Chloride

The serum chloride test (Cl, chloride ions) is used to assess electrolyte and water balance and to support the differential diagnosis of metabolic acidosis and metabolic alkalosis. Results generally track with serum sodium, but chloride may change independently in acid–base disorders. Elevated chloride occurs with dehydration, ammonium chloride administration, hyperchloremic (normal anion gap) metabolic acidosis such as renal tubular acidosis, and after excessive infusion of normal saline. Reduced concentrations are seen with overhydration, congestive heart failure, syndrome of inappropriate antidiuretic hormone secretion, vomiting or gastric suction, chronic respiratory acidosis, Addison disease, salt-losing nephritis, burns, metabolic alkalosis, and during some diuretic therapy. The test is also used to monitor disorders and treatments that may perturb electrolyte balance.

Chloride is a negatively charged electrolyte that partners with sodium and potassium to regulate extracellular fluid volume and acid–base homeostasis. It is present in all body fluids, with highest levels in blood and interstitial fluid. Dietary chloride, largely consumed as sodium chloride, is absorbed in the gastrointestinal tract. The kidney reabsorbs most filtered chloride, and urinary excretion varies with volume status and the accompanying cations. Gastrointestinal losses from vomiting, diarrhea, or enteric fistulas lower total body chloride, whereas diuresis increases urinary chloride output. In metabolic acidosis, chloride commonly rises as bicarbonate falls, producing hyperchloremic acidosis. Aldosterone promotes renal sodium reabsorption and secondarily influences chloride handling. Serum chloride concentrations in infants are typically higher than in adolescents and adults.