Reference data dictionary · version 1.0 · 11 September 2026
Short answer: do not read a headline overnight oxygen number without its definition, time window, event rule, and signal-quality information. SpO2 is an estimate, ODI is not AHI, and a minimum or time-below value cannot explain its own cause.
This reference is designed to be copied into a report, shared with a reader, or used as a checklist when two apps use similar labels for different calculations.
The CSV maps common fields to a plain-language definition, a method question, and a limitation.
The field-by-field dictionary
| Field | Plain-language definition | Question to ask | Common limitation |
|---|---|---|---|
| SpO2 | An optical estimate of peripheral oxygen saturation. | Where, how, and under what conditions was it measured? | Motion, low perfusion, temperature, skin pigmentation, nail polish, and fit can affect the estimate. |
| Pulse | The pulse rate detected by the same sensor. | Was the pulse signal stable at the same moment? | A plausible pulse does not validate every oxygen data point. |
| Recording time | The period during which the device was collecting data. | Does it represent time worn, recording time, or estimated sleep time? | Recording time is not automatically sleep time. |
| Minimum SpO2 | The lowest displayed value in the selected period. | How long did the value last and was the signal usable? | A single minimum can be artifact, movement, or a short event. |
| Average or median SpO2 | A summary of the values over a period. | Which values were included or excluded? | An average can hide short repeated drops or long invalid sections. |
| ODI 3% or ODI 4% | Desaturations meeting a selected percentage-drop rule per analyzed hour. | Which drop definition, baseline, duration, and denominator were used? | ODI is not AHI and is not calculated identically by every device. |
| Time below threshold | Minutes or a percentage below a displayed saturation threshold. | Which threshold and usable-time window are shown? | The number needs patient, altitude, device, and symptom context. |
| Signal quality | Usable, weak, missing, or artifact-affected data. | What did the software exclude? | A long recording with poor usable signal may be less informative than a shorter clean one. |
| Event marker | A software flag created when a rule is met. | What rule triggered the flag? | An event marker is a classification, not a confirmed cause. |
Why ODI and AHI must stay separate
ODI counts oxygen desaturations that meet a selected drop rule, often 3% or 4% from a baseline, per hour of analyzed time. AHI counts apneas and hypopneas per hour of sleep using a sleep-study method. The metrics may correlate in some settings, but they are not the same measurement and should not be swapped in a summary.
The systematic review of ODI for adult obstructive sleep apnea found substantial variation in study populations, thresholds, and diagnostic performance. A practical report should therefore include the rule behind the ODI rather than presenting a bare number. If a page says “ODI 12,” the reader still needs to know whether it means 3% or 4%, whether the denominator was recording time or estimated sleep time, and how artifact was handled.
The AASM also cautions that diagnosis, treatment efficacy, and treatment decisions should not be based only on automatically scored home sleep apnea data. The safe interpretation is that a report can provide structured evidence for clinical review, not that it can independently name the cause.
A method card to attach to every report
- Device and software: record the device model, sensor location, and app or firmware version where available.
- Recording window: write start time, end time, wear time, and whether the report estimates sleep time.
- Event rule: keep the ODI drop threshold, duration rule, and baseline definition.
- Quality: note usable signal, artifact handling, missing sections, and sensor-fit issues.
- Context: add symptoms, illness, altitude, sleep position, and the reason monitoring was performed.
- Comparison rule: do not compare two devices as though they were equivalent unless their methods are actually aligned.
Estimate
SpO2 is a sensor estimate. Preserve the reading but do not confuse it with a direct blood measurement.
Rule
ODI and event markers are rule-based outputs. Keep the threshold and denominator visible.
Window
Recording time, wear time, and sleep time are not automatically interchangeable.
Quality
Invalid or uncertain segments should remain visible instead of disappearing from the story.
Reuse and citation
Suggested citation: NightlyVitals, “Overnight Oxygen Report Field Definitions: A Reusable Data Dictionary,” September 11, 2026. Definitions are educational and do not replace clinical interpretation.
FAQ
What does SpO2 mean on an overnight report?
It is an estimate of peripheral oxygen saturation from an optical sensor. Review it with pulse, signal quality, time window, symptoms, and the device instructions.
Is ODI the same as AHI?
No. ODI counts selected oxygen drops; AHI counts breathing events under a sleep-study method. They are related in some settings but are not interchangeable.
Does time below a threshold prove a diagnosis?
No. It is a report metric whose meaning depends on the threshold, duration, usable signal, patient context, and clinical assessment.
How to quote a report without stripping away the method
A shareable report excerpt should carry more than one number. A compact format is: “ODI 3%: 8.2 events per hour, analyzed recording time 6 h 42 min, usable signal 93%, device and software version noted.” If the report does not expose one of those fields, say that it is unavailable instead of filling the gap with an assumption.
The same rule applies to minimum SpO2 and time below a threshold. Write the value, the duration or percentage, the analysis window, and the quality flag. “Minimum 88% for one sample, fair signal” is a different statement from “spent 4 minutes below 90% with good usable signal.” A careful reference page helps readers see that these are not interchangeable summaries.
Method card template
| Question | Record this | If missing |
|---|---|---|
| What device made the measurement? | Model, sensor site, app or firmware | Do not compare it as if it were identical to another device |
| What period was analyzed? | Start, end, wear time, sleep-time estimate | Label the denominator as unknown |
| What counts as an event? | Drop threshold, duration, baseline, recovery | Call the metric device-defined |
| What data was excluded? | Artifact, missing, weak, or invalid time | Keep the quality limitation beside the result |
Why definitions are a linkable asset
Publishers often need a neutral reference they can cite without endorsing a product or diagnosing a reader. A field-level data dictionary serves that need because it answers a narrow question, links to primary sources, and makes its limitations visible. It can be quoted in a patient education page, a research explainer, or a device-support article while preserving the difference between an estimate and a clinical conclusion.
Keep this page stable when the definitions do not change. If a device vendor changes its event rule, add the new rule and the date rather than silently rewriting the old explanation. That gives readers a clean citation trail and makes future updates easier to audit.
Related reading
Sources and verification
Last checked: September 11, 2026
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FDA: Pulse Oximeter Basics
Explains what SpO2 and pulse readings estimate, how to improve a home reading, and why symptoms and device limitations must be considered together.
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AASM: Clinical use of a home sleep apnea test
States that a medical provider must order and interpret testing for diagnosis and that automatically scored data should not be the sole basis for treatment decisions.
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The Value of Oxygen Desaturation Index for Diagnosing Obstructive Sleep Apnea: A Systematic Review
Reviews the relationship between ODI and AHI and reports important heterogeneity in populations, thresholds, and event definitions.