Urine pH and ammonium paediatric reference intervals in first morning spot urine

Urinary acidification plays a crucial role in acid-base homeostasis, its evaluation being necessary in the differential diagnosis of metabolic acidosis. The practical limitations of the functional tests required to confirm an acidification disorder point out the need for a simple screening method for urinary acidification assessment in spot urine. The best candidates are urine pH and ammonium (NH₄ ⁺), but there are few data on their reference values in spot urine in children.

Our study measured urine pH and NH₄ ⁺/creatinine ratio in the first morning fasting urine of 135 healthy children aged between 5 and 14 years. pH was measured by dipstick and potentiometry, concluding that only potentiometric measures were appropriated for assessing urinary acidification in acid-base diseases.

Median urine pH was 5.67 (interquartile range 5.44-6.01), ranging from 4.90 to 7.60. Median NH₄ ⁺/creatinine ratio was 4,869 μmol/mmol (interquartile range 3,528-5,918). The reference interval for urine pH ranged from 5.01 to 6.88 and the reference interval for NH₄ ⁺/creatinine ratio ranged from 1,646 μmol/mmol to 9,799 μmol/mmol.

These results supply, for the first time, paediatric reference values for both parameters simultaneously, standardizing the sample and method of choice and providing a clinically useful tool for a preliminary evaluation of urinary acidification.