Renal Tubule Function May Be Assessed by Determining Fractional Excretion and Fractional Reabsorption Rates
The percentage of a filtered substance that is excreted in the urine is called the fractional excretion rate. It is the net result of the tubular reabsorption and secretion of the filtered substance.
Mathematically, the fractional excretion rate of a substance X is the ratio of the urinary concentration of X (Ux) to the plasma concentration of X (Px) divided by the urinary/ plasma (UZP) ratio of a reference substance that is neither secreted nor reabsorbed. Relating Ux∕Px to the U/P ratio of a reference substance eliminates the confounding effect of water reabsorption on the urinary concentration of X. In experimental settings the urinary and plasma concentrations of inulin during a constant inulin infusion may be used for reference. However, it is more practical in clinical situations to use creatinine as the reference substance. Therefore the fractional excretion rate of X (FEx) is determined by the following equation:
where Uocatininc and Pocatininc are the urinary and plasma concentrations of creatinine, respectively. By multiplying FEx by 100, the fractional excretion rate is expressed as the percentage of filtered X that is excreted.
The fractional reabsorption rate of X (FRx) represents the proportion of filtered X that is reabsorbed by the tubule. FRx is mathematically determined by the following equation:
This can be expressed as a percentage by multiplying FRx by 100.
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