Acidosis, Metabolic, Decreased or Normal Anion Gap

Key Features

Essentials of Diagnosis

• Decreased HCO₃^– and hyperchloremia (usually) with acidemia

• Normal anion gap acidosis is mainly caused by gastrointestinal HCO₃^– loss or renal tubular acidosis (RTA); urinary anion gap may help distinguish between these causes

General Considerations

• 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

• Most common causes

  • Gastrointestinal (GI) HCO₃^– loss

  • Defects in renal acidification (renal tubular acidoses)

• The urinary anion gap can differentiate between these two causes

• Table 21–12 outlines anion gap in metabolic acidosis

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), which has significantly decreased the incidence of metabolic acidosis, though it can still occur when contact time between urine and mucosa is increased, typically as a result of an anastomotic stricture

Renal tubular acidosis (RTA)

• 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 the clinical setting: urinary pH, urinary anion gap (see below), ...