of simpler equations such as eCCr (male) = weight/ creatinine and eCCr (female) = weight x 0.84/creatinine 2.
• The MDRD study and Cockcroft-Gault equations provide useful estimates of GFR in adults, while the Schwartz and Counahan-Barratt equations are useful in children 3.

Comparison of Estimation Methods

• A study compared the use of estimated GFR with estimated creatinine clearance (CrCl) calculated using the Cockcroft-Gault equation in the dosing of drugs requiring adjustments in elderly patients with declining renal function 4.
• The study found that substantial mathematical differences are inherent in these two prediction equations, making clinical comparison difficult, and suggested that the Cockcroft-Gault equation should still be used for drug dosage adjustments 4.
• Another study reviewed the derivation of the Cockroft and Gault formula and found that it may have been mathematically invalid, but its role in estimating renal function at the bedside is established 5.

Factors Affecting Creatinine Clearance

• Age and body weight must be included for reasonable prediction of creatinine clearance 6.
• Serum creatinine concentration should not be used alone to assess the level of kidney function, and clinical laboratories should report an estimate of GFR using a prediction equation, in addition to reporting the creatinine measurements 3.
• Measurement of creatinine clearance using timed (e.g., 24-hour) urine collections does not improve the estimate of GFR over that provided by prediction equations, but may be useful in individuals with exceptional dietary intake or muscle mass 3.