How to interpret pulse oximeter readings

Pulse Oximetry: How to Use It, Interpret Readings, and Limitations — pulse oximeter readings (SpO2) are interpreted by comparing peripheral oxygen saturation percentage to expected ranges, with normal resting SpO2 typically 95–100% and clinical concern usually beginning below about 90%. Because the device estimates arterial hemoglobin saturation from red and infrared light absorption rather than measuring arterial PaO2 or oxygen content directly, trends, symptoms, and baseline lung disease must inform interpretation. Short-term dips during activity or measurement error are common, so confirm low values with repeat readings, proper technique, or arterial blood gas (ABG) when clinical decisions depend on accurate oxygenation.

A pulse oximeter works by photoplethysmography and the Beer–Lambert law: LED emitters (red and infrared) and a photodetector measure pulsatile absorbance to estimate oxygen saturation. SpO2 interpretation uses that percentage as a noninvasive surrogate for SaO2, while arterial blood gas with co-oximetry measures PaO2 and true SaO2. Named manufacturers and standards, such as Masimo, Nellcor, and FDA device specifications, set performance thresholds and testing protocols. In the evaluation and diagnostic testing context, the normal SpO2 range guides monitoring, but the chosen tool and method determine pulse oximeter accuracy and clinical use.

The most important nuance is that SpO2 is an estimate, not a direct measure of oxygen content or PaO2, and context changes thresholds: a patient with chronic obstructive pulmonary disease may have an established baseline SpO2 of 88% without acute distress, whereas a previously healthy adult at 92% could be developing hypoxemia. The oxygen–hemoglobin dissociation curve links PaO2 ≈ 60 mmHg with SpO2 ≈ 90%, so small percentage differences can reflect large PaO2 shifts. Factors affecting oximeter readings include skin pigmentation (several studies report overestimation in darker skin), low perfusion, motion artifact, nail polish, ambient light, and abnormal hemoglobins, all of which reduce pulse oximeter accuracy and may necessitate ABG confirmation.

Practical application requires correct technique—warm and still extremity, proper sensor placement, removal of nail polish, and repeated measurements—and interpretation against symptoms, baseline lung disease, and recent activity. Persistent SpO2 below about 90%, a new sustained fall from baseline, or discordance between readings and clinical appearance should prompt further evaluation with ABG or clinician assessment. The page provides a structured, step-by-step framework.