Blood Gases and Acid-base Profile
- An insight into the biochemistry of inborn errors of metabolism for a clinical neurologist

The second most important laboratory feature of many inherited neurometabolic disorders during episodes of illness is metabolic acidosis that is demonstrated by measuring arterial blood gases and bicarbonate. An increased anion gap (>16) is observed in many of these disorders due to the accumulation of fixed acids such lactic acid, ketoacids and other organic acids [Figure 3]. Diagnostically, identifying the unmeasured anion is the most important requirement in patients presenting with metabolic acidosis and increased anion gap. For this, lactate, 3-hydroxybutyrate, acetoacetate and organic acids are analyzed. The largest group consisting of organic acidemias, including entities such as methylmalonic, propionic and isovaleric acidemias are easily recognized by their typical organic acid profiles. In addition to specific organic acid intermediates, plasma lactate is often elevated in organic acidemias as a result of secondary interference with CoA metabolism. A flowchart for the evaluation of patients with metabolic acidosis with increased anion gap[5] is presented in [Figure 4].

Metabolic acidosis with a normal anion gap is noted in renal tubular acidosis or diarrhea. A history of diarrhea is usually sufficient to distinguish hyperchloremic metabolic acidosis due to excessive gastrointestinal bicarbonate losses from that arising from renal tubular dysfunction. Metabolic disorders associated with renal tubular acidosis include hereditary fructose intolerance, hepatorenal tyrosinemia, cystinosis, Fanconi-Bickel syndrome, Lowe syndrome, vitamin D dependency and congenital lactic acidosis due to cytochrome c oxidase deficiency.

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