AMMONIUM CHLORIDE LOADING TEST
INTRODUCTION
In the commoner and more severe form of Renal tubular acidosis (distal RTA, type I) there is failure of hydrogen ion (H+) excretion by the distal renal tubules, leading to metabolic acidosis. Following administration of ammonium chloride (which dissociates to ammonia and H+) there is an increase in the degree of the acidosis and a failure to acidify the urine to less than pH 5.2.
In the rarer form (proximal RTA, type II), there is excessive loss of bicarbonate from the proximal renal tubules with a subsequent fall in serum bicarbonate. Since H+ therefore combines with ammonium and phosphate ions (as bicarbonate is unavailable) in the distal tubule, administration of ammonium chloride reveals the ability to acidify urine to less than pH 5.2.
However, this test does not differentiate satisfactory between these two forms of renal tubular acidosis and should be used to confirm the diagnosis of distal RTA (type I) only.
PRECAUTIONS
Please refer to the Trust’s phlebotomy procedure for sample collection precautions.
Caution: this test is dangerous and is contraindicated in any patient with high plasma ammonia (eg. hepatic failure) or with metabolic acidosis due to any other obvious cause (eg. renal failure).
PATIENT PREPARATION
The patient should receive nil by mouth for the 8 hours preceding the test.
PROTOCOL
1. |
Collect minimum 5ml of venous blood for serum bicarbonate estimation (SST tube – yellow top). |
2. |
Ammonium Chloride (100mg/kg body weight) is given orally in water over a period of 10 minutes. |
3. |
Urine samples are collected every hour, for six hours, into a plain universal. Each urine sample should be sent immediately to the Clinical Biochemistry Department for pH measurement. |
4. |
After 6 hours collect a further 5ml blood for serum bicarbonate (SST tube – yellow top). |
INTERPRETATION
Normal response: The urine pH should fall to <5.2 in at least one specimen, and the serum bicarbonate should remain unchanged.
In patients with proximal RTA (type II) and those with generalised renal tubular damage, the urine pH falls to <5.2 at some point during the test ie. a normal response.
In patients with distal RTA (type I), the urine pH remains above 6.0 indicating an inability to acidify the urine in spite of a low serum bicarbonate concentration.
Reviewed by: Tony
Everitt, Consultant Biochemist
Last edited 07/08/08