Proportion of haemoglobin binding sites occupied by oxygen, reported as a percentage and commonly estimated noninvasively by pulse oximetry (SpO2); when measured invasively by co‑oximetry it yields arterial oxygen saturation (SaO2) and can distinguish dyshemoglobinemias.

Principle

Pulse oximetry estimates arterial oxygen saturation by measuring pulsatile differential light absorption of oxy‑ and deoxyhaemoglobin; valid readings require adequate arterial pulsatility and can be confounded by motion, poor perfusion, skin pigmentation, nail polish and dyshemoglobinemias (carboxyhaemoglobin, methemoglobin).

Demonstration

Situation: a patient with shortness of breath is triaged. Recognition: pulse oximeter probe is applied to a perfused fingertip and waveform quality is checked. Action: clinician records SpO2 and trend, and correlates with clinical signs and, if needed, arterial blood gas or co‑oximetry. Consequence: a sustained low SpO2 indicates hypoxaemia prompting oxygen therapy, further investigation of gas exchange, ventilation–perfusion mismatch or haemoglobin abnormalities; a normal SpO2 in the presence of hypoxia causes prompts alternative assessments (e.g., CO poisoning, severe anaemia).

Misapplication

Assuming a normal pulse oximeter percentage guarantees adequate oxygen delivery is a category error: oxygen content depends on haemoglobin concentration and cardiac output as well as saturation, and pulse oximetry cannot detect some forms of dysfunctional haemoglobin (e.g., carboxyhaemoglobin may falsely raise SpO2).

Consequence

Low arterial oxygen saturation reduces arterial oxygen content and can cause tissue hypoxia if compensatory mechanisms (cardiac output, oxygen extraction) are insufficient; false readings can produce inappropriate clinical reassurance or unwarranted interventions depending on the direction of error.

Reversal

Pulse oximetry reliably approximates arterial saturation only when arterial pulsatility is adequate and no dyshemoglobinemia is present; in states such as carbon monoxide poisoning, methemoglobinemia or severe anaemia, co‑oximetry, arterial blood gas analysis and clinical assessment are required to establish true oxygen status.

Boundary

Clearly within: functional arterial haemoglobin oxygen saturation estimated by pulse oximetry (SpO2) or measured by co‑oximetry (SaO2). Boundary case: discrepancy between SpO2 and SaO2 due to low perfusion or device limitations. Clearly outside: partial pressure of oxygen in blood (PaO2) which is related by the oxygen–hemoglobin dissociation curve but is a distinct physical variable.

Semantic Tension

Convenience and continuous noninvasive monitoring versus diagnostic specificity: pulse oximetry provides continuous, easy trend data but cannot replace targeted laboratory measurements when accuracy or differentiation of dyshemoglobinemias is required.

Synthesis

Oxygen saturation by pulse oximetry is a practical, continuous surrogate for arterial oxygenation that must be interpreted alongside perfusion, haemoglobin concentration and clinical context; where doubts exist, co‑oximetry or arterial blood gas analysis provides definitive assessment.