Oxygen saturation (SpO₂) is the percentage of haemoglobin binding sites occupied by oxygen within arterial blood and is one of the most frequently monitored physiological parameters in clinical practice. It provides a rapid, non-invasive assessment of oxygenation and plays a central role in the evaluation of respiratory, cardiovascular, and critical illness. Measurement of oxygen saturation is routinely performed across primary care, emergency medicine, hospital wards, ambulance services, perioperative care, and intensive care units.

Adequate oxygen delivery is essential for aerobic metabolism and normal cellular function. A reduction in oxygen saturation may indicate impaired pulmonary gas exchange, ventilation-perfusion mismatch, hypoventilation, or circulatory compromise, and should prompt clinical assessment to determine the underlying cause.

Measurement of Oxygen Saturation

Peripheral oxygen saturation is most commonly measured using pulse oximetry. A pulse oximeter uses red and infrared light wavelengths to estimate the proportion of oxygenated haemoglobin in pulsatile arterial blood. Sensors are typically applied to the finger, toe, or earlobe, allowing continuous, non-invasive monitoring.

Unlike arterial blood gas (ABG) analysis, pulse oximetry estimates oxygen saturation but does not directly measure arterial oxygen tension (PaO₂), carbon dioxide concentration, or acid-base status. Where a more comprehensive assessment of respiratory function is required, arterial blood gas analysis remains the reference standard.

Normal Oxygen Saturation

For healthy adults breathing room air at sea level, peripheral oxygen saturation generally falls within the following ranges:

• 95–100%: Normal oxygenation
• 90–94%: May indicate mild hypoxaemia, depending on the clinical context
• Below 90%: Consistent with significant hypoxaemia requiring urgent clinical assessment

Target oxygen saturation ranges should be individualised according to the patient’s underlying condition. Patients with chronic hypercapnic respiratory failure, including many individuals with chronic obstructive pulmonary disease (COPD), are commonly managed with lower target saturations of 88–92% to minimise the risk of oxygen-induced hypercapnia.

Causes of Reduced Oxygen Saturation

Hypoxaemia may occur in a wide variety of acute and chronic medical conditions, including:

• Pneumonia
• Acute exacerbations of asthma
• Chronic obstructive pulmonary disease (COPD)
• Pulmonary embolism
• Acute respiratory distress syndrome (ARDS)
• Heart failure
• Interstitial lung disease
• Sepsis
• Pneumothorax
• High-altitude exposure

The severity of oxygen desaturation should always be interpreted alongside the patient’s symptoms, respiratory effort, haemodynamic status, and overall clinical presentation.

Clinical Assessment

Although oxygen saturation is an important marker of respiratory function, it represents only one component of a comprehensive clinical assessment. Evaluation should also include:

• Respiratory rate
• Work of breathing
• Heart rate
• Blood pressure
• Level of consciousness
• Fraction of inspired oxygen (FiO₂)
• Arterial blood gas (ABG) analysis
• PaO₂/FiO₂ ratio
• Chest imaging where appropriate

Importantly, patients may exhibit significant respiratory distress despite relatively preserved oxygen saturation, particularly during the early stages of respiratory illness or in conditions associated with increased work of breathing.

Limitations of Pulse Oximetry

Pulse oximetry is a valuable monitoring tool but has recognised limitations that may affect measurement accuracy. Factors known to influence readings include:

• Poor peripheral perfusion
• Hypotension or vasoconstriction
• Patient movement
• Nail varnish or artificial nails
• Cold extremities
• Severe anaemia
• Bright ambient light
• Carbon monoxide poisoning
• Methaemoglobinaemia

Emerging evidence also suggests that pulse oximetry may demonstrate reduced accuracy in individuals with darker skin pigmentation, particularly at lower oxygen saturation levels. Consequently, SpO₂ measurements should always be interpreted within the wider clinical context and should not replace clinical judgement.

Oxygen Saturation in Clinical Scoring Systems

Oxygen saturation is incorporated into numerous validated clinical calculators and physiological scoring systems. It forms a key component of the National Early Warning Score 2 (NEWS2) and contributes to respiratory assessment tools used in emergency medicine, acute care, and critical care. Combined with arterial blood gas measurements, oxygen saturation also supports assessment of oxygenation, determination of respiratory failure severity, and decisions regarding supplemental oxygen and ventilatory support.

Clinical Significance

Oxygen saturation remains one of the most important bedside physiological measurements in modern healthcare. When interpreted alongside clinical examination, laboratory investigations, imaging, and validated clinical scoring systems, SpO₂ provides essential information for assessing respiratory function, identifying clinical deterioration, guiding oxygen therapy, and supporting evidence-based management across a broad spectrum of acute and chronic medical conditions.