Urinary Electrolytes

Urinary electrolyte measurement can be performed on a spot sample and can aid in the diagnosis of:

Interpretation of Spot Measured Urinary Electrolytes
Electrolyte Uses Interpretation
Na+
 Volume state <20mmol/L suggests (appropriate) conservation in hypovolaemia
Hyponatraemia
AKI Typically <20mmol/L
ATN Typically >20mmol/L
Free water clearance
Cl- Metabolic alkalosis

<10mmol/L: Appropriate renal chloride conservation, suggesting:

  • Gastric Cl- loss
  • Diuretic therapy

>20mmol/L: Inappropriate chloride loss, e.g.:

  • Corticosteroids
  • HTN
  • Hyperaldosteronism
K+ Hyperkalaemia
Hypokalaemia

<2mmol/L: Appropriate renal potassium conservation, suggesting:

  • Intracellular redistribution
    • Insulin
    • Alkalosis
  • GI loss
    • Diarrhoea
    • Laxative use
    • Ileostomy
  • Over-dialysis
  • Previous high urinary potassium
    e.g. Following loop diuretic administration, after resolution of drug effect.

>15mmol/L: Inappropriate renal potassium wasting, suggesting:

  • Renal tubular acidosis
  • Hyperaldosteronism
    • Corticosteroid excess
  • Upper GI loss
  • Spurious elevation in severe oliguria
    • Urinary potassium concentration will be ↑ in oliguria, but total potassium clearance may be low due to the low urine volume
    • Consider a 24-hour specimen to determine total renal potassium excretion
Free water clearance
Creatinine Fractional solute clearance
Osmolality Hyponatraemia
Polyuria
AKI
Osmolar Gap
(10-100mOsm/kg)		 Renal tubular acidosis By estimating NH4+ clearance.
Anion Gap
(20-90mmol/L)		 NAGMA Negative indicates GI base loss
Positive indicates a type 1 (distal) RTA

Use of urinary sodium in the context of hyponatraemia is covered under Hyponatraemia.

Derived Values

Fractional Excretion of Sodium

The fractional excretion of sodium describes the relationship between the amount of sodium filtered by the glomerulus and the amount excreted in urine. Accurate interpretation is difficult, however classically:

\(FE_{Na} = {Quantity \ of \ Na^+ \ Excreted \over Quantity \ of \ Na^+ Filtered} = {{U_{Na} \times P_{Cr} \over P_{Na} \times U_{Cr}} \times 100}\)

Where:

  • \(FE_{Na}\) is the fractional excretion, in %
  • \(U_{Na}\) is the urinary sodium concentration
  • \(P_{Cr}\) is the plasma creatinine concentration
  • \(P_{Na}\) is the plasma sodium concentration
  • \(U_{Cr}\) is the urine creatinine concentration
  • Hepatorenal syndrome has an FENa <0.1%
  • Pre-renal AKI has a FENa <1%
  • ATN has a FENa <2%
    Exceptions include:
    • Concurrent cirrhosis
    • Concurrent CHF
    • Radiocontrast administration
    • Rhabdomyolysis
  • Accuracy is improved with:
    • Simultaneous urine and plasma measurement
    • Oliguria
    • Absence of:
      • Acute volume loss
      • Diuretics
      • IV fluid administration

References

  1. Reddi AS. Interpretation of Urine Electrolytes and Osmolality. In: Fluid, Electrolyte and Acid-Base Disorders [Internet]. Cham: Springer International Publishing; 2023 [cited 2023 Oct 15]. p. 15–23.