[box type=”download”] Understanding of why creatinine is chosen for the clearance calculation Knowledge of the equation (Cu x V) / Cp in calculating clearance Knowledge of inulin and PAH as function indices is NOT required[/box]
Measurement of the glomerular filtration rate and the concept of clearance
If substance X is freely filtered and neither reabsorbed nor secreted in the nephron, the amount appearing in the urine per minute must equal the amount filtered per minute.
Thus, if the plasma concentration of X is Cp and the urine concentration is Cu, and the volume of urine passed per minute is V, then Cp × GFR = Cu × V, or GFR = (Cu × V)/Cp.
Creatinine, which is steadily released from skeletal muscle, is often used for clinical measurements of GFR because it is freely filtered and not reabsorbed; there is a little secretion, but this introduces only a small error, except when plasma creatinine or GFR is abnormally low.
More accurate measurements are made by infusing the polysaccharide inulin, which is neither reabsorbed nor secreted.
The term clearance is defined as the volume of plasma that would need to be completely cleared of a substance per minute in order to produce the amount found in the urine, or: clearance = (Cu × V)/Cp (i.e. the same equation as above).
Thus, the clearance of inulin is equal to GFR.
If a substance is reabsorbed in the nephron, its clearance will be less than the GFR and, if it is secreted, it will be greater than the GFR.
Some substances that are normally completely reabsorbed have zero clearance until the reabsorption mechanism becomes saturated.
Para-aminohippuric acid (PAH), at low concentrations is completely removed from renal blood by filtration and secretion, so that none remains in the venous outflow. The amount appearing in the urine must therefore equal the amount entering the kidney, and thus the clearance of PAH is equal to The renal plasma flow (RPF).
The filtration fraction (GFR/RPF) can therefore be estimated from inulin clearance/PAH clearance.
The renal blood flow is equal to RPF/(1 – haematocrit).