What do the measurements mean?

GFR is usually measured in ml/min/1.73 m².

The “ml/min” part means millilitres per minute, and is a measure of how much blood is filtered by all the glomeruli of both kidneys in one minute. For instance, a GFR of 100 ml/min means that 100 millilitres of blood are filtered every minute (that’s 6 litres an hour).

The “/1.73 m²” part means that the GFR is measured as a proportion of what the GFR would be in a person of normal body size. In medicine, body size is often expressed as “body surface area” and measured in square metres (m²). 1.73 m² is a “normal” body size. Bigger people naturally have higher GFR than smaller people, so a GFR that would be considered normal for a person of average body size (1.73 m²) would be abnormally high for a small person and abnormally low for a large person. To make up for this, the GFR is multiplied by:

1.73/Actual body surface area

NOTE: not all doctors always correct for body size like this. Until recently, it was standard practice to estimate GFR using the Cockcroft and Gault formula; this gave an estimate of GFR that was not normalised to body size. Using this formula, it was easy to under-diagnose CKD in big people and over-diagnose it in small people. Measuring GFR by measuring creatinine clearance using 24 hour urine samples did not take body size into account either.

WARNING: the MDRD formula, and every other way of estimating GFR apart from radioactive tracer methods, is less accurate when GFR is normal or near-normal than when it is low.

NKF Controlled Document No. 63, What do the measurements mean?, written 31 August 2006. Last reviewed 28 May 2009.

The National Kidney Federation cannot accept responsibility for information provided. The above is for guidance only. Patients are advised to seek further information from their own doctor.

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