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)


  • 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
Print Friendly


  1. Chrstian says

    Agree with above, but clinically the volume state is often difficult to assess, and hence does not help with the differentiation of Na. The urinary anion gap is normally -ve, because of the unmeasured NH4 (which, because unmeasured makes the term UC-UA negative). If a patient with an acidosis and a -ve urinary anion gap hence the cause must be extrarenal. If the urinary anion gap is positive the NH4 production must be low, which is most commonly due to a defect of the DCT cells. Given no or little NH4 is being secreted the urinary anion gap becomes positive. The lack of NH4 leads to poor Cl excretion, which in turn leads to a acidosis (reduced strong ion difference related rather than purely hyperchloraemic).