Outline the role of urinary electrolytes in the assessment of the critically ill patient.

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College Answer

Urinary electrolytes (sodium, potassium and chloride) can assist in the diagnosis of a number of electrolyte disturbances in ICU patients (especially where the intake of electrolytes is known and relatively controlled). This question does not refer to urinary pH or osmolality measurements. Some of the more commonly used example are included here. In assessing oliguria: a spot urinary sodium when low (<10 mmol/L) can indicate depleted extracellular volume and a pre-renal cause, whereas
>20 is more indicative of tubular damage. Hyponatraemia associated with extrarenal losses should be associated with a low spot urinary sodium (<10), whereas a higher level (>20) is more indicative of other causes (e.g. renal salt losing states, SIADH, and diuretic therapy). Fractional excretion of sodium can be calculated (100*UNa*PCr/PNa*UCr) but its ability to determine causes of oliguria (e.g. <1% implies pre-renal)is limited by sodium intake and diuretic therapy. Urinary chloride estimation is of most use when assessing normal anion gap metabolic acidosis. Renal tubular acidosis is associated with impaired urinary acidification (decreased ammonium excretion) and this is associated with a low urinary chloride (e.g. <10 mmol/L), a positive urinary anion gap (Na + K – Cl), and an inappropriately high urinary pH (e.g. >6). If the acidosis is due to extra-renal losses of bicarbonate, in the absence of renal failure the kidneys will excrete ammonium (and chloride) resulting in a negative urinary anion gap (as urinary Cl > Na + K). Urinary potassium concentration can also help with the cause of hypokalaemia. Renal loss is generally indicated by >20 mmol/L as iopposed to an extra-renal loss (<20 mmol/L).

Discussion

To present this topic systematically, one can either break it up into indications for urinary electrolyte testing, or into electrolytes tested (and the meaning of abnormal results). Both forms have a relevance.

A good article on this topic includes a table (Table 2.1) of urinary electrolyte results, their relevance, and the indications for the tests. I will repurpose some parts of this table for this answer.

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 or water conservation, 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
Negative: gastrointestinal causes of NAGMA
Urinary osmolal gap In an acidaemic patient with NAGMA:
Lower than 150 mOsm/kg = urinary acidification defect (renal tubular acidosis)
Higher than 400 mOsm/kg = appropriate renal response to a non-renal cause of acidosis, eg. to diarrhoea.
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: <5-10mmol/L
High urinary potassium: >15mmol/L
  • Renal tubular acidosis (Type 1 or 2)
  • Hyperaldosteronism
  • Corticosteroid excess
  • Diuretic therapy

Alternatively, you can organise it as a list. The list below is an adaptation of the table of contents for the chapter on urinary electrolytes, and clicking on the list items will take you to the Required Reading section where they are discussed in greater detail.

References

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.