What is the difference between SpO2 and SaO2?

What is the difference between SpO2 and SaO2?

• A. SpO2 is oxygen saturation measured by pulse oximetry; SaO2 is arterial oxygen saturation measured from an ABG or co-oximetry.
• B. SpO2 is arterial oxygen saturation; SaO2 is measured by pulse oximetry.
• C. SpO2 and SaO2 measure the same thing.
• D. SpO2 is measured in the blood gas; SaO2 is calculated.

Answer: (A)

SpO2 and SaO2 both describe how much hemoglobin in the blood is carrying oxygen, but they are obtained in different ways and can differ under certain conditions. SpO2 is what a pulse oximeter shows. It’s a noninvasive estimate of arterial oxygen saturation based on how light is absorbed by pulsatile arterial blood at two wavelengths. It gives a quick, continuous readout that tracks the oxygen saturation in arterial blood, but it’s an indirect measurement and can be affected by factors like poor perfusion, movement, skin color, nail polish, or dyshemoglobins (for example, methemoglobin or carboxyhemoglobin). Because it’s an estimate, it may deviate from the true arterial saturation in these situations. SaO2 is the true arterial oxygen saturation obtained from a blood sample. Measured with co-oximetry on an arterial blood gas (ABG) or directly by co-oximetry, it can distinguish various forms of hemoglobin (oxyhemoglobin, deoxyhemoglobin, methemoglobin, carboxyhemoglobin). This makes it more accurate in cases where abnormal hemoglobins are present or when precise saturation is needed. ABG-derived SaO2 can also be calculated from PaO2, but co-oximetry provides a direct measurement of saturation with the different hemoglobin species. So, SpO2 is a noninvasive, continuous estimate from pulse oximetry; SaO2 is the actual arterial saturation measured from blood (ABG or co-oximetry), with co-oximetry providing the most accurate assessment in the presence of dyshemoglobins.

SpO2 and SaO2 both describe how much hemoglobin in the blood is carrying oxygen, but they are obtained in different ways and can differ under certain conditions.

SpO2 is what a pulse oximeter shows. It’s a noninvasive estimate of arterial oxygen saturation based on how light is absorbed by pulsatile arterial blood at two wavelengths. It gives a quick, continuous readout that tracks the oxygen saturation in arterial blood, but it’s an indirect measurement and can be affected by factors like poor perfusion, movement, skin color, nail polish, or dyshemoglobins (for example, methemoglobin or carboxyhemoglobin). Because it’s an estimate, it may deviate from the true arterial saturation in these situations.

SaO2 is the true arterial oxygen saturation obtained from a blood sample. Measured with co-oximetry on an arterial blood gas (ABG) or directly by co-oximetry, it can distinguish various forms of hemoglobin (oxyhemoglobin, deoxyhemoglobin, methemoglobin, carboxyhemoglobin). This makes it more accurate in cases where abnormal hemoglobins are present or when precise saturation is needed. ABG-derived SaO2 can also be calculated from PaO2, but co-oximetry provides a direct measurement of saturation with the different hemoglobin species.

So, SpO2 is a noninvasive, continuous estimate from pulse oximetry; SaO2 is the actual arterial saturation measured from blood (ABG or co-oximetry), with co-oximetry providing the most accurate assessment in the presence of dyshemoglobins.