Urinary Electrolytes and the Urinary Anion Gap

Created on Sat, 07/11/2015 - 18:11
Last updated on Tue, 08/11/2015 - 18:53

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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).

In brief:

  • 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.

Urinary electrolytes

As for the urinary electrolytes...Why would you order any of those?

Urinary Electrolytes according to Indication
Indication

Electrolyte

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
- hypertension
- hyperaldosteronism 
Hypokalemia K+ Low urinary potassium: <2mmol/L
High urinary potassium: >5mmol/L
  • Renal tubular acidosis (Type 1 or 2)
  • Hyperaldosteronism
  • Upper gastrointestinal losses
  • Corticosteroid excess

 

References

LITFL has an excellent summary.

There seems to only be one free fulltext article on this matter!

Reddi, Alluru S. "Interpretation of Urine Electrolytes and Osmolality." Fluid, Electrolyte and Acid-Base Disorders. Springer New York, 2014. 13-19.

The rest, you people have to pay for.

Schrier, Robert W. "Diagnostic value of urinary sodium, chloride, urea, and flow." Journal of the American Society of Nephrology 22.9 (2011): 1610-1613.

Harrington, John T., and Jordan J. Cohen. "Measurement of urinary electrolytes-indications and limitations." The New England journal of medicine 293.24 (1975): 1241.

Kamel, K. S., et al. "Urine electrolytes and osmolality: when and how to use them." American journal of nephrology 10.2 (1990): 89-102.

Kirschbaum, Barry, Domenic Sica, and F. Phillip Anderson. "Urine electrolytes and the urine anion and osmolar gaps." Journal of Laboratory and Clinical Medicine 133.6 (1999): 597-604.

Batlle, Daniel C., et al. "The use of the urinary anion gap in the diagnosis of hyperchloremic metabolic acidosis." New England Journal of Medicine 318.10 (1988): 594-599.