2 Oxygen Saturation and Pulse Oximetry
Understand what pulse oximetry estimates, what it cannot measure, why readings may be unreliable, and how to interpret results alongside symptoms and clinical assessment.
What estimates
provides an estimate of and pulse rate without directly measuring arterial blood. A standard fingertip device primarily estimates the percentage of hemoglobin carrying oxygen in pulsating peripheral blood.
What the reading cannot tell you
does not directly measure , ventilation, carbon dioxide, or the total amount of oxygen carried in the blood. It also does not establish that oxygen delivery to body tissues is normal. A reading is an estimate rather than an exact measurement, so it cannot replace assessment of the person.
Interpreting a reading
For many healthy adults at sea level, is commonly between and . The appropriate range for an individual may differ because of health conditions, altitude, or a clinician-directed target. A value such as may fall within the commonly seen range for a healthy adult at sea level, but it does not by itself prove that ventilation or the overall respiratory assessment is normal.
Assess the person, not just the number
Consider the reading alongside symptoms, respiratory assessment, circulation, and the person’s usual oxygen-saturation range. Do not let a single number overrule concerning symptoms or clinical findings. An unexpected value should prompt assessment of the person and verification of sensor placement and signal quality; if appropriate, repeat the measurement at a suitable site. Escalate concerns based on the whole clinical picture and local protocol.
Factors that affect accuracy
Readings can be affected by the person and the measurement conditions. Factors that may interfere include skin pigmentation, poor circulation, cold skin, movement or shivering, and nail products such as dark nail polish or artificial nails. A still, well-perfused finger generally supports a more reliable signal.
Checking an unreliable reading
When a reading fluctuates or seems unreliable, assess the person and check sensor placement and signal quality. If feasible, reduce movement or shivering, warm a cold hand, wait for a stable signal, or repeat the measurement at another suitable site. Continue assessing the person rather than relying on the number alone.
When a normal value may mislead
can produce a falsely reassuring reading, even when a person has symptoms such as headache and confusion. A displayed value of does not rule out this concern or directly measure carboxyhemoglobin. Symptoms and exposure history matter, and may be needed to help identify abnormal hemoglobins.
When further testing may be needed
An may be used to assess oxygenation and ventilation when they remain uncertain or the reading conflicts with the clinical picture. can help identify abnormal hemoglobins such as carboxyhemoglobin. Further assessment or testing may be needed when a result is unreliable or does not fit the person’s condition.
Use trends and symptoms
A change over time can be more informative than one isolated value. For example, a fall in from to , together with worsening shortness of breath, is concerning even before the cause is known. Reassess promptly and respond according to the person’s condition and local clinical protocol; do not wait for a particular displayed number if the person is deteriorating.