Question

A patient with renal tubular acidosis has the following arterial blood gas (ABG): pH = 7.24, PaO2 = 80, PaCO2 = 36, Na = 131, HCO3 = 14, Cl = 90, base excess (BE) = -13, glucose = 135. The above ABG picture suggests:

• A. Respiratory acidosis
• B. Respiratory alkalosis
• C. Metabolic acidosis
• D. Metabolic alkalosis

Hint:

Low pH plus low HCO3 with a CO2 that is only mildly reduced points to a primary metabolic problem.

Answer 1

The Correct Option is C

Step 1: Read the pH. The pH is 7.24, which is below the normal 7.35 to 7.45, so the patient is acidotic.
Step 2: Find the primary driver. The HCO3 is 14 (normal 22 to 26) and the base excess is strongly negative at -13. A low bicarbonate with a low pH points to a metabolic acidosis.
Step 3: Check the respiratory side. PaCO2 is 36, sitting at the lower end of the normal 35 to 45 mmHg range. This drop is the expected respiratory compensation (blowing off CO2) for the metabolic acidosis, not a primary respiratory problem.
Step 4: Tie it to the clinical picture. Renal tubular acidosis causes loss of bicarbonate or failure to excrete acid, giving a normal anion gap metabolic acidosis, which fits this ABG perfectly.
So the picture is metabolic acidosis (option C). Ref: Harrison's Principles of Internal Medicine, 20e.