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

Work the ABG in order. First the pH 7.24 is acidemic. Now ask which system is to blame. In a respiratory acidosis the CO2 would be high; here PaCO2 is 36, which is low, so respiration is not the culprit. Instead the bicarbonate is the abnormal number: HCO3 14 and base excess -13 are both markedly low, the signature of a metabolic acidosis. The slightly low CO2 is simply Winter's compensation, the lungs hyperventilating to offset the acid load. Clinically this matches renal tubular acidosis, where the kidney cannot reclaim bicarbonate or secrete enough hydrogen ion, producing a non anion gap metabolic acidosis (here Na 131, Cl 90, HCO3 14 give a near normal gap). Eliminate alkalosis options outright since the pH is acidic, and eliminate respiratory acidosis since CO2 is not raised. The answer is metabolic acidosis.