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Clinical exam · General exam and vital signs

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

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

  1. Choose a warm, well-perfused finger with no dark polish or artificial nail; otherwise use the earlobe.
  2. Place the sensor so the light passes through the nail bed; avoid the arm with a blood pressure cuff inflating.
  3. Ask the patient to keep the hand still, supported at heart level.
  4. Wait for a steady, regular waveform before reading, usually a few seconds.
  5. Check that the displayed rate matches the palpated pulse.
  6. 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.

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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 (U.S. National Library of Medicine (MedlinePlus), 2025); 7.1 Indicators of Physiologic Functioning - Fundamentals of Nursing | OpenStax (OpenStax, Rice University, 2024); Blue discoloration of the skin: MedlinePlus Medical Encyclopedia (U.S. National Library of Medicine (MedlinePlus), 2025).

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Representative model, for educational purposes only.