Metabolic Acidosis or Respiratory Alkalosis? Evaluation of a Low Plasma Bicarbonate Using the Urine Anion Gap

Am J Kidney Dis. 2017 Sep;70(3):440-444. doi: 10.1053/j.ajkd.2017.04.017. Epub 2017 Jun 7.


Hypobicarbonatemia, or a reduced bicarbonate concentration in plasma, is a finding seen in 3 acid-base disorders: metabolic acidosis, chronic respiratory alkalosis and mixed metabolic acidosis and chronic respiratory alkalosis. Hypobicarbonatemia due to chronic respiratory alkalosis is often misdiagnosed as a metabolic acidosis and mistreated with the administration of alkali therapy. Proper diagnosis of the cause of hypobicarbonatemia requires integration of the laboratory values, arterial blood gas, and clinical history. The information derived from the urinary response to the prevailing acid-base disorder is useful to arrive at the correct diagnosis. We discuss the use of urine anion gap, as a surrogate marker of urine ammonium excretion, in the evaluation of a patient with low plasma bicarbonate concentration to differentiate between metabolic acidosis and chronic respiratory alkalosis. The interpretation and limitations of urine acid-base indexes at bedside (urine pH, urine bicarbonate, and urine anion gap) to evaluate urine acidification are discussed.

Keywords: Urine anion gap (UAG); acid-base disorders; arterial blood gas (ABG); chronic respiratory alkalosis; hypobicarbonatemia; nephrology; renal excretion; urinary ammonium; urine acidification; urine pH.

Publication types

  • Case Reports

MeSH terms

  • Acidosis / diagnosis*
  • Aged, 80 and over
  • Alkalosis, Respiratory* / blood
  • Alkalosis, Respiratory* / diagnosis
  • Alkalosis, Respiratory* / etiology
  • Diagnosis, Differential
  • Disease Management
  • Female
  • Humans
  • Hydrogen-Ion Concentration
  • Hyperventilation* / blood
  • Hyperventilation* / etiology
  • Stroke / complications*
  • Water-Electrolyte Imbalance* / blood
  • Water-Electrolyte Imbalance* / diagnosis
  • Water-Electrolyte Imbalance* / etiology