Editing Inulin (section)

==Calculation of glomerular filtration rate==
Inulin is uniquely treated by [[nephrons]] in that it is completely filtered at the [[Glomerulus (kidney)|glomerulus]] but neither secreted nor reabsorbed by the tubules.  This property of inulin allows the [[Clearance (medicine)|clearance]] of inulin to be used clinically as a highly accurate measure of glomerular filtration rate (GFR) — the rate of plasma from the [[afferent arteriole]] that is filtered into Bowman's capsule measured in ml/min.{{Cn|date=March 2021}}

It is informative to contrast the properties of inulin with those of [[para-aminohippuric acid]] (PAH).  PAH is partially filtered from plasma at the glomerulus and not reabsorbed by the tubules, in a manner identical to inulin.  PAH is different from inulin in that the fraction of PAH that bypasses the glomerulus and enters the nephron's tubular cells (via the [[peritubular capillaries]]) is completely secreted.  Renal clearance of PAH is thus useful in calculation of renal plasma flow (RPF), which empirically is (1-[[hematocrit]]) times [[renal blood flow]].  Of note, the clearance of PAH is reflective only of RPF to portions of the kidney that deal with urine formation, and, thus, underestimates the actual RPF by about 10%.<ref>Costanzo, Linda.  Physiology, 4th Edition.  Philadelphia: Lippincott Williams and Wilkins, 2007.  Pages 156–160.</ref>

The measurement of GFR by inulin or sinistrin is still considered the [[Gold standard (test)|gold standard]].  However, it has now been largely replaced by other, simpler measures that are approximations of GFR.  These measures, which involve clearance of such substrates as [[EDTA]], [[iohexol]], [[cystatin C]], [[125I|<sup>125</sup>I]]-[[iothalamate]] (sodium radioiothalamate), the chromium radioisotope [[chromium-51|<sup>51</sup>Cr]] (chelated with EDTA), and [[creatinine]], have had their utility confirmed in large cohorts of patients with chronic kidney disease.{{Cn|date=March 2021}}

For both inulin and creatinine, the calculations involve concentrations in the urine and in the serum. However, unlike creatinine, inulin is not naturally present in the body. This is an advantage of inulin (because the amount infused will be known) and a disadvantage (because an infusion is necessary).{{Cn|date=March 2021}}