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# Normal anion gap (hyperchloraemic) metabolic acidosis

Bicarbonate is lost through the gut or [kidney](https://durgical.com/en/anatomy/kidney/) and chloride rises; the most common cause is diarrhoea.

Updated 2026-11-01

## Structures involved

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- [Kidneys](https://durgical.com/en/anatomy/kidney/)
- [Ileum](https://durgical.com/en/anatomy/ileum/)
- [Ascending colon](https://durgical.com/en/anatomy/ascending-colon/)
- [Transverse colon](https://durgical.com/en/anatomy/transverse-colon/)
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## In biochemistry

- [Acid–base interpretation (step by step)](https://durgical.com/en/biochemistry/topics/acid-base-interpretation/)
- [Anion gap and delta ratio](https://durgical.com/en/biochemistry/topics/anion-gap-and-delta-ratio/)

## In physiology

- [Water and electrolyte absorption and how diarrhea happens](https://durgical.com/en/physiology/water-and-electrolyte-absorption-and-how-diarrhea-happens/)
- [Metabolic acidosis, the anion gap and compensation](https://durgical.com/en/physiology/metabolic-acidosis-the-anion-gap-and-compensation/)
- [Proximal tubule transport](https://durgical.com/en/physiology/proximal-tubule-transport/)
- [Renal handling of H⁺ and bicarbonate](https://durgical.com/en/physiology/renal-handling-of-h-and-bicarbonate/)
- [IV fluids: tonicity and where they go](https://durgical.com/en/physiology/iv-fluids-tonicity-and-where-they-go/)

## What it is

A metabolic acidosis from bicarbonate loss or failure of the kidney to excrete acid, with high chloride and a normal anion gap.

## Who gets it

Acute diarrhoea, very common in Latin American children, is its main cause; in hospital it is common after resuscitation with large volumes of saline. Renal tubular acidoses are less common.

## Causes

- Gut loss of bicarbonate: diarrhoea, pancreatic or biliary fistulas, high-output ileostomy, urinary diversion into bowel (ureterosigmoidostomy)
- Distal (type 1), proximal (type 2) or hypoaldosteronism (type 4) renal tubular acidosis
- Drugs: acetazolamide, topiramate, amiloride and spironolactone (type 4)
- Excess 0.9 % saline (dilutional hyperchloraemic acidosis)
- Recovery phase of diabetic ketoacidosis

## Risk factors

- Acute diarrhoea in infants and children
- Resuscitation with large volumes of saline
- Autoimmune disease (Sjögren) and tubulotoxic drugs
- Diabetes with hyporeninaemic hypoaldosteronism

## Pathophysiology

When bicarbonate is lost in stool or urine, or the tubule cannot excrete H⁺, HCO₃⁻ falls and the kidney retains chloride to keep neutrality: the anion gap does not change. In distal RTA H⁺ secretion fails in the collecting duct, urine cannot be acidified and there is hypokalaemia, nephrocalcinosis and stones; in proximal RTA HCO₃⁻ is lost through a low reabsorption threshold, often with Fanconi syndrome; in type 4 lack of aldosterone or of response to it causes hyperkalaemia, which suppresses ammonium production. The urine anion gap (Na⁺ + K⁺ − Cl⁻) estimates urine ammonium: negative when the kidney responds (diarrhoea), positive when it cannot excrete acid (RTA).

## Classification

- Extrarenal (gut): negative urine anion gap
- Renal: RTA type 1 (distal), type 2 (proximal), type 4 (hyperkalaemic); positive urine gap
- Dilutional or chloride-driven (saline)

## Symptoms and signs

- Often only those of the cause (diarrhoea, dehydration)
- Fast, deep breathing if severe
- Muscle weakness from hypokalaemia (diarrhoea, RTA 1 and 2)
- Poor growth and rickets in children with RTA
- Kidney stones (distal RTA)

## Red flags

- Severe dehydration in an infant with diarrhoea
- Hypokalaemia with paralysis or arrhythmias
- Hyperkalaemia with ECG changes (type 4)

## Diagnosis

**Clinical**
History of diarrhoea, stomas, drugs or autoimmune disease; signs of dehydration.

**Tests**
Gases and electrolytes with high chloride and normal (albumin-corrected) anion gap, potassium, urine anion gap, urine pH, glycosuria and phosphaturia with normal blood glucose (Fanconi), aldosterone and renin in type 4.

**Imaging**
Renal ultrasound for nephrocalcinosis or stones in distal RTA.

## Treatment

**Medical**
Rehydrate and replace bicarbonate or its precursors (citrate) in gut losses

**Medical**
Continuous oral alkali in RTA types 1 and 2, with potassium when needed

**Medical**
In type 4: correct hyperkalaemia, stop causative drugs and, when indicated, give a mineralocorticoid

**Follow-up**
Prefer balanced solutions over saline in large resuscitations and monitor chloride and gases

## Complications

- Severe hypokalaemia and paralysis
- Nephrocalcinosis and stones
- Rickets and poor growth
- Bone loss

## Prognosis

Good in treated diarrhoea; chronic RTAs need lifelong treatment and monitoring of growth and kidneys.

## Prevention

- Early oral rehydration salts in childhood diarrhoea
- Sensible use of saline
- Electrolyte monitoring with drugs that affect the tubule

## Test yourself

Think of the answer, then tap the question to see it.

## What is the most common cause of normal anion gap acidosis?

Diarrhoea, through gut loss of bicarbonate.

## What is the urine anion gap used for?

It estimates urine ammonium: negative when the kidney excretes acid (diarrhoea), positive when it cannot (renal tubular acidosis).

## Which RTA comes with hyperkalaemia?

Type 4, from lack of or resistance to aldosterone, common in diabetes.

## Why is distal RTA linked to kidney stones?

Persistently alkaline urine, low urine citrate and hypercalciuria favour calcium phosphate precipitation.

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## Related conditions

- [High anion gap metabolic acidosis](https://durgical.com/en/conditions/high-anion-gap-metabolic-acidosis/)
- [Hypokalaemia](https://durgical.com/en/conditions/hypokalaemia/)
- [Hyperkalaemia](https://durgical.com/en/conditions/hyperkalaemia/)
- [Acute (viral) gastroenteritis](https://durgical.com/en/conditions/acute-gastroenteritis/)
- [Chronic kidney disease](https://durgical.com/en/conditions/chronic-kidney-disease/)
- [Adrenal insufficiency](https://durgical.com/en/conditions/adrenal-insufficiency/)

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Metabolic acidosis: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000335.htm) (MedlinePlus, US National Library of Medicine, 2026); [Distal renal tubular acidosis: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000493.htm) (MedlinePlus, US National Library of Medicine, 2026); [Proximal renal tubular acidosis: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000497.htm) (MedlinePlus, US National Library of Medicine, 2026); [Chloride Blood Test: MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/chloride-blood-test/) (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/conditions/normal-anion-gap-metabolic-acidosis/