Normal anion gap acidosis

Normal anion gap acidosis
Other names	Non-anion gap acidosis
Specialty	Endocrinology, nephrology

Normal anion gap acidosis is an acidosis that is not accompanied by an abnormally increased anion gap. ^[1]

The most common cause of normal anion gap acidosis is diarrhea with a renal tubular acidosis being a distant second.

Differential diagnosis

The differential diagnosis of normal anion gap acidosis is relatively short (when compared to the differential diagnosis of acidosis):

• Hyperalimentation (e.g. from TPN containing ammonium chloride)
• Chloride administration, often from normal saline
• Acetazolamide and other carbonic anhydrase inhibitors
• Renal tubular acidosis ^[2]
• Diarrhea: due to a loss of bicarbonate. This is compensated by an increase in chloride concentration, thus leading to a normal anion gap, or hyperchloremic, metabolic acidosis. The pathophysiology of increased chloride concentration is the following: fluid secreted into the gut lumen contains higher amounts of Na⁺ than Cl⁻; large losses of these fluids, particularly if volume is replaced with fluids containing equal amounts of Na⁺ and Cl⁻, results in a decrease in the plasma Na⁺ concentration relative to the Cl⁻concentration. This scenario can be avoided if formulations such as lactated Ringer’s solution are used instead of normal saline to replace GI losses. ^[3]
• Ureteroenteric fistula – an abnormal connection (fistula) between a ureter and the gastrointestinal tract
• Pancreaticoduodenal fistula – an abnormal connection between the pancreas and duodenum
• Spironolactone
• High ostomy output ^[2]
• Hyperparathyroidism – can cause hyperchloremia and increase renal bicarbonate loss, which may result in a normal anion gap metabolic acidosis. Patients with hyperparathyroidism may have a lower than normal pH, slightly decreased PaCO2 due to respiratory compensation, a decreased bicarbonate level, and a normal anion gap. ^[4]

As opposed to high anion gap acidosis (which involves increased organic acid production), normal anion gap acidosis involves either increased production/administration of chloride (hyperchloremic acidosis) or increased excretion of bicarbonate.

See also

• High anion gap metabolic acidosis

References

1. Kharsa, Antoine; Vashisht, Rishik; Rout, Preeti; Meseeha, Marcelle (2025), "Anion Gap and Non-Anion Gap Metabolic Acidosis", StatPearls, Treasure Island (FL): StatPearls Publishing, PMID   28846305 , retrieved 2025-11-03
2. "Metabolic Acidosis: Acid-Base Regulation and Disorders: Merck Manual Professional" . Retrieved 2008-12-04.
3. Jean-Louis Vincent; Abraham Edward; Kochanek Patrick (8 July 2011). "Acid-base disorders". Textbook of Critical Care . Elsevier. ISBN   143771367X.
4. Coe FL (August 1974). "Magnitude of metabolic acidosis in primary hyperparathyroidism". Arch. Intern. Med. 134 (2): 262–5. doi:10.1001/archinte.1974.00320200072008. PMID   4843192.