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A CPAP machine measures airflow and pressure. It cannot see your blood oxygen. An overnight oximeter, a small sensor on a finger or a ring, can, and putting the two on the same timeline answers questions neither can alone.
Key takeaways
- ODI counts desaturations per hour; ODI₃ uses a 3 percent drop and ODI₄ a 4 percent drop, so compare like with like.
- T90 is the time spent below 90 percent saturation; it complements ODI.
- There are no standard severity bands for ODI. Severity is graded from AHI, and your clinician reads oximetry alongside it.
What the numbers mean
SpO₂ is the percentage of your haemoglobin carrying oxygen. Overnight, apnea and hypopnea can pull it down in dips that recover when breathing resumes. ODI (oxygen desaturation index) is the number of those dips per hour of recording, usually reported at a 3 percent or 4 percent threshold. T90 is the total time, or percentage of the night, spent below 90 percent. Our explainer on SpO₂, oxygen desaturation, ODI and T90 goes deeper.
| Measure | What it is | Why it matters alongside CPAP |
|---|---|---|
| ODI₃ | Dips of 3 percent or more per hour | Sensitive; always at least as high as ODI₄ |
| ODI₄ | Dips of 4 percent or more per hour | Stricter; closer to some scoring rules |
| T90 | Time below 90 percent | Shows how long oxygen stayed low, not just how often it dipped |
| Lowest SpO₂ | The deepest dip | Context for how severe an individual dip was |
Why pair it with CPAP data
If your machine says AHI 2 and your oximeter shows frequent dips, the two are telling different stories, and that is useful for your clinician to see. If both are quiet, that is reassuring. Matching timestamps lets you see whether a dip lines up with a leak spike, a position change or an event the machine flagged.
Getting the two on one chart
Apnealytics imports data from the Wellue O2Ring and the Contec CMS50 family, and a ResMed oximeter's SAD.edf file, then overlays SpO₂ and pulse on the same night with ODI and T90. Overlays are a Pro feature, and our guide to importing oximeter data has the steps for each device. The oximeter overview lists what is supported.
Caveats of home oximetry
Finger probes can read low with cold hands, poor circulation, nail polish or movement. Wrist and ring devices differ in accuracy. A single night is a snapshot, so repeat before drawing conclusions. None of this is a diagnosis: it is information to bring to a clinician.
Not medical advice. Apnealytics is a tracking and coaching tool, not a diagnostic device. Machine-estimated numbers are not a clinical diagnosis; talk to your clinician about your therapy.
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Written by
Haniel Singh
Founder of Apnealytics. A CPAP user who builds software, not a sleep physician.
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