Fractional bicarbonate excretion can be useful in the diagnosis of the proximal form of renal tubular acidosis. In the proximal form of renal tubular acidosis there is hyperchloremic acidosis associated with defective bicarbonate reabsorption in the proximal renal tubule. This may be inherited or acquired, with the inherited form associated with growth retardation.


Spectrum of findings in the proximal form of renal tubular acidosis:

(1) With normal or minimally decreased bicarbonate levels there is considerable loss of bicarbonate in the urine, with increased fractional bicarbonate excretion and increased urine pH.

(2) With more severe acidosis there is decreased plasma bicarbonate levels but less urinary bicarbonate loss so that the urine pH may be below 5.4

(3) The proximal tubules have a reduced threshold for bicarbonate reabsorption while the distal tubules function normally. At high bicarbonate levels in the glomerular filtrate, bicarbonate wastage occurs when the capacity of the proximal tubules for reabsorption is exceeded. At lower bicarbonate levels the proximal tubules are able to reabsorb sufficient bicarbonate so that wastage does not occur.


fractional bicarbonate excretion as a percent =

= ((urine bicarbonate in mEq/L) / (plasma bicarbonate in mEq/L)) / ((urine creatinine in mg/dL) / (plasma creatinine in mg/dL)) * 100%



• At normal plasma bicarbonate levels the fractional bicarbonate excretion is > 15% in the proximal form of renal tubular acidosis.


Reference Range

arterial pH

7.35 to 7.45

venous plasma bicarbonate

22 to 29 mEq/L (mmol/L)

plasma chloride

98 to 107 mEq/L (mmol/L)


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