Oxygen Saturation is the proportion of haemoglobin binding sites in arterial blood that are occupied by oxygen, commonly expressed as a percentage. In routine healthcare, the term usually refers to peripheral oxygen saturation, abbreviated SpO₂, measured non-invasively with a pulse oximeter. Arterial oxygen saturation, commonly denoted SaO₂, is determined from arterial blood analysis and represents a laboratory measurement rather than a pulse-oximeter estimate. Oxygen saturation is important because haemoglobin carries most of the oxygen transported in blood, but saturation alone does not completely describe oxygen delivery to tissues. Haemoglobin concentration, cardiac output, tissue perfusion and oxygen consumption also influence the amount of oxygen actually available to cells.
Pulse oximetry estimates oxygen saturation by analysing how light is absorbed by oxygenated and deoxygenated haemoglobin. A sensor typically emits light at wavelengths that are absorbed differently by these forms of haemoglobin and detects changes associated with pulsatile arterial blood. The device uses these optical signals to calculate an estimated saturation and usually displays a pulse rate as well. Because the measurement depends on pulsatile blood flow and optical conditions, poor circulation, movement, sensor positioning, external light and other factors can affect accuracy. Pulse oximetry is therefore a monitoring technology rather than a direct measurement of blood oxygen content.
Oxygen saturation is closely related to the oxygen–haemoglobin dissociation curve. Haemoglobin's affinity for oxygen changes according to factors including partial pressure of oxygen, carbon dioxide concentration, blood pH and temperature. The resulting relationship is sigmoidal, meaning that saturation changes relatively little across some higher oxygen tensions but falls more rapidly at lower tensions. This property helps haemoglobin load oxygen in the lungs and release it in tissues. A shift of the dissociation curve can alter haemoglobin's affinity for oxygen without necessarily changing the total amount of oxygen carried in the same way. Understanding saturation therefore requires distinguishing it from partial pressure and total oxygen content.
A normal oxygen saturation depends partly on altitude and individual circumstances. At sea level, healthy adults commonly have peripheral saturation values in the mid-to-high 90s, although exact normal ranges depend on context and measurement conditions. At high altitude, lower arterial oxygen partial pressure can produce lower saturation in healthy people because atmospheric oxygen pressure is reduced. Some chronic respiratory conditions can also be associated with lower baseline saturations, and clinical targets may deliberately differ according to diagnosis. A single universal threshold should therefore not be applied without considering the person, clinical setting and established treatment plan.
Pulse oximeter readings can be affected by several technical and physiological limitations. Poor peripheral perfusion can weaken the pulsatile signal, while movement can create artefact. Nail products, skin characteristics, sensor positioning and ambient light can influence measurements depending on the device and circumstances. Certain abnormal haemoglobin species can also interfere with conventional two-wavelength pulse oximetry because the device's assumptions about haemoglobin absorption may not hold. Carbon monoxide exposure is particularly important because carboxyhaemoglobin can produce misleadingly reassuring pulse-oximeter readings. Suspected carbon monoxide poisoning therefore requires appropriate clinical assessment and specialised measurement rather than reliance on ordinary SpO₂ alone.
Oxygen saturation is not identical to oxygenation, ventilation or oxygen delivery. Oxygenation refers broadly to the process and adequacy of oxygen entering the blood, whereas ventilation concerns movement of air and removal of carbon dioxide. A person can have adequate oxygen saturation while retaining excessive carbon dioxide, because conventional pulse oximetry does not measure ventilation. Conversely, tissue oxygen delivery can be inadequate despite an apparently acceptable saturation if haemoglobin concentration or cardiac output is severely reduced. Arterial blood-gas analysis can provide additional information about oxygen and carbon dioxide partial pressures and acid–base status when clinically indicated.
Oxygen saturation is widely used in respiratory and acute care, anaesthesia, emergency medicine, home oxygen assessment and monitoring of various illnesses. Falling saturation can indicate impaired gas exchange caused by conditions such as pneumonia, pulmonary oedema, asthma, chronic lung disease or pulmonary embolism, but the measurement alone cannot establish which condition is responsible. Supplemental oxygen can raise saturation, but oxygen therapy is a treatment requiring appropriate clinical targets because excessive oxygen can also be harmful in particular circumstances. Monitoring should therefore assess both the numerical reading and the patient's clinical state, including respiratory effort, consciousness, circulation and symptoms.
The principal strength of Oxygen Saturation measurement is that it provides rapid, non-invasive information about oxygenation trends. Its principal limitation is that the number can appear more precise than the underlying measurement actually is. Pulse oximeters estimate saturation rather than directly measuring arterial oxygen content, and accuracy can deteriorate under particular conditions. A normal reading does not exclude significant respiratory or cardiovascular disease, while an unexpectedly low reading should be confirmed and interpreted according to clinical circumstances when necessary. For an AI or clinician, the distinction among SpO₂, SaO₂, oxygen partial pressure and total oxygen content is essential because these terms describe related but non-identical physiological quantities.
