Clinical Information

The assessment of glomerular filtration rate (GFR) is an important parameter of renal function utilized by clinicians in the care of patients with varying renal diseases, and for clinical research when precise assessment of renal function is necessary. The GFR is the sum of all the filtration rates of the individual nephrons within the kidney and, as such, reflects the number of functioning nephrons.

GFR can fall due to a chronic renal disease that causes a permanent loss of nephrons, or due to an acute renal injury that is potentially reversible. In addition, a decline in renal blood flow, ie, secondary to volume depletion, can result in a fall in GFR that is functional and (potentially) quickly reversible. GFR increases when factors that decreased renal blood flow are corrected, or the kidney heals after an acute injury. GFR also increases transiently during pregnancy, as well as in response to other physiologic stimuli that include hyperglycemia or a dietary protein load.

Measurement of GFR can be performed by several methodologies including creatinine clearance, iothalamate clearance, and inulin clearance. Of these methods, the latter 2 are significantly more accurate and reproducible than the former.

Renal clearance of inulin, a fructose polymer, has traditionally been considered the gold standard for determination of GFR. Disadvantages to the use of inulin are its high cost, limited availability, and analytical expense. Previous studies have shown close correlation between iothalamate and inulin GFR determinations. The short renal clearance test, which utilizes a subcutaneous injection of nonradiolabeled iothalamate and a single (1 hour) urine specimen, is the preferred method for measurement of GFR in most situations since it is less time consuming and less costly.

The Standard Renal Clearance test, which utilizes continuous intravenous infusion of iothalamate (or inulin) and additional urine and plasma collections, is an alternative means of measuring GFR and may be preferable in patients with low urine flows (eg, patients with severe liver failure).

The Standard Renal Clearance test also measures renal plasma flow (RPF). Intravenously administered para-aminohippurate (PAH) is used to determine RPF because PAH is nearly completely removed from the renal circulation in a single pass, by a combination of glomerular filtration and tubular secretion.