From the Guidelines
To figure out creatinine clearance, you can estimate it using the Cockcroft-Gault equation: CrCl = [(140 - age) × weight (kg)] / (72 × serum creatinine [mg/dL]), multiplied by 0.85 for females, as recommended by 1 and 1. This equation provides a superior estimate of renal function compared to using serum creatinine alone, and is widely used in clinical practice.
- The Cockcroft-Gault equation is a useful tool for estimating creatinine clearance, but it has its limitations, such as underestimating GFR for normal and moderately reduced levels of renal function, and overestimating renal function for patients with significantly impaired renal function 1.
- For more accuracy, use actual body weight for underweight patients, ideal body weight for normal weight patients, and adjusted body weight for obese patients.
- Creatinine clearance is important for assessing kidney function and adjusting medication dosages that are cleared by the kidneys, as highlighted in 1 and 1.
- The normal range is typically 90-140 mL/min for men and 80-125 mL/min for women, with values decreasing with age.
- It is essential to consider the patient's age, sex, weight, and serum creatinine level when estimating creatinine clearance, as these factors can significantly impact the accuracy of the calculation 1.
From the FDA Drug Label
Renal clearance (see Table 4) is approximately 3. 5 times greater than creatinine clearance, which indicates that tubular secretion is the major route of metformin elimination. Table 4: ... Renal Clearance (mL/min) ... Healthy, nondiabetic adults: 500 mg single dose (24) ... 600 (±132) ... Subject Groups: ... Renal-impaired adults: 850 mg single dose Mild (CLcr g 61 to 90 mL/min) (5) ... 384 (±122)
To figure out a creatinine clearance, we can use the information that renal clearance is approximately 3.5 times greater than creatinine clearance.
- If the renal clearance is 600 mL/min, then the creatinine clearance would be approximately 600 / 3.5 = 171 mL/min.
- For a mild renal impairment with a creatinine clearance of 61 to 90 mL/min, the renal clearance would be approximately 384 mL/min, which is consistent with the given data.
- For other values of renal clearance, we can use the same formula: creatinine clearance = renal clearance / 3.5. Some key points to consider:
- Renal clearance is the rate at which the kidneys clear a substance from the blood.
- Creatinine clearance is a measure of kidney function, which can be used to estimate the glomerular filtration rate (GFR).
- The relationship between renal clearance and creatinine clearance can be used to estimate creatinine clearance when renal clearance is known 2.
From the Research
Calculating Creatinine Clearance
To figure out a creatinine clearance, several factors and formulas must be considered, as highlighted in various studies 3, 4, 5.
- The formula developed in 1976 predicts creatinine clearance (Ccr) from serum creatinine (Scr) in adult males, with a note that females have 15% less clearance 3.
- The Cockroft and Gault formula, also used for estimating creatinine clearance, incorporates body weight and has been validated by long usage, despite its derivation appearing logically insecure 4.
- It is essential to distinguish between creatinine clearance and glomerular filtration rate, as different equations, such as the CKD-EPI Collaboration Equation, estimate these values differently and may yield varying results for the same patient 5.
Key Factors in Calculation
When calculating creatinine clearance, the following factors should be taken into account:
- Serum creatinine levels
- Age
- Body weight (for certain formulas, such as the Cockroft and Gault formula)
- Sex (with females having 15% less clearance than males, according to the 1976 formula) 3
- The specific equation or formula being used, as different equations may produce different estimates of renal function 5.