Urine acidification test
Last updated: Friday, 14, May, 2004
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Timed urine specimens sent immediately to the laboratory.
5 mL blood in lithium heparin tube.
pH, blood gas analysis: selective electrodes
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
Diagnosis of Renal tubular acidosis
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.
Dubose TD and Alpern RJ. In: Scriver CR et al eds. The Metabolic and Molecular Bases of Inherited Disease. 7th ed. McGraw Hill 1995.