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

摘要

Hypobicarbonatemia or a reduced level of bicarbonate in plasma is a finding seen in three 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, the arterial blood gas (ABG) and the 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 the urine anion gap (UAG), as a surrogate marker of urine ammonium, in the evaluation of a patient with low plasma bicarbonate to differentiate between metabolic acidosis and chronic respiratory alkalosis. The interpretation and limitations of urine acid-base indices at bedside (urine pH, urine bicarbonate and urine anion gap) to evaluate urine acidification are discussed.