What is the difference between SpO2 (peripheral capillary oxygen saturation) and SO2 in a medical context?

Difference Between SpO2 and SO2

SpO2 is oxygen saturation measured non-invasively by pulse oximetry, while SO2 (specifically SaO2) is the actual arterial oxygen saturation measured directly from arterial blood samples using laboratory spectrophotometry. 1, 2

Measurement Methods

SaO2 (Arterial Oxygen Saturation)

• SaO2 represents the actual percentage of hemoglobin bound to oxygen in arterial blood, measured directly from arterial blood samples using CO-oximetry. 1, 2
• This is the gold standard measurement requiring invasive blood sampling. 2
• The measurement is performed using laboratory spectrophotometry. 1

SpO2 (Peripheral Capillary Oxygen Saturation)

• SpO2 is a non-invasive estimation of arterial hemoglobin oxygen saturation using pulse oximetry. 1, 2
• Pulse oximeters detect variations in light transmission at two wavelengths (typically 660 and 990 nm) that occur with arterial pulsations in an extremity, usually the finger or ear lobe. 2
• This provides continuous, real-time monitoring without requiring blood draws. 2

Accuracy and Clinical Discrepancies

Expected Differences

• Pulse oximeters generally have 95% confidence limits of 4-5% compared to directly measured SaO2, meaning SpO2 can differ from SaO2 by this amount even under ideal conditions. 2
• In critically ill patients, the mean difference between SpO2 and SaO2 is approximately 2.75-3.8%, with SpO2 typically overestimating the true SaO2. 3, 4
• The standard deviation of differences ranges from 2.1-3.1%, indicating substantial variability between measurements. 3, 5

Factors That Worsen SpO2 Accuracy

• Hypoxemia (SaO2 <90%) significantly affects accuracy, with SpO2 overestimating SaO2 by a mean of 4.9% compared to 1.89% in non-hypoxemic patients. 4
• Poor peripheral perfusion yields falsely low SpO2 readings. 2
• Vasoactive drug requirements are associated with greater SpO2-SaO2 discrepancies. 5, 4
• Carboxyhemoglobin from carbon monoxide exposure or smoking causes SpO2 to overestimate true oxygenation, as standard pulse oximeters cannot differentiate carboxyhemoglobin. 2, 6
• Dark skin color can interfere with signal detection. 2
• Movement and stray light cause artifacts in readings. 2
• Older age, current smoking, and lower forced expiratory volume are associated with larger SpO2-SaO2 differences. 6

Critical Clinical Pitfall

When SpO2 reads 90-93%, the actual SaO2 is below 90% in 50% of cases, particularly in patients with severe sepsis and septic shock. 4 This means:

• A SpO2 threshold above 94% is necessary to ensure SaO2 is actually ≥90% with good sensitivity. 5
• In critically ill patients requiring precise oxygenation management, arterial blood gas analysis should be performed before adjusting FiO2 settings rather than relying solely on pulse oximetry. 3

Normal Values

SaO2 Normal Ranges by Age

• Adults aged 18-24 years: mean 96.9% (range 96.1-97.7%). 1
• Adults aged >64 years: mean 95.5% (range 92.7-98.3%). 1
• Normal range for healthy adults is approximately 95-98%. 2

SpO2 Normal Ranges by Age

• Adults aged 18-24 years: mean 98.0% (median 98%, IQR 97-99%). 1
• Adults aged ≥65 years: mean 95.8% (median 96%, IQR 95-98%). 1

When Each Measurement Should Be Used

• SpO2 is useful for continuous monitoring of oxygenation status in stable patients. 2
• SaO2 from arterial blood gas provides more accurate information when precise oxygenation assessment is critical, such as in severe sepsis, septic shock, hypoxemia, or when adjusting ventilator settings. 2, 3, 4
• In carbon monoxide poisoning or when carboxyhemoglobin is suspected, laboratory-based SaO2 measurements must be used as pulse oximetry gives falsely high readings. 2
• In neurocritical care, both periodic SaO2 measurements and continuous SpO2 monitoring should be used together to guide airway and ventilator management. 2