Serum Electrolytes

Biochemistry

Na, K, Cl, Ca, Mg, PO4 — interpret with volume status and ABG.
NaKClCaMgPO4

Hyponatraemia (Na <135)

Common causes

  • • Hypovolaemic: GI loss, diuretics
  • • Euvolaemic: SIADH, hypothyroid
  • • Hypervolaemic: CHF, cirrhosis, nephrosis

Significance: Classify by volume status and serum/urine osmolality.

Differential diagnosis

  • • True hypoNa
  • • Pseudohyponatraemia
  • • SIADH
  • • Adrenal insufficiency

Next investigations

  • • Serum & urine osmolality
  • • Urine Na
  • • TSH, cortisol
  • • Volume assessment

Red flags

  • • Na <120 with seizures / coma
  • • Acute drop >10 mmol/L in 24h
  • • Risk of central pontine myelinolysis if corrected too fast

Hyperkalaemia (K >5.5)

Common causes

  • • AKI/CKD
  • • ACEi/ARB/spironolactone
  • • Tissue breakdown
  • • Acidosis
  • • Addison's

Significance: Cardiac arrhythmia risk increases >6.5.

Differential diagnosis

  • • True hyperK
  • • Pseudohyperkalaemia (haemolysed sample)

Next investigations

  • • Urgent ECG
  • • Repeat unhaemolysed sample
  • • ABG
  • • RFT

Red flags

  • • K >6.5
  • • ECG changes (peaked T, wide QRS)
  • • Bradyarrhythmia

Educational support only — not a substitute for clinical judgment.

WardRound

WardRound

Clinical Decisions in Seconds