Urine Electrolytes

Urinary Sodium

Increased Urinary Sodium: Salt and water lost through kidneys

  • HYPOnatremia with hypovolaemia
    • Renal failure
    • ATN
    • Addison’s
    • Osmotic diuresis (Raised glucose and urea)
    • Diuretics (Thiazides)
    • Cerebral salt wasting
  • HYPOnatraemia with euvolaemia
    • Elevated urine osmolality (SIADH)
  • HYPOnatraemia with oedema
    • Renal failure
    • Hypertonic saline

Decreased Urinary Sodium: Increased interstitial salt attempted Na retention

  • HYPOnatraemia with oedema
    • Liver failure
    • Cirrhosis
    • CCF
    • Nephrotic syndrome
  • HYPOnatremia with dehydration (increased urine osmolality)
    • Burns, fistula, heat illness
    • Diarrhoea, sweat, vomiting

Urinary anion gap (UAG)

  • Differentiate renal or GIT cause of HYPERchloraemic metabolic disorders
  • or Normal anion gap metabolic acidosis (NAGMA)


  • Urine must maintain electrical neutrality
  • Urinary anion gap provides rough measure of urinary sodium excretion (Positively charged cation)
  • Low anion gap associated with increased NH4+ excretion

Urinary AG = (UA – UC) = [Na+] + [K+] – [Cl]

Hyperchloraemic metabolic acidosis associated with

  • neGUTive UAG
    • Loss of base by GIT (Diarrohea)
      • → hypovolemia and metabolic acidosis
      • → ↑ ­NH4+ excretion
      • → appropriate urine ↓Na but ‘inappropriate’ urine ­↑Cl
    • Loss of base by the kidney (RTA)
    • Impaired renal acidification and kidney unable to excrete enough NH4+
    • NOTE: However in the case of ‘gastric loss‘ the UAG is usually Positive
      • → hypovolemia and metabolic acidosis
      • → HCO3- excretion
      • → ‘inappropriate’ urine ­↑Na but appropriate urine ↓Cl

Critical Care


Associate Professor Curtin Medical School, Curtin University. Emergency physician MA (Oxon) MBChB (Edin) FACEM FFSEM Sir Charles Gairdner Hospital.  Passion for rugby; medical history; medical education; and asynchronous learning #FOAMed evangelist. Co-founder and CTO of Life in the Fast lane | Eponyms | Books | Twitter |

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