GFR (Glomerular Filtration Rate) is a test of how much the kidneys are filtering. It is normally about 100 mls/minute. This means that the kidneys are removing all the creatinine found in 100mls of blood every minute - almost 150 litres per day! Most of this is absorbed back into the body, so that only 1-2% of the filtrate appears as urine. Creatinine clearance gives quite a good measure of GFR, but requires a 24 hour urine collection for measurement.
Two other ways of measuring GFR are:
eGFR - Using blood tests, age, sex, and sometimes other information to estimate the GFR from the MDRD equation (eGFR). This isn't as good as measuring it, but is much simpler as it requires just one blood test. It is being used increasingly to spot kidney disease earlier than previously, and earlier than would be possible using just creatinine measurements.
Measured GFR - Injecting a tiny amount of a radioactive substance and measuring how quickly it disappears from the blood, or appears in the urine, is used to calculate GFR.
Why It’s Done?
Knowing your GFR is very useful. It can give you an approximate % kidney function, and be used to show your CKD stage - more info about low GFR and CKD stages.