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  • Decreased HCO3 with acidemia (low blood pH).

  • Classified into increased anion gap acidosis and normal anion gap acidosis.

  • Lactic acidosis, ketoacidosis, and toxins produce metabolic acidoses with the largest anion gaps.

  • Normal anion gap acidosis is mainly caused by GI HCO3 loss or RTA.


The hallmark of metabolic acidosis is low serum bicarbonate concentration from loss of bicarbonate or gain of acid (Table 21–12); the anion gap detects an increase in plasma anions other than from measured bicarbonate and chloride.

Table 21–12.Anion gap in metabolic acidosis.1

Many clinicians use 12 mEq/L as the normal serum anion gap (range 4–12 mEq/L due to differences in analyzer methods).

Anion Gap = Na+ – (HCO3 + Cl)

If serum potassium is included in the formula, the range for anion gap increase by about 4 mEq/L:

Anion gap = (Na+ + K+) – (HCO3 + Cl)

The principle unmeasured anion usually responsible for the anion gap is albumin. The expected anion gap must be adjusted for hypoalbuminemia; the anion gap decreases by approximately 2.5 mEq/L for every 1 g/dL reduction in the serum albumin concentration.

Corrected serum anion gap = (measured serum anion gap) + (2.5 × [4.0 – serum albumin])

In metabolic acidosis from a gain of acid, the anion gap will increase because the addition of acid includes the addition of anions. In nongap or hyperchloremic metabolic acidosis, the anion gap is normal because the rise in chloride parallels the fall in bicarbonate.

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