Pulse Oximetry: Interpretation, Accuracy, and Clinical Use

Pulse oximetry is one of the most common bedside assessments in nursing, but it is also one of the most misinterpreted. This note reviews how to read SpO2 values correctly, what can make the reading unreliable, and how to correlate the number with the patient in front of you.

Definition

• Pulse oximetry is a non-invasive method to measure arterial oxygen saturation (SpO2) — the percentage of hemoglobin bound with oxygen.
• Normal SpO2 on room air is 95–100%. Values below this range indicate hypoxemia.
• SpO2 measures oxygenation only — it does not measure ventilation or carbon dioxide levels.

Normal and Abnormal Ranges

• Hypoxemia is defined as SpO2 below 95%; values below 90% require immediate intervention.
• In COPD patients, the acceptable SpO2 target is often 88–92%. High oxygen levels may suppress their hypoxic respiratory drive.
• A normal SpO2 does not rule out respiratory failure — the patient may have normal oxygenation but elevated CO2 (hypercapnia).

Factors Affecting Accuracy

• Remove nail polish from the monitoring fingernail; dark polish affects accuracy more than light polish.
• Poor perfusion from cold hands, hypovolemia, or hypotension weakens the pulsatile signal — try an earlobe or toe.
• Carbon monoxide poisoning causes a falsely high SpO2 because standard pulse oximetry cannot distinguish COHb from oxyhemoglobin.
• Methemoglobinemia causes SpO2 to trend toward 85% regardless of the patient's true saturation.
• Patient movement creates motion artifact — immobilize the site or use a motion-tolerant sensor.

Proper Placement Technique

• Select a site with adequate perfusion — finger, toe, or earlobe.
• Remove nail polish from the fingernail. If polish cannot be removed, place the probe sideways or use a different site.
• Warm the site if the patient's hands are cold; cold causes vasoconstriction and a poor signal.
• Position the sensor so the light emitter and detector are directly opposite each other.
• Allow several seconds for the signal to stabilize before recording.
• Do not place the sensor on an arm with a blood pressure cuff — cuff inflation will interrupt the signal.
• Do not place the sensor on a finger with an IV infusion — risk of inaccurate readings or tissue damage.

Clinical Correlation

• A patient with normal SpO2 can still be in respiratory failure if they are retaining CO2 (e.g., COPD exacerbation, opioid overdose).
• Always correlate SpO2 with clinical presentation: a patient with labored breathing, confusion, or cyanosis needs intervention even if SpO2 is normal.
• Early signs of hypoxia: restlessness, anxiety, tachycardia, tachypnea.
• Late signs of hypoxia: cyanosis, bradycardia.
• Do not rely on SpO2 alone — also assess work of breathing, mental status, and breath sounds.

Pulse Oximetry Waveform

• The plethysmograph (pleth wave) displayed on the oximeter indicates signal quality.
• A strong, regular waveform = good perfusion and a reliable reading.
• A weak, flat, or chaotic waveform = unreliable number; check sensor placement and perfusion before acting.

Common NCLEX Traps

• A normal SpO2 does not mean the patient is stable — assess work of breathing and mental status.
• Never ignore a low SpO2 without first checking the patient — verify clinically.
• Always remove nail polish before placing the probe.
• Never place the sensor on an arm with a BP cuff or a finger with an IV.
• In carbon monoxide poisoning, SpO2 is falsely high — do not trust it.
• Always check the waveform; a poor waveform means the number is not reliable.
• Remember COPD baseline SpO2 may be 88–92% — know the patient's baseline.

Key takeaways

• Normal SpO2 is 95–100% on room air; <90% requires immediate intervention; COPD target is often 88–92%.
• Pulse oximetry measures oxygenation only, not ventilation — a normal SpO2 does not rule out hypercapnic respiratory failure.
• CO poisoning falsely raises SpO2; nail polish, poor perfusion, and motion falsely lower it; methemoglobinemia drives the reading toward 85%.
• Never place the probe on an arm with a BP cuff or a finger with an IV; remove nail polish first.
• Always correlate the number with clinical signs: early hypoxia = restlessness, tachycardia, tachypnea; late = cyanosis, bradycardia.
• If the waveform is flat or chaotic, the SpO2 number cannot be trusted — troubleshoot before treating.