What does it mean if arterial oxygen saturation is low but pulse oximetry (SpO2) is high?

Understanding the Discrepancy Between Low Arterial Oxygen Saturation and High SpO2

When arterial oxygen saturation (SaO2) measured by blood gas is low but pulse oximetry (SpO2) is high, this represents a measurement error or technical limitation of the pulse oximeter—the SpO2 reading is falsely elevated and should not be trusted. 1, 2

Why This Discrepancy Occurs

Technical Limitations of Pulse Oximetry

Pulse oximeters require adequate pulsatile arterial flow to function accurately, and when this is compromised, readings become unreliable. 2

Key mechanisms causing falsely high SpO2 readings include:

• Poor peripheral perfusion from shock, hypotension, or vasoconstriction prevents adequate signal detection, making the device unable to accurately measure oxygen saturation 1, 2, 3
• Inadequate surface contact from probe malposition, movement artifact, or anatomical variations (such as severe finger clubbing) interferes with proper light transmission 2
• Standard two-wavelength pulse oximeters cannot distinguish carboxyhemoglobin (COHb) or methemoglobin (MetHb), leading to falsely elevated readings in carbon monoxide poisoning or methemoglobinemia 2, 4
• Dark skin pigmentation systematically overestimates oxygen saturation and interferes with signal detection 2

Inherent Accuracy Limitations

• Even under optimal conditions, pulse oximeters have accuracy limitations of ±4-5% compared to directly measured arterial oxygen saturation 2, 5
• In critically ill ICU patients, the mean difference between SpO2 and SaO2 can be -0.02% with standard deviation of ±2.1%, but large individual variations occur with poor reproducibility 5
• Pulse oximetry commonly overestimates SaO2, with 23% of patients at rest showing SpO2 ≥5% higher than actual SaO2 4

Immediate Clinical Actions

Verify Signal Quality First

Before trusting any SpO2 reading, verify that the heart rate displayed on the pulse oximeter matches the ECG or palpated pulse rate—if these don't match closely, the reading is unreliable. 2

Steps to improve measurement accuracy:

• Reposition the probe and repeat measurements to ensure adequate surface contact 1, 2
• Suction the patient's nose and oral airway if necessary before reassessing 1
• Use an ear lobe probe as an alternative site, removing any jewelry and gently rubbing the lobe to improve local perfusion 2
• Ensure the patient's hand is still and not gripping objects tightly, as movement artifact causes significant errors 2

Obtain Arterial Blood Gas Analysis

The presence of a normal SpO2 does not negate the need for blood gas measurements, especially if the patient is on supplemental oxygen therapy. 1

When to obtain ABG:

• All critically ill patients require blood gas analysis regardless of SpO2 readings 1
• Any unexpected or inappropriate fall in SpO2 below 94% in patients breathing air or oxygen warrants blood gas measurement 1
• If adequate pulse oximeter signal cannot be obtained despite repositioning maneuvers, obtain arterial blood gas analysis immediately 2
• For critically ill patients with shock or hypotension (systolic BP <90 mmHg), the initial blood gas measurement should be obtained from an arterial sample 1

Clinical Significance

Why Arterial Blood Gas is Superior

Pulse oximetry measures saturation (SpO2) rather than partial pressure (PaO2), and PaO2 is more relevant for assessing pulmonary gas exchange effects. 2

Critical distinctions:

• Pulse oximetry will be normal in a patient with normal oxygen tension (PaO2) but abnormal blood pH or carbon dioxide tension (PCO2) 1
• A patient can have normal SpO2 but low blood oxygen content due to anemia, which only blood gas and full blood count can detect 1
• Despite a fall in arterial PaO2 to 70 mmHg, saturation would still remain above 93% because the oxygen dissociation curve at this point is insensitive to changes in PO2 2

High-Risk Situations

Never rely solely on pulse oximetry when clinical assessment suggests respiratory compromise, especially in patients with known perfusion issues. 2

Patients requiring particular caution:

• Those requiring vasoactive drugs show decreased accuracy of SpO2 measurements 5
• Current smokers with elevated carboxyhemoglobin levels have significantly higher SpO2-SaO2 discrepancies 4
• Patients with severe hypoxemia (SpO2 <88%) have less reliable pulse oximetry accuracy, particularly in Black patients 2
• Premature infants and those with bronchopulmonary dysplasia or congenital heart disease require closer monitoring and should not rely on pulse oximetry alone 1, 6

Practical Threshold for Action

A SpO2 above 94% appears necessary to ensure a SaO2 of 90%, meaning high SpO2 thresholds are required to detect significant hypoxemia with good sensitivity. 5

• For most patients not at risk of hypercapnic respiratory failure, target SpO2 is 94-98% 1, 6
• For patients at risk of hypercapnic respiratory failure (severe COPD, neuromuscular disease, severe obesity, cystic fibrosis), target SpO2 is 88-92% 1, 6
• A SpO2 level below 94% should prompt the assumption that the patient is hypoxic until proven otherwise by arterial blood gas 7