The apnea-hypopnea index (AHI) determines the severity level on a sleep apnea test, while the other numbers help put that score in context. At Longevity Benchmark, the most common mistake we see is interpreting a home test result as if it came from a sleep lab. Home devices calculate events based on how long the recorder was running, not how long you were actually asleep, so the same night can produce a lower score.
The Verdict
What Each Sleep Apnea Test Measurement Counts
Seven figures carry almost all the information on a sleep apnea report, and each one counts something different.
| Measurement | What it counts | What it tells you |
|---|---|---|
| AHI (apnea-hypopnea index) | Apneas plus hypopneas per hour of measured sleep | Sets the severity band on an in-lab study |
| RDI (respiratory disturbance index) | Apneas, hypopneas and respiratory effort related arousals per hour of sleep | Sits at or above the AHI, and can be abnormal on a night the AHI is normal |
| REI (respiratory event index) | Apneas plus hypopneas per hour of recording time on a home test | Reads lower than a lab AHI from the same night, because the divisor is bigger |
| ODI (oxygen desaturation index) | Drops in blood oxygen of 3% or 4% below baseline, counted per hour | Shows how often your breathing cost you oxygen as well as sleep |
| Nadir SpO2 (lowest oxygen saturation) | The single lowest saturation reached across the night | One bad moment. Read it alongside how long you spent low |
| Time below 88% | Minutes spent with oxygen saturation under 88% | The figure a clinician weighs when deciding how urgent this is |
| Supine AHI | Events per hour counted only while you were on your back | A supine figure far above the whole-night AHI means the apnea is positional |
The three indices at the top answer how often your breathing was interrupted. The three oxygen figures answer what those interruptions cost your blood oxygen. Supine AHI answers whether the events only happen on your back, which changes what a clinician is likely to offer you.
The AHI Ranges That Set Your Severity Band
An AHI counts apneas and hypopneas per hour of measured sleep, and the four bands below are the scale the AASM publishes for adults.
| AHI (events per hour) | Severity band | What usually follows |
|---|---|---|
| Under 5 | Normal for an adult | No diagnosis from this number by itself. Persistent symptoms still need an explanation |
| 5 to under 15 | Mild | Diagnosis when symptoms or related conditions are present. Treatment is discussed rather than assumed |
| 15 to under 30 | Moderate | Diagnosis without needing symptoms. CPAP (continuous positive airway pressure), an oral appliance or positional therapy is offered |
| 30 and above | Severe | Diagnosis and prompt treatment, usually CPAP with a pressure titration step |
Two diagnostic thresholds sit behind those bands. An adult diagnosis needs 5 or more events per hour with symptoms or an associated condition, or 15 or more on its own. Children run on a completely different scale, where an obstructive AHI of 1 or more events per hour is already abnormal. Read a child's report against adult bands and a treatable problem looks fine. The American Academy of Pediatrics guideline sets out the paediatric pathway.
Why RDI Runs Higher Than AHI
The RDI counts everything the AHI counts plus respiratory effort related arousals (RERAs), so it is never lower and is often meaningfully higher. A RERA is a stretch of at least ten seconds where breathing effort rises against a narrowing airway and you surface into lighter sleep. A hypopnea needs an airflow drop or an oxygen dip, and a RERA carries neither. The scoring rules for all three event types are set out in the Journal of Clinical Sleep Medicine update to the AASM scoring manual.
That gap has a practical consequence. You can walk out of a laboratory with an AHI of 3, which is squarely normal, and an RDI of 18, which is not. Your airway is closing enough to wake you eighteen times an hour and not quite enough to be scored as apnea. Repeated arousals break up sleep architecture the way full events do. That is one reason someone whose report reads as normal still cannot stay awake at four in the afternoon. Our guides on rapid eye movement (REM) sleep and how much deep sleep you need cover what those interruptions take out of a night.
Scoring a RERA requires electroencephalography (EEG), because an arousal is a brain-wave event. Home devices carry no EEG channel and cannot score arousals at all, which means a home report has no usable RDI even when it prints one.
Home Tests Divide by Recording Time Rather Than Sleep Time
A home sleep apnea test reports an REI, and the REI divides events by how long the recorder ran instead of how long you slept. Without EEG the device cannot separate sleep from lying awake in the dark, so it has no measured sleep time to divide by. Every quiet minute you spent staring at the ceiling lands in the denominator as though you had been asleep through it.
