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For further information, see CMDT Part 21-17: Metabolic Acidosis

For further information, see CMDT Part 21-19: Normal Anion Gap Acidosis

Key Features

Essentials of Diagnosis

  • Decreased HCO3 and hyperchloremia (usually) with acidemia

  • Normal anion gap acidosis is mainly caused by gastrointestinal HCO3 loss or RTA. Urinary anion gap may help distinguish between these causes

General Considerations

  • 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

Table 21–12.Anion gap in metabolic acidosis.1
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, serum K+ level

  • Type I (distal H+ secretion defect)

    • Characterized by selective deficiency in H+ secretion by alpha intercalated cells in the collecting tubule

    • Enhanced K+ excretion occurs probably due to less competition from H+ in the distal nephron transport system

    • Develops as a consequence of paraproteinemias, autoimmune disease, and drugs and toxins such as amphotericin

  • Type II (proximal HCO3 reabsorption defect)

    • Due to a defect in the ability of the proximal tubule to reabsorb filtered HCO3

    • Hypokalemia results when a patient is treated with HCO3 without adequate K+ supplementation

    • Can exist with other proximal reabsorption defects resulting in glucosuria, aminoaciduria, phosphaturia, and uricosuria (Fanconi syndrome)

    • Causes include plasma cell myeloma and nephrotoxic drugs and carbonic anhydrase inhibitors (acetazolamide)

  • Type IV (hyporeninemic hypoaldosteronism)

    • Most common RTA in clinical practice

    • Due to a defect in aldosterone action that impairs distal nephron Na+ reabsorption and therefore K+ and H+ excretion

    • Common causes include diabetic nephropathy and tubulointerstitial renal diseases

    • In ...

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