Sleep test results: what AHI, RDI, ODI and oxygen numbers mean

What’s going on

A sleep test report is full of short codes. This page explains what they mean, so you can follow the conversation with your physician.

It doesn’t replace that conversation. Only a physician can read your raw data, diagnose sleep apnea and decide on treatment. The same numbers can mean different things for different people.

Your report will likely come from one of two tests. An in-lab study (polysomnography) records brain waves, breathing, oxygen and more. A home sleep apnea test records breathing and oxygen, but usually not sleep itself.

The numbers on your report

AHI: apnea-hypopnea index

AHI is the headline number. It’s the average number of apneas and hypopneas per hour of sleep.

• An apnea is a pause in breathing.
• A hypopnea is a stretch of shallow, reduced breathing.

Cleveland Clinic lists the severity bands used for adults:

• Under 5: below the cutoff for sleep apnea in adults
• Mild: 5 to 14 events per hour
• Moderate: 15 to 29 events per hour
• Severe: 30 or more events per hour

The AASM’s 2009 guideline uses nearly the same cutoffs, though it counts exactly 30 as moderate. A number near a boundary, like 14 or 16, isn’t very different in practice. Your physician weighs it with your symptoms and health.

REI: respiratory event index

Home tests often report REI instead of AHI. The AASM uses REI for home tests because most don’t measure sleep. They divide events by the hours the device was recording, not the hours you were asleep.

RDI: respiratory disturbance index

RDI counts apneas and hypopneas, plus a third kind of event. These are breaths that strain against a narrowed airway and briefly wake your brain, without a full pause.

So RDI is usually the same as AHI or a little higher. The AASM notes that the term has been used in different ways over time. Ask which definition your report uses.

ODI: oxygen desaturation index

ODI is how many times per hour your blood oxygen dropped by a set amount, usually 3% or 4%. It tracks how much your breathing events affect your oxygen.

A 2024 study found ODI can differ by how it’s counted. It came out lower when drops during wake time were left out. It also came out lower when home-test recording time was used.

Oxygen nadir and time at low oxygen

The oxygen nadir (sometimes “SpO2 low” or “lowest saturation”) is the single lowest oxygen reading of the night. MedlinePlus says a normal oxygen reading is 95% to 100%.

A single brief dip matters less than how often and how long your oxygen stays low. Some reports list the minutes or percent of the night spent below 90%. Ask your physician how your numbers fit together.

Other lines you may see

• Supine vs non-supine AHI: events on your back compared with other positions. A big gap can point to positional sleep apnea.
• REM AHI: events during REM (dreaming) sleep, reported on their own.
• Central apneas: pauses where your brain doesn’t send the signal to breathe. These are a different problem from obstructive apnea and change the treatment plan.

Why a home test can read low

A home test is a good tool for the right person. The AASM’s diagnostic guideline explains why it can undercount:

1. It counts recording time, not sleep time. Time you lie awake still goes into the hours, so the hourly rate shrinks.
2. It misses some hypopneas. Most home devices can’t see brain arousals. Shallow breaths that wake your brain without a large oxygen drop go uncounted.
3. It’s one night. A single night may not match your usual sleep.

The guideline calls home testing less sensitive than a lab study. It recommends an in-lab study if a home test is negative, inconclusive or technically poor.

The scoring rule matters too. The AASM’s recommended rule counts a hypopnea with an arousal or a 3% oxygen drop. Some payers require the stricter 4% rule, which gives a lower AHI from the same night.

What a dentist or doctor can do

Your sleep physician reads the full report, not just one number, and decides on treatment. A dentist trained in dental sleep medicine can make a custom oral appliance, but only after a physician’s diagnosis and prescription.

Treatment depends on more than severity. Symptoms, other health conditions, how your airway collapses and your own preferences all count. See CPAP alternatives and oral appliance vs CPAP for the options.

Questions to ask your physician

1. What is my AHI (or REI), and which hypopnea rule was used to score it?
2. Is my apnea mostly obstructive, or are there central events?
3. How low did my oxygen go, and how long did it stay low?
4. Is my apnea worse on my back or in REM sleep?
5. If this was a home test, could it have undercounted? Do I need a lab study?
6. Which treatments fit my results, and why?
7. How will we check that treatment is working? Will I need a follow-up sleep test?

What not to do

• Don’t self-diagnose from one number. A “mild” AHI with heavy daytime sleepiness can still need treatment.
• Don’t take a negative home test as the final word if you still have symptoms. Ask about a lab study.
• Don’t rely on a watch or app to track your AHI. Wearables can flag possible apnea, but they don’t diagnose it or measure treatment.