[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Clinical exam](https://durgical.com/en/clinical-exam/) › [General exam and vital signs](https://durgical.com/en/clinical-exam/#a-general) › Pulse oximetry [Clinical exam](https://durgical.com/en/clinical-exam/) · [General exam and vital signs](https://durgical.com/en/clinical-exam/#a-general) # Pulse oximetry - Vital sign A finger sensor estimates how much oxygen the blood carries; quick and useful, but with pitfalls worth knowing. Updated 2026-11-01 ## Key points - Normal: 95 to 100% on room air; somewhat lower values may be acceptable in COPD or at altitude. - The reading may differ by 2 to 4 points from true arterial saturation; when in doubt, confirm with arterial blood gases. - Dark skin, poor perfusion, cold, dark nail polish and artificial nails all affect the reading. - In carbon monoxide poisoning SpO2 can look normal even though available oxygen is low. - Check that the waveform is good and that the oximeter's rate matches the palpated pulse. ## What it is Pulse oximetry estimates the oxygen saturation of arterial hemoglobin (SpO2) by measuring how red and infrared light are absorbed in a pulsating bed such as a finger or earlobe. It is non-invasive and continuous, and many units treat it as the fifth vital sign. It measures saturation, not ventilation or oxygen content: a normal value does not exclude hypoventilation, anemia or carbon monoxide poisoning. ## Reference values - Normal: 95 to 100%. - Below 95% is considered abnormal and is investigated (OpenStax). ## How to measure it - Choose a warm, well-perfused finger with no dark polish or artificial nail; otherwise use the earlobe. - Place the sensor so the light passes through the nail bed; avoid the arm with a blood pressure cuff inflating. - Ask the patient to keep the hand still, supported at heart level. - Wait for a steady, regular waveform before reading, usually a few seconds. - Check that the displayed rate matches the palpated pulse. - Record the SpO2 and whether the patient is on room air or supplemental oxygen (and how much). ## What abnormal values mean - Low SpO2: pneumonia, asthma or COPD exacerbation, pulmonary embolism, pulmonary edema, hypoventilation and shunting heart disease. - Low reading with a poor waveform: usually artefact (cold, movement, poor perfusion); reposition before concluding. - A value around 85% that does not improve with oxygen, with chocolate-coloured blood: methemoglobinemia. - Normal SpO2 with marked dyspnea: does not exclude hypoventilation with high carbon dioxide, severe anemia or carbon monoxide. ## Common errors - Reading the value before the waveform settles. - Measuring on a cold finger or on the arm with the cuff inflated. - Forgetting that dark skin can overestimate saturation and hide hypoxemia. - Not recording whether the patient was on oxygen, which makes the number uninterpretable. ## Test yourself Think of the answer, then tap the question to see it. ## What is the normal SpO2 range on room air? 95 to 100%; somewhat lower values may be acceptable in patients with COPD or who live at altitude. ## Why does a normal SpO2 not rule out carbon monoxide poisoning? Because a standard oximeter cannot tell carboxyhemoglobin from oxyhemoglobin and shows a falsely normal saturation. ## What should be checked before trusting an oximeter reading? That the waveform is steady and regular and that the displayed rate matches the palpated pulse. ## Which patient factors can distort the reading? Poor perfusion, cold, movement, dark nail polish, artificial nails and dark skin pigmentation. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/clinical-exam/pulse-oximetry/) ## Related - [Respiratory rate and breathing patterns](https://durgical.com/en/clinical-exam/respiratory-rate-and-breathing-patterns/) - [Pallor, cyanosis and jaundice](https://durgical.com/en/clinical-exam/pallor-cyanosis-and-jaundice/) - [Early warning scores: NEWS2 and qSOFA](https://durgical.com/en/clinical-exam/early-warning-scores-news2-and-qsofa/) - [Vital signs: what they are and how to record them](https://durgical.com/en/clinical-exam/vital-signs-what-they-are-and-how-to-record-them/) ## Related conditions - [Acute respiratory failure (hypoxaemic and hypercapnic)](https://durgical.com/en/conditions/acute-respiratory-failure/) - [Chronic obstructive pulmonary disease (COPD)](https://durgical.com/en/conditions/chronic-obstructive-pulmonary-disease/) - [Methemoglobinemia](https://durgical.com/en/conditions/methemoglobinemia/) [← All of the clinical exam](https://durgical.com/en/clinical-exam/) Educational content on the clinical exam; it is learned and practised under supervision. It does not replace assessment by a health professional and is not for self-diagnosis. Sources: [Pulse Oximetry: MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/pulse-oximetry/) (U.S. National Library of Medicine (MedlinePlus), 2025); [7.1 Indicators of Physiologic Functioning - Fundamentals of Nursing | OpenStax](https://openstax.org/books/fundamentals-nursing/pages/7-1-indicators-of-physiologic-functioning) (OpenStax, Rice University, 2024); [Blue discoloration of the skin: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003215.htm) (U.S. National Library of Medicine (MedlinePlus), 2025). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/clinical-exam/pulse-oximetry/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/clinical-exam/pulse-oximetry/