ABG results show PaO2 88 mm Hg, PaCO2 32 mm Hg, pH 7.2, HCO3- 12, Cl- 115, Na+ 138. The anion gap is normal. Which acid-base disorder is most likely responsible?

ABG results show PaO2 88 mm Hg, PaCO2 32 mm Hg, pH 7.2, HCO3- 12, Cl- 115, Na+ 138. The anion gap is normal. Which acid-base disorder is most likely responsible?

• A. Renal tubular acidosis
• B. Lactic acidosis
• C. Diabetic ketoacidosis
• D. Nasogastric suction

Answer: (A)

Normal anion gap metabolic acidosis (hyperchloremic) is characterized by a low bicarbonate with a normal anion gap, meaning there is no increase in unmeasured anions to raise the gap. In this case, the ABG shows pH 7.2 with HCO3- 12, and the chloride is elevated (Cl- 115). The anion gap calculation is Na - (Cl + HCO3) = 138 - (115 + 12) = 11, which is within the normal range. This pattern points to a non-gap metabolic acidosis due to bicarbonate loss or impaired acid excretion rather than accumulation of acids with unmeasured anions. Renal tubular acidosis fits perfectly because it causes bicarbonate loss or defective acid secretion in the kidneys, leading to a hyperchloremic, normal-AG metabolic acidosis. The other options produce different patterns: lactic acidosis and diabetic ketoacidosis cause high anion gap metabolic acidosis; nasogastric suction causes metabolic alkalosis from loss of HCl. Thus the presentation aligns best with renal tubular acidosis.

Normal anion gap metabolic acidosis (hyperchloremic) is characterized by a low bicarbonate with a normal anion gap, meaning there is no increase in unmeasured anions to raise the gap. In this case, the ABG shows pH 7.2 with HCO3- 12, and the chloride is elevated (Cl- 115). The anion gap calculation is Na - (Cl + HCO3) = 138 - (115 + 12) = 11, which is within the normal range. This pattern points to a non-gap metabolic acidosis due to bicarbonate loss or impaired acid excretion rather than accumulation of acids with unmeasured anions.

Renal tubular acidosis fits perfectly because it causes bicarbonate loss or defective acid secretion in the kidneys, leading to a hyperchloremic, normal-AG metabolic acidosis. The other options produce different patterns: lactic acidosis and diabetic ketoacidosis cause high anion gap metabolic acidosis; nasogastric suction causes metabolic alkalosis from loss of HCl. Thus the presentation aligns best with renal tubular acidosis.