Pulse Oximetry Terms and Definitions: A Practical Glossary for Device Developers A Common Language for Pulse Oximetry Design and Testing

Pulse oximetry is broadly adopted across clinical care and wearable technologies. Standardized terminology shared by engineering, clinical research, and regulatory teams promotes aligned protocols, reproducible analyses, and more efficient FDA and CE reviews. 

This glossary provides clear definitions of core pulse oximetry terms commonly encountered during pulse oximetry testing, controlled desaturation studies following ISO 80601-2-61, and regulatory-grade CRO engagements. It is intended for product developers, R&D teams, and decision-makers working in physiological monitoring. 

Core Pulse Oximetry Concepts 

Pulse Oximetry 

A noninvasive optical method used to estimate oxygen saturation by measuring changes in light absorption caused by pulsatile blood flow. Pulse oximetry relies on photoplethysmography and the different absorption of red and infrared light by oxygenated and deoxygenated hemoglobin. 

SpO₂ (Oxygen Saturation estimated by Pulse Oximetry) 

An estimate of arterial oxygen saturation derived from optical measurements at the tissue surface. It reports the proportion of hemoglobin that is oxygenated (functional oxygen saturation). SpO₂ is not a direct measurement; it is calculated using calibration models developed during controlled desaturation clinical research studies. 

SaO₂ (Arterial Oxygen Saturation) 

The percentage of hemoglobin saturated with oxygen as measured directly from arterial blood. SaO₂ is used as the reference comparator obtained by co-oximetry on arterial blood during pulse oximetry studies conducted in a hypoxia lab under IRB oversight. 

ISO 80601-2-61 

The international standard for pulse oximeter clinical accuracy and essential performance. It describes controlled desaturation methods, reference measurements, data pairing, and how to report the pulse oximeter’s performance.  

Optical and Hardware Terminology 

Photoplethysmography (PPG) 

An optical sensing technique that detects changes in blood volume by measuring variations in light absorption or reflection over time. In pulse oximetry, the pulsatile component of the PPG signal corresponds primarily to arterial blood flow. 

Red Wavelength 

Light typically centered around 660 nm, where deoxygenated hemoglobin exhibits higher absorption relative to oxygenated hemoglobin. 

Infrared (IR) Wavelength 

Light typically ranging from 880–940 nm, where oxygenated hemoglobin absorbs more light than deoxygenated hemoglobin. 

Transmissive Pulse Oximetry 

An optical configuration in which the light source and photodetector are placed on opposite sides of the tissue. This design is commonly used at the fingertip or earlobe. 

Reflective (Reflectance) Pulse Oximetry 

An optical configuration in which the light source and photodetector are positioned on the same surface of the skin.  

Signal Processing and Measurement Terms 

AC Component 

The pulsatile portion of the PPG signal associated with arterial blood volume changes during each cardiac cycle. 

DC Component 

The non-pulsatile portion of the PPG signal caused by static tissue, venous blood, bone, and baseline light absorption. 

Ratio of Ratios (R-Value or Modulation Ratio) 

A value calculated from the relative AC and DC components of red and infrared signals of a pulse oximeter. The R-value serves as the input to empirical calibration curves used to estimate SpO₂. 

Calibration Curve 

A mathematical relationship that maps the optical R-value to SpO₂ based on reference measurements. Calibration curves are device- and algorithm-specific and are supported by appropriate clinical research studies. 

Averaging Time (Averaging Window) 

The time window a device uses to calculate SpO₂ values; longer windows reduce noise but may delay response to rapid changes. 

Update Period  

How often the device updates the displayed SpO₂ value. 

Motion Artifact 

Unwanted signal distortion caused by movement, pressure changes, or sensor displacement.  

Percent Modulation (% mod) or Perfusion Index (PI) 

Indicator of pulsatile signal strength or good blood flow at the sensor site.

Signal Quality Index (SQI) 

A device or analysis metric that flags segments with poor signal (e.g., motion, low perfusion) so they are excluded from accuracy calculations. 

Study Terminology 

Controlled Desaturation 

A study method in which participant oxygen saturation is gradually and safely lowered under controlled conditions to generate calibration and verification data. Controlled desaturation studies are central to regulatory-grade pulse oximetry testing. 

Hypoxia Lab 

A specialized research environment designed to safely manipulate inspired oxygen levels while continuously monitoring physiological parameters. Hypoxia labs support repeatable, optimal, high-fidelity data collection for pulse oximetry studies. 

Arterial Line (A-line) 

An indwelling catheter (placed in the radial artery) used to obtain repeated arterial blood samples and monitor arterial pressure during hypoxia studies. 

Transfer Standard  

Pulse oximeter equipment used to provide a non-invasive estimate of SaO₂, directly traceable to SaO₂.  Suitable for development-phase accuracy assessment of a device under test, using a verified error-transfer equation, but not permitted as the reference in controlled desaturation studies for verification of accuracy. 

Plateau 

A stable oxygen saturation level maintained long enough to collect paired device readings and arterial samples with minimal physiological drift. 

Pairing (Time Alignment) 

The process of matching each device SpO₂ value to the simultaneous arterial SaO₂ sample per the standard’s rules—critical for accuracy statistics. 