Suppose the device logged 90 obstructive events across 8 hours of recording. The REI is 90 divided by 8, which is 11.3 events per hour, and that lands in the mild band. Now suppose you were genuinely asleep for 6 of those hours, which is an ordinary night for someone whose breathing keeps waking them. The same 90 events over 6 hours give an AHI of 15, the bottom of the moderate band. Your breathing was identical in both calculations, and only the divisor changed.
The dilution runs in one direction only: it shrinks the index and can never raise it. That is why a home test under-calls severity and can read as negative in someone who has moderate disease. The effect grows the worse you sleep, so a broken night dilutes the score most. The AASM diagnostic testing guideline also requires at least four hours of technically adequate airflow and oximetry data before a home study can be scored at all. Our breakdown of what a sleep apnea test costs covers what a repeat study adds when the first one misses that bar.
Two Hypopnea Rules Produce Two Different AHIs
The same night can be scored into two different AHIs depending on which hypopnea rule the laboratory applied. Rule 1A, the one the AASM recommends, scores a hypopnea when airflow drops by at least 30% of baseline for at least 10 seconds. The event must also produce either a 3% oxygen desaturation or an arousal. Rule 1B uses the same 30% drop over the same 10 seconds, requires a 4% desaturation, and ignores arousals completely.
Every event that wakes you without pulling your oxygen down a full four points counts under 1A and disappears under 1B. The 4% figure is therefore always the lower of the two. One night of data can put a person in the mild band under one rule and the moderate band under the other. Both reports are correctly scored under the rule each laboratory used.
Your Oxygen Numbers Can Matter More Than the AHI
Oxygen tells a clinician what the events did to you, while the AHI only says how many there were. Three figures do that work. The ODI counts desaturations per hour, and the nadir gives the lowest saturation of the night. Time below 88% gives the minutes spent under the saturation line CMS uses to qualify people for home oxygen.
A high AHI with a flat oxygen trace and a low AHI with deep, prolonged drops are different clinical situations, and the second one often gets acted on faster. Research published in the European Heart Journal in 2019 measured the hypoxic burden of sleep apnea, which is the area under each desaturation rather than a count of events. Across two large cohorts, it predicted cardiovascular mortality where the AHI did not. That finding is why a clinician may spend longer on your oxygen columns than on the headline index.
What a Negative Sleep Apnea Test Does Not Rule Out
A negative result means the events the device could count stayed under the threshold on one particular night. Four things routinely hide behind a clean report.
- Arousal-driven disease. With no EEG there is no arousal scoring, so a night dominated by RERAs produces a low REI and a normal-looking report.
- Positional apnea. If your events cluster on your back and you slept on your side, the recorder never saw the problem. The supine AHI line tells you how much time you spent in that position.
- REM-related apnea. Obstructive events cluster in REM sleep, when muscle tone is lowest. A short or fragmented night gives the scorer less REM to find them in.
- Central events. A device without effort belts cannot separate a central pause from an obstructive one, and central sleep apnea has a different cause and a different treatment.
Symptoms that persist after a negative test are a reason to go back for a laboratory study rather than to accept the number. The AASM recommendation is explicit: when a single home test comes back negative, inconclusive or technically inadequate, a polysomnogram should follow. Wearables add a useful pattern here without settling anything. If overnight oxygen dips or a sagging heart rate variability (HRV) trend show up on the nights you feel worst, that is a prompt to test again. Our page on sleep apnea detection on wearables covers what those devices can and cannot flag.
Symptom overlap sends some people down the wrong path entirely. Daytime exhaustion, low mood and reduced libido push readers toward a hormone workup, and the signs of low testosterone overlap almost completely with untreated obstructive sleep apnea. Testing the breathing first is usually cheaper and faster.
What Happens After a Sleep Apnea Test at Each Band
The severity band sets the pathway after a report, and the options widen as the number climbs. A normal AHI with continuing symptoms sends you toward an in-lab study, or toward a different explanation for the tiredness. Mild results open a conversation rather than an automatic prescription, and weight, alcohol timing and positional therapy are usually on the table before a machine is.
Moderate and severe results lead to treatment, and CPAP is the usual first offer. Finding your pressure takes a titration step. A laboratory night has a technologist adjust the pressure while you sleep. An auto-adjusting machine does the same job at home across the first few weeks. Some adults cannot tolerate CPAP, and some prefer an alternative from the start. The joint AASM and American Academy of Dental Sleep Medicine guideline recommends a custom mandibular advancement device for that group. A dentist trained in sleep medicine fits it, and an off-the-shelf mouthguard is not the same device.
