Urine acidification test

Last updated: Friday, 14, May, 2004

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Item Process
Specimen

Timed urine specimens sent immediately to the laboratory.

5 mL blood in lithium heparin tube.

Method

pH, blood gas analysis: selective electrodes

Ammonium: spectrophotometry

Protocol

If the patient is acidotic, a 1 hour urine specimen is collected for pH and ammonium.

If the patient is not acidotic, a 6% solution of arginine hydrochloride (0.6 g/kg body weight for adults, 30 g/m2 body surface area for children - maximum 45 g) is infused over 2-3 hours.

Blood and urine specimens are collected at hourly timed intervals.

Urine pH and ammonium are measured.

Blood gases are done to determine when the patient has become acidotic.

Reference Interval

pH: <5.3 (<5.5 in children)

Ammonium: >35 µmol/min/m2

Application

Diagnosis of Renal tubular acidosis

Interpretation

Patients with hyperchloraemic metabolic acidosis, not due to renal tubular acidosis, have a urine pH of <5.3 and normal urine ammonium. Patients with distal renal tubular acidosis produce ammonium at a subnormal rate and are unable to lower urine pH to <5.3. Patients with proximal renal tubular acidosis or hypoaldosteronism can lower urine pH normally but have subnormal urine ammonium production.

Reference

Dubose TD and Alpern RJ. In: Scriver CR et al eds. The Metabolic and Molecular Bases of Inherited Disease. 7th ed. McGraw Hill 1995.