Co-Oximeter 

A multi-wavelength optical blood analyzer that quantifies total hemoglobin and individual hemoglobin species (e.g., oxyhemoglobin, deoxyhemoglobin, carboxyhemoglobin, methemoglobin) from an arterial sample. It reports SaO₂ (functional hemoglobin) and serves as the laboratory reference for pulse oximeter accuracy in ISO 80601-2-61 aligned controlled desaturation studies 

Total Hemoglobin (tHb) 

Sum of the concentrations of all hemoglobin species in blood as reported by co-oximetry, including: oxyhemoglobin (O₂Hb), deoxyhemoglobin (HHb), carboxyhemoglobin (COHb), methemoglobin (MetHb).  

Functional Oxygen Saturation  

Percentage of hemoglobin that is oxygenated: O₂Hb ÷ (O₂Hb + HHb) × 100. Dyshemoglobins are excluded from the denominator. Pulse oximeters report functional saturation. 

Dyshemoglobins (COHb, MetHb) 

Hemoglobin variants that alter light absorption. Standard two-wavelength pulse oximeters are not designed to quantify these; are excluded following ISO 80601-2-61. Carboxyhemoglobin (COHb) is hemoglobin bound to carbon monoxide, reducing oxygen-carrying capacity.  Methemoglobin (MetHb) is hemoglobin with iron in the ferric (Fe³⁺) state and it cannot carry oxygen. 

Fractional Oxygen Saturation 

Percentage of hemoglobin saturated with oxygen when all hemoglobin species are included in the denominator: O₂Hb ÷ (O₂Hb + HHb + COHb + MetHb) × 100. Fractional oxygen saturation should not be used for pulse oximetry comparison.  

Analysis and Performance Characteristics 

Accuracy 

Closeness of agreement between a test result and an accepted reference value. In pulse oximetry, accuracy describes how well device SpO₂ matches laboratory SaO₂ (co-oximetry) across the tested range, typically summarized as bias, precision, and RMS error (A_rms) per ISO 80601-2-61. 

Bias 

The systematic difference between SpO₂ readings and reference SaO₂ values; also called mean bias.  

Precision 

A measure of variability in SpO₂ readings across repeated measurements under similar conditions.  

Root-mean-square difference (A_rms) 

A single accuracy metric that combines bias and precision across paired SpO₂–SaO₂ comparisons: Arms = √(mean[(SpO₂ − SaO₂)²]/total number of data points). A lower A_rms indicates better overall accuracy. Pulse oximeter equipment must achieve a root-mean-square difference (Arms) ≤ 3.0 %SpO₂ to meet ISO 80601-2-61 expectations. 

Skin Tone and Inclusivity Terms 

Skin Tone 

A factor influencing light absorption and scattering due to variations in melanin concentration. Skin tone may affect pulse oximetry performance and is addressed through inclusive participant recruitment. Skin tone is not a proxy for race or ethnicity.  

Monk Scale 

A standardized skin tone classification system used to characterize participant diversity in optical device studies. Use of the Monk scale supports transparent reporting. 

Individual Typology Angle (ITA) 

An objective colorimetry metric used to quantify skin pigmentation; higher ITA values indicate lighter skin and lower values indicate darker pigmentation. In pulse oximetry and other light-based device studies, ITA is used alongside the Monk Skin Tone scale to document skin-tone diversity 

Regulatory and Study Oversight Terms 

Informed Consent 

A documented process by which study participants voluntarily agree to participate after being informed of study procedures and risks. Informed consent is required for all pulse oximetry clinical research studies. 

Institutional Review Board (IRB) 

An oversight body responsible for protecting participant rights and welfare in clinical research. IRB approval is mandatory prior to initiating controlled desaturation or hypoxia-based studies. 

Good Clinical Practice (GCP) 

An international ethical and scientific quality standard for designing and conducting clinical research. GCP compliance is essential for generating regulator-ready clinical endpoints. 

ALCOA+ Data Integrity 

Framework ensuring data are Attributable, Legible, Contemporaneous, Original, Accurate + Complete, Consistent, Enduring, and Available. 

ISO 14155 

International standard for the design, conduct, recording, and reporting of clinical investigations of a medical device. Pulse oximetry studies intended for FDA or CE mark submissions must align with ISO 14155 expectations. 

Open Oximetry Project 

Independent initiative promoting inclusive pulse oximetry research and resources for skin tone representation and equity. 

Why Terminology Alignment Matters 

Clear, consistent use of pulse oximetry terminology reduces friction across engineering, clinical, and regulatory teams. It supports trial protocol alignment, improves CRO transparency, and helps mitigate risks that can lead to FDA submission delays. For developers navigating pulse oximetry testing, a shared vocabulary is essential to efficient, defensible verification. 

Conclusion 

Pulse oximetry depends on a tightly integrated set of optical principles, signal-processing techniques, and clinical research practices. Understanding the terms that define these elements is a prerequisite for effective medical device validation. 

Parameters Research Laboratory (PRL) supports pulse oximetry studies through controlled desaturation, hypoxia lab expertise, and regulatory-grade CRO services, helping teams translate technical concepts into high-quality clinical evidence. Contact us today!

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