Urine Electrolytes

Increased Urinary Sodium: Salt and water lost through kidneys

• HYPOnatremia with hypovolaemia
  • Renal failure
  • ATN
  • Addisons
  • 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)

Nature

• 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
• POSITIVE UAG
  • 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

Print PDF