Annales de pédiatrie
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In normal adults eating diets with standard protein contents, urinary excretion of NH4 approximates 40 mmol/24 hours and urinary pH is variable. In patients with metabolic acidosis, a urinary pH under 5.5 suggests an extra-renal cause whereas a urinary pH above 5.5 suggests a renal disorder, although there are many exceptions to this rule of thumb. However, urinary excretion of NH4 is always above 70 mmol/24 hours in extra-renal acidosis and less than or equal to 40 mmol/24 hours in renal acidosis; the two situations can readily be differentiated by determining the urinary anion gap which is absent in the former case and present in the latter. ⋯ In the latter case, decreased serum potassium levels suggest distal tubular acidosis due to defective H(+)-ATPase or H+/K(+)-ATPase pump function (no increase in urinary PCO2 after oral NaHCO3 loading) or to inability of the kidney to develop a normal H+ gradient (normal increase in urinary PCO2). Increased serum potassium levels suggest conditions involving either hypoaldosteronism or alterations in transepithelial voltage or pseudo-hypoaldosteronism. The incidence of distal tubular acidosis with increased serum potassium levels is rising, whereas tubular acidosis with low serum potassium levels remains infrequent.