Nursing care
Pulse Oximetry: the method, the errors, and the exam
Written and reviewed by Dana Whitfield, RN, MSN · 5 min read · Updated September 2026
Short answer
Pulse oximetry is a noninvasive measure of arterial oxygen saturation, read by light absorption through a pulsating capillary bed. It measures how saturated the haemoglobin is with oxygen, not how well the patient is ventilating, which is why a patient retaining carbon dioxide can show a reassuring 98% while becoming acidotic and drowsy.
What the skill is for
Pulse oximetry gives a continuous, noninvasive estimate of arterial oxygen saturation by shining light of two wavelengths through a pulsating vascular bed, usually a fingertip or earlobe, and calculating the ratio of oxygenated to deoxygenated haemoglobin from the absorption pattern. It is used to monitor patients during and after procedures, on oxygen therapy, with respiratory or cardiac disease, and as an early trend indicator during deterioration.
The number it produces is oxygen saturation, SpO2, and nothing more. It says nothing about the effectiveness of ventilation, meaning how well carbon dioxide is being cleared, and nothing about tissue perfusion or oxygen delivery beyond the sensor site. A patient can have a normal or near-normal SpO2 while hypoventilating and accumulating carbon dioxide, because haemoglobin remains well saturated long after ventilation has become inadequate. This is the single fact that separates a nurse who trusts the number blindly from one who reads it in context.
The method, step by step
Choose a well-perfused site — a finger is standard, with earlobe or forehead sensors as alternatives when peripheral perfusion is poor. Remove nail polish or artificial nails if they interfere with light transmission, and warm a cold extremity if perfusion appears sluggish, since a cool, vasoconstricted finger gives an unreliable or absent waveform.
Apply the sensor so the light source and detector sit directly opposite each other across the tissue bed, and confirm a good pulsatile waveform on the monitor before trusting the number, not just a displayed percentage. Compare the displayed pulse rate against the patient's actual palpated or auscultated pulse; a mismatch means the reading is unreliable and should not be charted as accurate. Reposition or change sites periodically on continuous monitoring to prevent pressure injury to the digit.
Where it goes wrong
Poor peripheral perfusion from hypotension, hypothermia, or vasoconstrictive medications produces a weak or absent signal and an unreliable reading, often before the monitor flags it as such. Motion artefact from shivering, tremor, or an uncooperative patient can produce a falsely low or erratic reading that looks like desaturation but is not.
Carbon monoxide poisoning is the classic trap: carboxyhaemoglobin is read by standard pulse oximetry as though it were oxyhaemoglobin, producing a falsely reassuring high saturation in a patient who is severely hypoxic. Nail polish, especially dark blue, green, or black, and acrylic nails can attenuate the signal and lower the reading. Most importantly, oxygen saturation can remain in the high 90s in a patient who is hypoventilating and retaining carbon dioxide, because the oxyhaemoglobin dissociation curve is flat at high saturations — a large drop in ventilation can occur before saturation falls meaningfully. A rising respiratory rate, drowsiness, or confusion in a patient with a reassuring SpO2 is not reassuring at all.
Practising it deliberately
Practise applying the sensor and reading the waveform, not just the number, on every patient encounter, so an unreliable trace becomes instantly recognisable rather than something read past. Deliberately compare SpO2 against respiratory rate, work of breathing, and mental status on real patients, training the habit of cross-checking rather than trusting a single data point in isolation.
When possible, correlate a low or borderline SpO2 with an arterial blood gas to build an intuition for how the two relate and where they diverge, particularly in patients with chronic lung disease whose baseline saturation runs lower than the textbook normal. Practise recognising the clinical picture of hypercapnia — headache, drowsiness, flushed skin, reduced consciousness — independently of the oximeter reading, since this is the scenario where the device will mislead you if it is all you check.
Applying it on the exam
NCLEX questions on pulse oximetry commonly test recognition of unreliable readings: a scenario describing a hypothermic, shivering, or hypotensive patient with a low SpO2 is testing whether you identify poor perfusion or motion artefact as the cause before jumping to oxygen therapy. Expect a question pairing a normal or near-normal SpO2 with signs of respiratory distress or altered mental status, testing whether you recognise that saturation alone does not rule out respiratory failure.
Questions may also test correct technique — removing nail polish, confirming waveform quality, matching pulse rate to the patient's actual pulse — as the prerequisite to trusting any reading. A stem describing carbon monoxide exposure with a reassuringly high SpO2 is testing the false-normal trap directly, and the correct response is to treat the clinical picture, not the number.
A worked example
A patient with a COPD exacerbation and known carbon dioxide retention is on 2 litres of oxygen by nasal cannula. The pulse oximeter reads 98%, but the patient is increasingly drowsy, has a rising respiratory rate that then begins to slow, and is difficult to rouse. A nurse anchored to the oximeter alone would see a reassuring number and move on.
The correct read is that a falling respiratory drive and worsening drowsiness in a CO2 retainer point toward hypercapnic respiratory failure regardless of what the saturation shows, because oxygen delivery can mask hypoxaemia from developing while ventilation quietly worsens. The nurse should assess mental status and respiratory effort directly, obtain an arterial blood gas, and notify the provider promptly rather than waiting for the SpO2 to fall, since by the time it does, the patient may already be in extremis.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our respiratory practice questions are the closest set to what this page covers.
Common questions
Why does pulse oximetry read normal in a patient retaining CO2?
SpO2 measures how saturated haemoglobin is with oxygen, not how well the patient is ventilating or clearing carbon dioxide. The oxyhaemoglobin dissociation curve is flat at high saturations, so a patient can hypoventilate significantly before the SpO2 drops noticeably.
What causes a falsely high pulse oximetry reading?
Carbon monoxide poisoning is the most important cause, since carboxyhaemoglobin is misread by standard pulse oximeters as oxyhaemoglobin. Treat the clinical picture and, where available, use co-oximetry rather than trusting standard SpO2 in suspected exposure.
What should the nurse do if the pulse oximeter waveform looks poor?
Do not trust the displayed number until the waveform is corrected. Check the site for perfusion, warm a cold extremity, remove interfering nail polish, reduce motion, or relocate the sensor to a more reliable site such as the earlobe.
Does nail polish actually affect pulse oximetry readings?
Yes, particularly dark blue, green, black, and some metallic shades, which can attenuate the light signal and lower the reading. Remove polish where possible or use an alternate site such as the earlobe if it cannot be removed.