99 Oxygen saturation in arterial blood (SaO2)

Arterial oxygen saturation: target ranges, pulse oximetry limitations and causes of hypoxaemia.

Arterial oxygen saturation (SaO₂) is the percentage of haemoglobin binding sites in arterial blood that carry oxygen. Each haemoglobin molecule has four haem groups, each able to bind one oxygen molecule. Saturation reflects how well the lungs oxygenate the blood, but not how much oxygen the blood carries: that also depends on the haemoglobin concentration. SaO₂ is measured in an arterial blood sample with a co-oximeter; pulse oximetry (SpO₂) estimates it non-invasively through the skin and is one of the most widely used vital signs.

OxygenBlood gasesPulse oximetry

Healthy adult at sea level≈ 95–100%

Target in acute illness (BTS)94–98%

Target with risk of CO₂ retention88–92%

Reference methodarterial blood co-oximetry

Figure 1. Oxygen saturation zones and BTS 2017 target ranges. Pulse oximeters are less accurate below about 90% and can overestimate saturation in people with darker skin.

Measurement

Pulse oximeters compare the absorption of red and infrared light by oxygenated and deoxygenated haemoglobin. Their readings can be inaccurate with poor circulation (cold hands, shock), movement, nail polish, carbon monoxide poisoning (falsely normal), methaemoglobin and at low saturations. Studies have shown that pulse oximetry more often misses low arterial oxygen in Black patients than in White patients, so an arterial blood gas should be checked when doubt arises.

Causes of low saturation (hypoxaemia)

Lung disease: pneumonia, COPD and asthma exacerbations, pulmonary oedema, pulmonary embolism, interstitial lung disease, ARDS
Hypoventilation: opioids, sedatives, neuromuscular weakness, obesity hypoventilation
Shunts: some congenital heart defects
Low inspired oxygen: high altitude
Sleep apnoea: repeated drops during sleep

Too much oxygen is also harmful: in patients with COPD it can raise CO₂, and very high oxygen levels may damage tissues, which is why oxygen is prescribed to a target range.

Seek emergency care

for saturation below about 92% at rest in a person without known chronic lung disease, or a sudden drop, especially with breathlessness, confusion, chest pain or bluish lips.

References

O’Driscoll BR, Howard LS, Earis J, Mak V; BTS Emergency Oxygen Guideline Group. BTS guideline for oxygen use in healthcare and emergency settings. Thorax. 2017;72(Suppl 1):ii1–ii90.
Sjoding MW, Dickson RP, Iwashyna TJ, Gay SE, Valley TS. Racial bias in pulse oximetry measurement. N Engl J Med. 2020;383(25):2477–2478. doi:10.1056/NEJMc2029240

This article is for general information and does not replace a consultation with a doctor. Measurements must be interpreted by a physician together with symptoms and other tests.

Published on 5 May 2024