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Lessons · Medicine foundations · creatinine clearance by Cockcroft-Gault

Cockcroft-Gault: an equation you substitute into

Creatinine clearance is estimated as (140 minus the age) times the weight in kilograms, divided by 72 times the serum creatinine in mg/dL, and then multiplied by a further factor of 0.85 where the problem says to apply it.

Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.

What it is for

In a pharmacokinetics tutorial the equation is on the slide the whole time, and nobody loses marks for forgetting it. They lose marks for rounding the top before dividing, for putting a weight in pounds into a formula built for kilograms, and for applying a factor the question never mentioned.

How to think about it

Build the top on one line and the bottom on the next, then divide once, at the end. Only apply the 0.85 when the problem states it, and write a line saying that you did, so whoever reads your working can see the decision rather than guess at it.

Worked example

CrCl = (140 − age) × weight in kg / (72 × serum creatinine in mg/dL)
The equation, as a problem gives it.
Age 60, weight 70 kg, creatinine 1.0 mg/dL
Three figures, all from the question.
Top: (140 − 60) × 70 = 5600, the numerator
The subtraction happens inside the brackets, before the multiplication.
Bottom: 72 × 1.0 = 72, the denominator
The 72 is part of the equation, not a figure about this person.
5600 / 72 = 77.8 mL/min
One division, at the end, with the decimals kept until now.
Where the problem says to apply the 0.85 factor: 77.8 × 0.85 = 66.1 mL/min
Only when it says so, and the line records that it did.

Your turn

Age 50, weight 80 kg, serum creatinine 1.2 mg/dL, and the problem does not ask for any further factor. Write the line that gives the estimate.

(140 − 50) × 80 / (72 × 1.2) =  mL/min

The trap

Rounding the numerator and the denominator before dividing. Round 5600 to 5600 and 86.4 to 86 and the answer moves by half a millilitre a minute for no reason at all. One rounding, at the very end.

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