A 55-year-old man with acute pancreatitis and sepsis has pH 7.28, HCO₃⁻ 12 mEq/L, PaCO₂ 28 mmHg, and anion gap 18 mEq/L. A 48-year-old woman with chronic kidney disease stage 4 has pH 7.30, HCO₃⁻ 16 mEq/L, PaCO₂ 38 mmHg, and anion gap 16 mEq/L. Which finding best distinguishes high anion gap metabolic acidosis from normal anion gap metabolic acidosis?