Renal function tests

Published on 13/02/2015 by admin

Filed under Anesthesiology

Last modified 13/02/2015

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Renal function tests

C. Thomas Wass, MD

Glomerular function is characterized by glomerular filtration rate (GFR), whereas tubular functions include concentrating ability, water conservation, and electrolyte and pH homeostasis. For practical purposes, renal function tests can be stratified into clearance techniques that estimate GFR, tubular function tests, and assays that are largely used for clinical and laboratory investigation.

The glomerular filtration rate

The GFR is the amount of plasma filtered through glomeruli per unit of time and is the single best index of functioning renal mass. Inulin is a sugar that is completely filtered by the glomerulus but is neither secreted nor reabsorbed by the tubule. Thus, the volume of intravenously administered inulin cleared from the plasma can be used to calculate the GFR. However, inulin clearance is seldom used clinically because the assay is cumbersome and time consuming to perform. Creatinine (Cr) is a metabolic end product of creatine phosphate in skeletal muscle that is cleared by the kidney in a manner similar to that of inulin. Creatinine clearance (CrCl), the most clinically useful measure of GFR, is estimated using the following formula:

< ?xml:namespace prefix = "mml" />CrCl = (140–age)(body weight in kg)(serum creatinine)(72)

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More precise measurement requires timed collections (over a period of 24 h) of urine and plasma samples and requires use of the following formula:

CrCl=(UCr×V)PCr

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where U is the urinary Cr concentration in mg/dL, V is the volume of urine in mL/min, and P is the plasma Cr concentration.

Normal GFR is 120 ± 25 mL/min in men and 95 ± 20 mL/min in women. Mild, moderate, and severe impairment have corresponding values of approximately 40 to 60 mL/min, 20 to 40 mL/min, and less than 20 mL/min, respectively. Serial GFR measurements are important in determining the severity of renal dysfunction, as well as in monitoring disease progression.

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