URINE ACIDIFICATION TEST

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Specimen: Timed urine specimens sent immediately to the laboratory; 5 mL blood in lithium heparin tube.

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.

Method:

pH, blood gas analysis: selective electrodes

Ammonium: spectrophotometry

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.

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