Difference Between Arterial Oxygen Saturation (SaO2) and Peripheral Capillary Oxygen Saturation (SpO2)
SaO2 is the direct measurement of oxygen bound to hemoglobin in arterial blood obtained through invasive blood sampling, while SpO2 is a non-invasive estimation of arterial oxygen saturation measured by pulse oximetry that may differ from the actual arterial value by up to ±4%. 1, 2
Definitions and Measurement Methods
Arterial Oxygen Saturation (SaO2)
- SaO2 represents the actual amount of oxygen bound to hemoglobin in a given volume of arterial blood, expressed as a percentage of the total capacity for oxygen binding of the hemoglobin in that blood volume 1
- Measured directly from arterial blood samples using laboratory spectrophotometry (CO-oximetry) 1
- Considered the gold standard for measuring oxygen saturation 2
- Modern blood gas analyzers with CO-oximeters directly measure the concentrations of oxyhemoglobin, deoxyhemoglobin, carboxyhemoglobin, and methemoglobin through spectrophotometry 1
Peripheral Capillary Oxygen Saturation (SpO2)
- SpO2 is a non-invasive estimation of arterial hemoglobin oxygen saturation using pulse oximetry 1
- Utilizes the principles of spectrophotometry and pulse plethysmography to estimate oxygen saturation 1, 2
- Pulse oximeters detect the variation in transmission of light at two different wavelengths (typically 660 and 990 nm) that occur with arterial pulsations in an extremity (usually finger or ear lobe) 1
- The probe (sensor) can be placed on the ear lobe or fingertip 1
Accuracy and Limitations
Accuracy of SpO2 Compared to SaO2
- Pulse oximeters generally have reasonable accuracy with 95% confidence limits of 4-5% compared to directly measured SaO2 1
- Manufacturers typically claim an accuracy of ±2% (standard deviation of differences between SpO2 and SaO2) 2
- In clinical studies, the actual error is often 3-4%, which means SpO2 readings can differ from SaO2 by this amount 2
- Recent studies show a mean difference (bias) between SpO2 and SaO2 of:
Factors Affecting SpO2 Accuracy
- Poor peripheral perfusion can yield falsely low SpO2 readings 1
- Movement and stray light can cause artifacts in SpO2 readings 1
- Dark skin color can interfere with signal detection 1
- Standard pulse oximeters cannot detect carboxyhemoglobin or methemoglobin 1
- Smoking status affects accuracy - increased carboxyhemoglobin levels are independently associated with differences between SpO2 and SaO2 4
- The accuracy appears to be influenced by the type of oximeter, presence of hypoxemia, and requirement for vasoactive drugs 5
Clinical Implications
- SpO2 commonly overestimates the actual SaO2, particularly in smokers and patients with elevated carboxyhemoglobin levels 4
- Pulse oximetry has high accuracy in estimating oxygen saturation when SpO2 is >90% 6
- For critical clinical decisions, especially in critically ill patients receiving supplemental oxygen, arterial blood gas analysis should be performed 2
- A SpO2 above 94% appears necessary to ensure a SaO2 of 90% in critically ill patients 5
Normal Values
- Normal SaO2 ranges from approximately 95-98% in healthy adults 1
- Mean SaO2 in adults aged 18-24 years is about 96.9%, while in adults >64 years it is about 95.5% 1
- Normal SpO2 values are similar but may show slight variations from SaO2 1
- Both SaO2 and SpO2 tend to decrease slightly with age 1
Clinical Applications
- SpO2 is useful for continuous monitoring of oxygenation status 1
- SaO2 provides more accurate information but requires invasive blood sampling 1
- In neurocritical care, periodic measurements of PaO2 and SaO2 and continuous SpO2 measurements should be used to guide airway and ventilator management 1
- In cases of carbon monoxide poisoning, standard pulse oximeters may give falsely high readings as they cannot differentiate carboxyhemoglobin 1
- For patients being considered for hyperbaric oxygen therapy, laboratory-based SaO2 measurements should confirm pulse oximetry readings 1