The rate at which a particular chemical is removed from the plasma indicates kidney efficiency. This rate of removal is called renal clearance.
Tests of renal clearance can detect glomerular damage or judge the progress of renal disease. One such test, the inulin clearance test, uses inulin (not to be confused with insulin), a complex polysaccharide found in certain plant roots. In the test, a known amount of inulin is infused into the blood at a constant rate. The inulin passes freely through the glomerular membranes, so that its concentration in the glomerular filtrate equals that of the plasma. In the renal tubule, inulin is not reabsorbed to any significant degree, nor is it secreted. Consequently, the rate at which it appears in the urine can be used to calculate the rate of glomerular filtration.
Similarly, the kidneys remove creatinine, which is produced at a constant rate as a result of muscle metabolism, from the blood. Like inulin, creatinine is filtered, but neither reabsorbed nor secreted by the kidneys. Thus, the creatinine clearance test, which compares a patient's blood and urine creatinine concentrations, can also be used to calculate the GFR. A significant advantage is that the bloodstream normally has a constant level of creatinine. Therefore, a single measurement of plasma creatinine levels provides a rough index of kidney function. For example, significantly elevated plasma creatinine levels suggest that GFR is greatly reduced. Because nearly all of the creatinine the kidneys filter normally appears in the urine, a change in the rate of creatinine excretion may reflect a renal disorder.
Another plasma clearance test uses para-aminohippuric acid (PAH), a substance that filters freely through the glomerular membranes. However, unlike inulin, any PAH remaining in the peritubular capillary plasma after filtration is secreted into the proximal convoluted tubules. Therefore, essentially all PAH passing through the kidneys appears in the urine. For this reason, the rate of PAH clearance can be used to calculate the rate of plasma flow through the kidneys. Then, if the hematocrit is known, the rate of total blood flow through the kidneys can be calculated.