How is GFR measured?
GFR is measured as the clearance of an ideal filtration marker. Clearance refers to the amount of plasma that is completely cleared of a substance over a set amount of time. For example, if a person has substance X in his blood at a concentration of 2 g X per liter and he excretes 1 g of X in the urine, he will have theoretically removed all the X from half a liter of blood. If the person took 1 day to produce enough urine to get rid of 1 g of X, then his clearance will be 0.5 L (of plasma cleared of X) per day. The equation for calculating clearance is as follows:
where Cl x is the clearance of substance x, Ur x is the urine concentration of X, and V is the urine flow rate.
Plasma clearance is an alternative to urinary clearance for measurement of GFR. Plasma clearance is performed by measurement timed plasma levels following a bolus intravenous injection of an exogenous filtration marker computed from the following:
where A x is the amount of the marker administered and Px is the plasma concentration computed from the entire area under the disappearance curve.
An ideal filtration marker is one that is freely filtered at the glomerulus but not reabsorbed, secreted, or metabolized by the kidney. Clearance of an ideal filtration marker can therefore be used to measure GFR. Inulin is the ideal filtration marker. Inulin is freely filtered by the glomerulus but is neither secreted nor reabsorbed by the tubules. However, inulin is rarely used, and alterative markers include iohexol and iothalamate and are more commonly used. In addition to exogenous substances, endogenous molecules that are filtered by the glomerulus can be measured to assess GFR. In particular, creatinine clearance assessed using timed urine collections is often used to estimate GFR.
