Urinary Electrolytes and the Urinary Anion Gap
Urinary anion gap
The urinary anion gap is extensively explored in the chapter on the diagnosis of renal tubular acidosis. Its role in the SAQs has been limited to the diagnosis of renal tubular acidosis (i.e where it is used to discriminate between renal and non-renal causes of a normal anion gap acidosis, such as in Question 3.4 from the second paperof 2013).
- Urinary anion gap is the difference between the excreted chloride and the excreted cations.
- The formula is (Na+ + K+) - Cl-
- If there is more chloride than cations, i.e. a "negative" urinary anion gap, it means another cation - namely ammonium - is being excreted
- Increased ammonium excretion is the appropriate renal reaction to acidosis
- Thus, a negative urinary anion gap demonstrates that the cause of a normal anion gap metabolic acidosis is not related to renal tubular function. In other words, its not an RTA, its diarrhoea.
As for the urinary electrolytes...Why would you order any of those?
Meaning of results
|Oliguria||Na+||Na+ < 20mmol/L: appropriate conservation of sodium in the context of hypovolemia|
|Na+ >20mmol/L: renal failure, eg. ATN|
|Hyponatremia||Na+||Na+ < 20mmol/L: appropriate conservation of sodium in the context of hyponatremia|
|Na+ >20mmol/L: renal salt wasting, eg:
- cerebral salt wasting or SIADH
- adrenal insufficiency
- diuretic use
- osmotic diuresis eg. mannitol or glucose
|Normal anion gap metabolic acidosis||Urinary anion gap||Positive: renal causes of NAGMA|
|Normal or Negative: gastrointestinal causes of NAGMA|
|Metabolic alkalosis||Cl-||0-10: appropriate renal chloride conservation
- gastric chloride losses
- diuretic therapy (between doses)
- post hypercapnea alkalosis
|>20: inappropriate renal chloride loss
- corticosteroid excess
|Hypokalemia||K+||Low urinary potassium: <2mmol/L
|High urinary potassium: >5mmol/L