Creatinine

There are many different ways to estimate GFR using the measurement of creatinine. 3 of the commonest ways are discussed here. Please see the 'Resources' section for further details. 

Creatinine clearance

A creatinine clearance test involves looking at the amount of creatinine in a timed urine collection (usually 24hrs) as compared to plasma. So, a blood sample is taken for measurement of plasma creatinine and the patient given a urine bottle to collect urine in over the timed period. This method is subject to several inaccuracies though (mainly related to collecting the urine sample!)

Cockcroft-Gault equation

The Cockcroft-Gault equation is used to estimate GFR without the need for a urine collection. It is used to calculate drug dosing.

GFR = weight x (140 - age) / plasma creatinine x 0.82    (Male)
GFR = weight x (140 - age) / plasma creatinine x 0.96    (Female)

MDRD equation

The MDRD (Modification of Diet in Renal Disease) is an equation which in modified form is now recommended for use in primary care for the detection of chronic kidney disease by calculating an estimated GFR (eGFR). The formula itself is fairly complicated but its advantage lies in it only requiring knowledge of the patient's plasma creatinine, age, gender and race, without the need for urine or weight measurement, and has been validated in many clinical situations.

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Urine
creatinine
(umol/l)

Urine
volume
(ml)

X

Plasma
creatinine
(umol/l)

X

Time
period
(mins)

(Normally around 100 ml/min)