At the far end sit airway surgery and hypoglossal nerve stimulation, an implant that moves the tongue forward in time with each breath. The US Food and Drug Administration approved the Inspire system under premarket approval P130008. In the Stimulation Therapy for Apnea Reduction (STAR) trial, published in the New England Journal of Medicine, the median AHI fell from 29.3 to 9.0 events per hour at twelve months. None of those routes is something to pick for yourself. Which one fits depends on your anatomy, your band and what you have already tried, and that is a conversation with the clinician who ordered the test.
Who Should Not Read a Report Against These Bands
Parents holding a child's report should ignore the adult scale entirely. Paediatric criteria treat an obstructive AHI of 1 or more events per hour as abnormal. A result that looks reassuring against adult bands can describe a child who needs treatment. That report belongs with a paediatric sleep specialist.
Anyone with heart failure, serious lung disease, a neuromuscular condition, suspected hypoventilation or chronic opioid use sits outside this scale too. The AASM guideline sends those groups to an in-lab polysomnogram instead of a home test in the first place. An REI generated at home is the wrong instrument for that question, and a normal one should not be treated as an answer.
One change at CMS would change our answer. If Medicare updated its coverage rule to accept the 3% desaturation-or-arousal definition the AASM recommends, one night would stop producing two defensible AHIs. A report from 2019 would then be comparable with one from this year. Until that happens, take your sleep apnea test results back to the clinician who ordered them and ask which hypopnea rule was scored and what the total recording time was.
Frequently Asked Questions
How long do sleep apnea test results take?
Results usually take days rather than minutes. The raw recording is scored event by event by a technologist and then read and signed by a sleep physician, and that second step sets the pace. A home test adds return shipping or a device drop-off before scoring starts. The clinic that issued the device knows its own turnaround, and handover is the moment to settle that rather than a week of silence later.
What happens after a sleep apnea test?
The report goes to whoever ordered it, and the follow-up depends on which severity band the apnea-hypopnea index landed in. Under 5 with ongoing symptoms usually means a second look, either an in-lab study or a search for a different cause. Mild leads to a conversation about whether to treat at all. Moderate and severe lead to treatment, most often continuous positive airway pressure, with a titration step to find your pressure. Our page on the home sleep apnea test covers what the device itself records.
What does a sleep apnea test measure?
Airflow at the nose and mouth, breathing effort at the chest and abdomen, blood oxygen from a fingertip sensor, heart rate, snoring and body position. An in-lab polysomnogram adds brain-wave, eye-movement and leg sensors, which is how it measures real sleep and scores arousals. A home test leaves the brain-wave channel out. That single omission is why home results are reported against recording time.
What does a negative sleep apnea test mean?
It means the events the device could count did not reach the diagnostic threshold on that night. The question stays open. The American Academy of Sleep Medicine recommends a polysomnogram when a single home test comes back negative, inconclusive or technically inadequate and suspicion remains. The home format under-calls severity in several predictable ways. Arousal-driven events, central events and a night with little REM sleep all read as a low score.
Can a sleep apnea test be wrong?
Yes, and it errs low far more often than high. A dislodged nasal cannula, a finger sensor that loses contact, a night spent mostly awake, or sleeping on your side when your apnea is positional all pull the number down. False positives happen too, usually from artifact that scoring software counted as an event, and a physician reading the raw traces catches most of those. When the report and the symptoms disagree, the report is the part to re-examine.
What counts as a positive sleep apnea test?
An apnea-hypopnea index of 5 or more events per hour alongside symptoms, or 15 or more regardless of symptoms, is the usual line for a diagnosis in adults. Medicare uses a similar shape for coverage. It funds a trial at an index of 15 or more, or at 5 to 14 alongside documented sleepiness, insomnia, a mood disorder, high blood pressure or a history of stroke. A positive result opens a treatment conversation rather than settling it.
Why is my RDI higher than my AHI?
Because the respiratory disturbance index counts respiratory effort related arousals and the apnea-hypopnea index does not. One of those arousals is a run of breaths where effort climbs against a narrowing airway and you surface into a lighter stage. There is no airflow drop and no oxygen dip, so it is never scored as a hypopnea. Those events fragment sleep without registering on the AHI. Someone with an AHI of 3 and an RDI of 18 is being woken roughly six times as often as the AHI suggests.
What oxygen level is concerning on a sleep apnea test?
Clinicians read three oxygen figures together: the oxygen desaturation index, the lowest saturation reached, and the minutes spent under 88%. The 88% line matters because it is the saturation threshold Medicare uses for home oxygen coverage, so it shapes what happens next. A low point of 82% for ten seconds and a low point of 82% held across forty minutes are different problems. The time figure therefore carries more weight than the single worst reading.