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GFR Estimation : Cockroft-Gault Equation

GFR Estimation : Cockroft-Gault Equation
GFR estimation. Direct measurement of GFR by endogenous creatinine is the most accurate but inconvenient to do. On the other hand, use of radio-labeled agent is undesirable for environmental reason and for safety reason. And therefore we would like to use a serum marker if possible to estimate GFR. And there are several formulas available for this purpose. Including Cockroft-Gault method, MDRD, Jelliffe Method and the Schwartz formula. And we will talk about each and every one of them. First the Cockroft and Gault equation. Now this equation says, creatinine clearance is equal to 140 minus age times body weight divided by 72 times serum creatinine.
So here again Clcr is the creatinine clearance, Ccr is the serum creatinine, age in year and body weight in kilogram. And the body weight is gross weight in the original formula. And very interestingly, based on this formula. The maximum life expectancy is about 140 years for men or women. Now the Cockroft-Gault Estimate. The formula was published in 1976, and it was derived from 249 normal white males age 18-92. The original formula was based on gross weight. And the correction factor is 0.85 for female subject, assuming that there’s a 15% difference in muscle mass between male and the female subject. And some physicians actually believe that this formula provides a more accurate result than the 24-hour creatinine clearance.
Because of the potential under collection of urine particularly in an outpatient situation. There are some ground rules for using the Cockroft-Gault equation. First, use gross weight if height is not available. Use ideal body weight if both the weight and height are available. This is to avoid overestimated particularly in obese patient. If the gross weight is less than ideal body weight, then use the gross weight. This could be the situation in malnutrition or muscle wasting in AIDS patient. This provide reasonably accurate estimate for mild to normal kidney function. Ideal body weight calculation. Ideal body weight can be calculated by these formulas for male. And for female. And you can find this equations of formula from the literature source.
Here I like to introduce another body weight term. It’s called adjusted body weight. Adjusted body weight is for obese patient whose actual weight is greater than 30% of idea body weight. And here is the formula. Adjusted body weight is equal to ideal body weight plus 0.4 times total body weight minus ideal body weight. The difference between total body weight and the ideal body weight essentially is the fat content. Therefore adjusted body weight takes into consideration ideal body weight and the 40% of the fat content. Adjusted body weight is used for dosing adjustment. Ideal body weight is used for both creatinine clearance estimation and the dosing adjustment. Another body weight term.
Lean body weight is rarely used for clinical application except as a weight loss measure in the gym or gymnastics

Continuing from the previous step, Prof. Lee illustrates Cockroft-Gault Equation this time.

To begin with, we have to know that direct measurement of GFR by endogenous creatinine is most accurate but inconvenient, and the use of radio-labeled agent is undesirable. Consequently, we would use Ccr to estimate GFR.

There are four formulas available, including Cockroft-Gault, MDRD, Jelliffe Method, and Schwart. Prof. Lee clarifies Cockroft-Gault first since it is the most common one.

While using this formula, it is necessary for us to know that correction factor is 0.85 for female, assumed 15% lower in muscle mass.

Ultimately, we need to know when to use ideal or adjusted body weight, and we must understand how to calculate ideal body weight and adjusted body weight.

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Pharmacokinetics: Drug Dosing in Renal Disease

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