A 32-year-old woman with chronic diarrhea presents with pH 7.32, HCO₃⁻ 14 mEq/L, PaCO₂ 32 mmHg, and normal anion gap. A 28-year-old man with COPD presents with pH 7.32, HCO₃⁻ 32 mEq/L, PaCO₂ 58 mmHg, and normal anion gap. Which feature best distinguishes metabolic acidosis from respiratory acidosis in these two patients?