There are a number of different methods for predicting (calculating) GFR from serum creatinine. All methods make allowance for the different rates of creatinine generation in individual patients.

Creatinine generation rate is influenced by dietary protein intake, muscle mass and activity. The GFR prediction methods attempt to allow for this variation by using the patients sex and age. The accuracy of the prediction is increased by serum albumin in certain versions of the MDRD equations and by weight in the Cockroft and Gault equation.

If the patient has higher than average (for age and sex) muscle mass or protein intake then the predicted GFR will be underestimated. The measured GFR (using the fall in blood concentration of an injected tracer) is the most accurate. GFR measured from urea and creatinine concentrations 24h urine collections are accurate if the collection is performed properly (start collection from an empty bladder, collect all urine passed up to and including a complete voiding exactly 24 hours after start). Patients may incorrectly start the collection with a full bladder which results in a significant overestimation.

A common mistake is to confuse creatinine clearance with GFR. Creatinine clearance is higher than GFR (due to tubular excretion of creatinine). The difference depends on GFR. At normal renal function, creatinine clearance is around 20% higher than GFR. At around 10% of normal renal function (the level at which dialysis may be considered), creatinine clearance is around twice GFR (as tubular creatinine excretion is relatively preserved).

Another cause of confusion is that GFR or creatinine clearance should be corrected for the patient's surface area (calculated from height and weight). The correction may change the result by a huge amount in large or small patients. Depending on the method for calculating GFR or creatinine clearance, surface area correction may or may not be included.

Methods for calculating GFR or creatinine clearance from 24h urine collections need height and weight to correct for surface area. The laboratory usually reports UNCORRECTED values. Methods based on the fall in concentration of an injected tracer in blood do not need height or weight for surface area correction and the laboratory will usually report CORRECTED values.

The MDRD method for predicting GFR from serum creatinine is already corrected for surface area and does not require height and weight. The Cockcroft and Gault prediction method calculates creatinine clearance (NOT GFR) and is UNCORRECTED for surface area.

Some laboratories still report creatinine clearance rather than GFR when receiving 24h urine samples.

I am told that, when reporting creatinine clearance from 24h urine collections, some laboratories apply an arbitrary correction so that it is reduced by around 20% to better reflect GFR. None of the laboratories I have used does this so I do not know how widespread this practice is.

James Tattersall, Leeds, UK January 2003

This page created 8th January 2003. Modified 7/6/07. Contact us with feedback