Oxygen saturation (SpO₂) scales are used to assess the degree of haemoglobin oxygenation and provide a rapid, non-invasive measure of a patient’s respiratory status. Measured using pulse oximetry, peripheral oxygen saturation (SpO₂) is an essential physiological parameter routinely monitored in primary care, emergency medicine, inpatient wards, perioperative settings, and critical care.
SpO₂ represents the percentage of haemoglobin binding sites occupied by oxygen and serves as an indirect indicator of arterial oxygenation. In healthy adults breathing room air at sea level, oxygen saturation is typically between 95% and 100%, although acceptable values may vary according to age, altitude, and underlying cardiopulmonary disease.
Oxygen saturation is a key component of clinical assessment and is incorporated into numerous early warning systems, respiratory assessment tools, and physiological scoring systems. A reduction in SpO₂ may indicate impaired pulmonary gas exchange, ventilation-perfusion mismatch, hypoventilation, circulatory compromise, or progression of acute respiratory illness. Serial measurements are particularly valuable for detecting trends and identifying patients at risk of clinical deterioration.
Target oxygen saturation ranges are used to guide oxygen therapy and minimise the risks associated with both hypoxaemia and hyperoxaemia. For most acutely ill adults, a target SpO₂ of 94–98% is recommended. Patients with chronic hypercapnic respiratory failure, including many individuals with chronic obstructive pulmonary disease (COPD), are commonly managed to a lower target range of 88–92% in order to reduce the risk of oxygen-induced hypercapnia. Oxygen therapy should always be individualised and delivered in accordance with local clinical guidelines and the patient’s underlying condition.
Although pulse oximetry is a valuable monitoring tool, clinicians should recognise its limitations. Measurement accuracy may be reduced by poor peripheral perfusion, vasoconstriction, hypothermia, excessive patient movement, nail cosmetics, ambient light interference, dyshemoglobinaemias such as carboxyhaemoglobin or methaemoglobin, severe anaemia, and device-related factors. Emerging evidence also suggests that pulse oximetry accuracy may vary between individuals with different skin pigmentation, particularly at lower oxygen saturation levels. Consequently, SpO₂ values should always be interpreted within the broader clinical context.
Oxygen saturation should never be considered in isolation. Interpretation should include assessment of respiratory rate, work of breathing, heart rate, blood pressure, temperature, mental status, and the overall clinical presentation. Where oxygenation remains uncertain or significant respiratory compromise is suspected, arterial blood gas analysis may be required to provide direct measurement of arterial oxygen tension and acid-base status.
Oxygen saturation scales are integral to the assessment and management of numerous acute and chronic conditions, including pneumonia, asthma, chronic obstructive pulmonary disease, pulmonary embolism, heart failure, sepsis, and acute respiratory failure. They also contribute to risk stratification within early warning scores and support decisions regarding supplemental oxygen, escalation of care, and ongoing patient monitoring.
As a rapid, objective measure of oxygenation, pulse oximetry has become a cornerstone of modern clinical practice. When interpreted alongside clinical examination and appropriate investigations, oxygen saturation measurements provide valuable information that supports timely diagnosis, therapeutic decision-making, and the early recognition of patient deterioration.