Hyperchloraemia and Hypochloraemia

Chloride is the principal extracellular anion and moves largely in parallel with sodium and inversely with bicarbonate.

Chloride abnormalities are rarely a primary problem in their own right; their value is as a pointer to the underlying disorder, particularly in acid-base interpretation, where the chloride level distinguishes normal anion gap from raised anion gap acidosis.

Hyperchloraemia occurs with large-volume 0.9% saline administration (which contains 154 mmol/L of chloride, well above physiological), gastrointestinal bicarbonate loss from diarrhoea, renal tubular acidosis, carbonic anhydrase inhibitors, and dehydration with water loss exceeding solute loss.

Hypochloraemia occurs with vomiting or nasogastric aspiration (loss of gastric hydrochloric acid), loop and thiazide diuretics, and any cause of metabolic alkalosis, alongside dilutional causes such as SIADH and heart failure.

• Chloride should always be interpreted alongside sodium, bicarbonate and the anion gap rather than in isolation, since the same chloride value carries different meaning at different sodium levels

• Calculate the anion gap: Na − (Cl + HCO₃). A raised chloride with a normal anion gap and low bicarbonate indicates a hyperchloraemic (normal anion gap) metabolic acidosis

• The chloride-to-sodium ratio is a useful refinement, since it corrects for changes in free water and identifies chloride excess or deficit that the absolute value obscures

• Urine chloride is the more informative test in metabolic alkalosis, distinguishing saline-responsive causes (urine chloride low, typically under 20 mmol/L, vomiting, nasogastric loss, prior diuretic use) from saline-resistant causes (urine chloride high: hyperaldosteronism, current diuretic use, Bartter and Gitelman syndromes).

• Urine sodium is unreliable for this purpose in alkalosis, because bicarbonate excretion obligates sodium loss

• Review fluid prescriptions and drug charts, since iatrogenic causes dominate hyperchloraemia in hospital practice

• Venous blood gas gives chloride and bicarbonate together for rapid assessment

Management is directed at the underlying disorder rather than at the chloride value itself.

Hyperchloraemia:

• Reduce or stop the chloride load: switch from 0.9% saline to a balanced crystalloid such as Hartmann's or Plasma-Lyte, which have a chloride content closer to plasma and produce less acidosis in large-volume resuscitation
• Treat gastrointestinal losses and the underlying cause of diarrhoea
• Address renal tubular acidosis where identified
• Bicarbonate replacement is reserved for severe or symptomatic acidosis rather than used to normalise a number

Hypochloraemia:

• Chloride-containing fluid replacement, usually as 0.9% saline, which is the correct choice here precisely because of its chloride content
• Potassium chloride where hypokalaemia coexists, which it usually does in vomiting and diuretic-related cases
• Treat the cause: antiemetics and management of gastric outlet obstruction where relevant, review of diuretic dosing, and investigation for hyperaldosteronism in saline-resistant cases
• In diuretic-induced hypochloraemic alkalosis, acetazolamide is sometimes used, and potassium-sparing agents may reduce ongoing loss

Referral: nephrology for renal tubular acidosis, unexplained persistent chloride disturbance, or suspected Bartter or Gitelman syndrome; endocrinology if hyperaldosteronism is suspected.