Noninvasive, continuous, real-time monitoring Pulse oximetry gives an immediate, continuous readout of hemoglobin oxygen saturation (SpO2) without needing arterial blood sampling. It's cheap, portable, easy to apply (finger, ear, forehead probe), and requires minimal training, which is why it has become a near-universal "fifth vital sign" in EDs, ORs, ICUs, wards, and outpatient settings - Miller's Anesthesia, 10e.

Rapid detection of hypoxemia in room air If a patient is breathing room air, pulse oximetry detects a fall in alveolar ventilation "rather quickly," making it a genuinely useful early-warning tool in that specific setting - Roberts and Hedges' Clinical Procedures in Emergency Medicine.

Good general safety adjunct Combined with capnography, ECG, and blood pressure monitoring, it meaningfully improves the safety of procedural sedation and anesthesia by flagging desaturation before it becomes clinically obvious - Roberts and Hedges' Clinical Procedures in Emergency Medicine.

Useful in specific screening contexts It has documented utility beyond routine vital signs monitoring, such as screening for peripheral artery disease via toe-brachial/segmental oximetry indices, and general perioperative/critical care monitoring.

It measures oxygenation, not ventilation This is the most clinically important limitation. Pulse oximetry only reflects hemoglobin saturation, not CO2 clearance. On supplemental oxygen, a patient can be apneic for 4-5 minutes before SpO2 drops significantly, because the reservoir of oxygen in the lungs and blood delays desaturation even while ventilation has completely stopped. Capnography detects hypoventilation and apnea far earlier (rising or absent end-tidal CO2) and has been shown across multiple studies to identify respiratory depression earlier and more frequently than pulse oximetry during sedation - Roberts and Hedges' Clinical Procedures in Emergency Medicine.

Nonlinear relationship with PaO2 Because the oxyhemoglobin dissociation curve is sigmoidal, a "normal" or high SpO2 cannot distinguish normoxia from hyperoxia - relevant when trying to limit oxygen exposure/toxicity in neonates or critically ill patients. Conversely, on the steep part of the curve small drops in SpO2 can reflect large drops in PaO2 - Miller's Anesthesia, 10e.

Dyshemoglobinemias cause false readings Standard two-wavelength pulse oximeters cannot distinguish oxyhemoglobin from other hemoglobin species:

Technical/physical interference Accuracy degrades with motion artifact, poor peripheral perfusion (hypotension, vasoconstriction, cold extremities, shock), low-amplitude or irregular pulse signals, ambient light interference, and nail polish or artificial nails (especially dark colors).

Skin pigmentation bias - an important and current issue This is a well-documented and evolving concern. Recent systematic reviews and meta-analyses show pulse oximetry tends to overestimate true arterial oxygen saturation in patients with darker skin pigmentation compared to lighter skin, meaning occult hypoxemia can be missed in these patients - a finding with real implications for equitable clinical decision-making (e.g., triggering supplemental oxygen or hospital admission).

These are recent (2024-2025) systematic reviews/meta-analyses; if your clinical context involves patients with darker skin tones, treat any borderline-normal SpO2 with more caution than the number alone suggests, and corroborate with clinical exam or arterial blood gas when in doubt.

Pulse oximetry is an excellent, cheap, continuous screen for hypoxemia but is not a substitute for ventilation monitoring (capnography) or a definitive measure of oxygenation status in the presence of dyshemoglobinemias, poor perfusion, or darker skin pigmentation. It should be interpreted alongside clinical assessment and, when accuracy is in question, arterial blood gas analysis.