A 52-year-old man with sepsis and metabolic acidosis (pH 7.22, HCO₃⁻ 16 mEq/L) presents with tissue hypoxia despite adequate hemoglobin (14 g/dL) and normal SaO₂ (98%). His mixed venous oxygen saturation (SvO₂) is 58% (normal 75%). To determine whether his tissue hypoxia is due to a leftward shift of the oxygen dissociation curve impairing oxygen unloading, which investigation is most appropriate?