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For further information, see CMDT Part 23-16: Metabolic Acidosis

For further information, see CMDT Part 23-18: Normal Anion Gap Acidosis

KEY FEATURES

Essentials of Diagnosis

  • Decreased HCO3 and hyperchloremia (usually) with acidemia

  • Normal anion gap acidosis is mainly caused by loss of bicarbonate in the gastrointestinal (GI) track or renal tubular acidosis (RTA)

General Considerations0.

  • The hallmark of this disorder is that the low HCO3 of metabolic acidosis is associated with hyperchloremia, so that the anion gap remains normal

  • Decreased HCO3 is seen also in respiratory alkalosis, but the pH distinguishes between the two disorders

  • Two major causes

    • Bicarbonate loss from the GI tract

    • Defects in renal acidification (RTA)

  • The compensatory increase in serum chloride (hyperchloremia) maintains electroneutrality and a normal anion gap

  • The urinary anion gap can differentiate between these two causes

  • Table 23–12 outlines anion gap in metabolic acidosis

Table 23–12.Anion gap in metabolic acidosis.1
Gastrointestinal HCO3 Loss

  • The GI tract secretes bicarbonate at multiple sites

  • The most common cause of a non-anion gap metabolic acidosis from the GI tract is diarrhea (loss of bicarbonate rich stool fluid)

  • An infrequent cause is a ureterosigmoidostomy, where ureters are implanted into the sigmoid colon for urinary diversion

    • Unlike the bladder, colonic mucosa secretes bicarbonate in exchange for chloride, resulting in metabolic acidosis

    • This procedure is now rarely performed in the United States, though remains popular in other countries

  • More commonly, a neobladder is created using a loop of bowel (generally ileum or colon)

    • This procedure has significantly decreased the incidence of metabolic acidosis

    • Non-anion gap metabolic acidosis, however, can still occur when contact time between urine and mucosa is increased, typically as a result of an anastomotic stricture

Renal Tubular Acidosis

  • Defined as hyperchloremic acidosis with a normal anion gap and normal or near-normal glomerular filtration rate, in the absence of diarrhea

  • Three major types of RTA can be differentiated by ...

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