What can cause a patient to have a low oxygen saturation reading but normal PaO2 on ABG?

A low oxygen saturation reading, while having a normal arterial blood gas (ABG) PaO2, can occur in cases of methemoglobinemia. This condition arises when hemoglobin is oxidized to methemoglobin, which is unable to bind oxygen effectively. Consequently, the total oxygen content in the blood may remain normal (hence the normal PaO2), as the amount of dissolved oxygen in plasma is not affected. However, the pulse oximeter may give a misleading low reading of oxygen saturation since it cannot differentiate between normal hemoglobin and methemoglobin.

In contrast, conditions like acute lung injury, pneumonia, or asthma exacerbation usually present with both low oxygen saturation and low PaO2 on ABG due to impaired gas exchange or ventilation-perfusion mismatch, making them less likely explanations for the scenario described.