Oxygen results can describe different things. SpO₂ and SaO₂ are expressed as percentages, while PaO₂ is a pressure. Similar-looking numbers on a screen and a blood-test report should not be treated as interchangeable or used to choose an oxygen setting without clinical interpretation.
SpO₂: an estimate from a sensor
SpO₂ is the oxygen-saturation estimate produced by a pulse oximeter, often clipped to a finger. It estimates how much haemoglobin is carrying oxygen using light signals from pulsating blood. It does not measure the oxygen concentration coming out of a concentrator or the amount of carbon dioxide in the blood.
The FDA explains that pulse oximeters have limitations. Factors including circulation, skin pigmentation, movement and nail products can affect readings. An advertised accuracy specification is not a guarantee that every individual reading is within a fixed number of percentage points.
SaO₂ and PaO₂: information from arterial blood
SaO₂ describes arterial oxygen saturation. Depending on the test and analyser, a reported saturation may be directly measured using co-oximetry or calculated from other measurements. The clinical team can explain which method was used and whether abnormal forms of haemoglobin need consideration.
PaO₂ is the partial pressure of oxygen in arterial blood, reported in units such as mmHg or kPa. An arterial blood gas can also provide information about carbon dioxide and acidity. The ATS oxygen guide explains why clinicians may use blood testing as well as oximetry when assessing oxygen needs.
Why the numbers do not translate directly
The relationship between oxygen pressure and saturation is not linear and changes with physiological conditions. A percentage from a finger sensor therefore cannot reliably be converted into one arterial oxygen pressure. Saturation also does not describe haemoglobin concentration or the entire process of delivering oxygen to tissues.
A result must be interpreted alongside symptoms, the reason for testing, whether oxygen was being given and other clinical information. A reassuring saturation does not exclude carbon dioxide retention, anaemia or every cause of serious illness. A single unexpected result also should not be used to diagnose a condition at home.
Make home readings useful
Follow the device instructions and the monitoring plan given by the treating team. Record the time, symptoms, activity and prescribed oxygen setting when asked to keep a log. Use the sensor at its intended site; do not improvise another position or add a fixed correction to every reading.
Ask what range and changes require contact for this individual. If symptoms are severe or the person appears unwell, seek help even if the displayed number seems satisfactory. Do not increase, reduce or stop prescribed oxygen solely from an online chart. For the related clinical decision, see .