What factors can cause falsely low pulse‑oximeter (SpO₂) readings?

Factors Causing Falsely Low Pulse Oximetry Readings

Poor peripheral perfusion is the primary cause of falsely low SpO₂ readings because pulse oximeters require adequate pulsatile arterial flow to function accurately. 1, 2

Technical and Physiological Factors

Poor Perfusion States

• Cold extremities, hypotension, shock, or low cardiac output states produce falsely low readings because the device cannot detect adequate pulsatile blood flow necessary for measurement 3, 2
• Cold-induced vasoconstriction dramatically reduces peripheral blood flow, making it difficult or impossible for the device to detect adequate arterial pulsatile signals 2
• Severe finger clubbing with extensive lipodermatosclerosis significantly hinders accuracy by compromising detection of adequate pulsatile arterial flow 1

Anatomical and Structural Interference

• Pulsating arteries directly beneath the sensor can cause falsely low readings when the photodiode is positioned over a superficial artery (such as the temporal artery), resulting in SpO₂ values 5.8% lower in adults and 7.5% lower in neonates 4
• Altered tissue architecture in clubbed fingers changes the normal light transmission pathway, interfering with the device's ability to accurately measure the pulsatile component 1
• Inadequate surface contact from severe clubbing or abnormal finger anatomy prevents proper probe seating and signal detection 1, 3

Motion and Electrical Interference

• Movement artifact during measurement causes significant errors and data dropout because pulse oximeters assume arterial blood is the only pulsatile absorber 3, 2
• Concomitant use of peripheral nerve stimulators or evoked-potential stimulators on the same limb creates artificial pulsatile signals that the device misinterprets as desaturation 5

Pharmacological Interference

• Methylene blue injection causes falsely low SpO₂ readings by interfering with light absorption at the wavelengths used by pulse oximeters 6

Clinical Verification and Alternative Approaches

Signal Quality Assessment

• Verify signal quality by confirming 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 1, 2
• Ensure adequate surface contact and perfusion by repositioning the probe and repeating measurements 1

Alternative Measurement Sites

• Use an ear lobe probe as the preferred alternative site when an adequate signal cannot be obtained on the finger, ensuring any jewelry is removed and gently rubbing the lobe to improve local perfusion 1, 2
• Avoid placing the sensor directly over pulsating arteries, as this positioning systematically produces falsely low readings 4

When to Obtain Arterial Blood Gas

• If adequate signal cannot be obtained despite repositioning maneuvers, obtain arterial blood gas analysis because pulse oximetry measures saturation (SaO₂) rather than partial pressure (PaO₂), and PaO₂ is more relevant for assessing pulmonary gas exchange 1
• Never rely solely on pulse oximetry when clinical assessment suggests respiratory compromise, especially in patients with known perfusion issues 1, 2

Important Caveats

• Pulse oximeters have inherent accuracy limitations of ±4-5% even under optimal conditions, meaning a reading of 95% could represent anywhere from 90-100% 1, 3, 2
• Dark skin pigmentation causes systematic overestimation (not underestimation) of oxygen saturation, but can interfere with signal detection leading to unreliable readings 1, 3, 2
• Accuracy is less reliable at saturations below 88%, which is further exacerbated in Black patients 3, 2