Answer: By measuring para-aminohippurate (PAH) clearance.
How does it work? Ideally, one would simply choose a substance which is neither synthesized nor metabolized by the kidney; the amount of such a substance entering the kidney per unit time via the renal artery should equal the amount leaving via the renal vein. However, it is not trivial to obtain blood samples from the renal vein in humans and therefore another approach is needed.
PAH works because although it is not filtered at the glomerulus, it undergoes tubular secretion in an extremely efficient manner: about 90% of all PAH is removed from the plasma via secretion while 10% remains in the renal vein. This 10% is ignored, and it is then assumed that the amount entering the kidneys equals the amount showing up in the urine. Mathematically, this can be expressed as the formula:
RPF (in cc/min) x [PAH] in plasma = [PAH] in urine x urine flow rate V (in cc/min).
Rearranging, RPF = [PAH] in urine x urine flow rate V (in cc/min)/[PAH] in plasma.
Although this test is rarely used in clinical practice, it is essential for research studies in which the hemodynamic effects of various drugs can be measured. For example, ACE-inhibitors are associated with a measurable increase in RPF.