Why Your Home Sleep Test Reports an REI Instead of an AHI and How the Two Scores Differ

Why Your Home Sleep Test Reports an REI Instead of an AHI and How the - Back2Sleep
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REI vs AHI and why a home sleep report scores your breathing over recording time

A plain-language guide to the respiratory event index on your home sleep report, how it differs from the AHI, and when a low score still needs a closer look.

6-minute readAI-assisted writing

The short answer on REI vs AHI

The short answer is that your home sleep test reports an REI, a respiratory event index, because it cannot measure how long you actually slept. A sleep lab counts breathing events per hour of confirmed sleep and reports an apnea-hypopnea index, or AHI. A home device divides the same kind of events by monitoring time, an estimate of sleep built without EEG. That change of denominator is the heart of REI vs AHI and can make a home result look milder than the same night in a lab. Our guide to what your AHI score actually means covers the thresholds.

Why a different score? A home sleep apnea test, or HSAT, records airflow, breathing effort and blood oxygen through pulse oximetry (your SpO2). It has no EEG sleep staging. So it cannot tell sleep from lying awake, and it spreads your events across the whole recording. That dilutes the number.

Some simple signs deserve a clinician's attention even when the home score looks low:

  • witnessed pauses in breathing during sleep
  • loud snoring most nights
  • repeated awakenings
  • morning headaches
  • excessive daytime sleepiness
Key Takeaway
  • REI counts events over recording time, AHI over confirmed sleep time.
  • A low REI alongside real symptoms deserves a second look, not reassurance.
REI versus AHI
Infographic: REI versus AHI
Compares the two scores: REI counts events over recording time from a home test, AHI over sleep time from an in-lab study.

What each index counts and why the denominators differ

Each index counts similar breathing events but divides them by a different clock. The AASM scoring manual (2023) defines the respiratory event index as respiratory events multiplied by 60, divided by monitoring time. Monitoring time is the total recording time minus the artefact and awake periods the device can estimate from body position, breathing pattern or a diary. Because it has no EEG, that estimate stays rough, so quiet wakefulness with normal breathing often stays inside the denominator. The apnea-hypopnea index divides by total sleep time instead, and only a study with EEG can confirm that.

An apnea is a near-complete stop in airflow lasting at least 10 seconds. A hypopnea is a partial drop lasting at least 10 seconds, paired with an oxygen desaturation or a brief arousal. A respiratory effort-related arousal, or RERA, is a stretch of rising breathing effort that ends in a brief arousal. The respiratory disturbance index, or RDI, adds RERAs to the count, as the obstructive sleep apnea reference explains.

Scoring a RERA, or a hypopnea that ends only in an arousal, needs EEG. A full in-lab polysomnography records brain waves, eye movement, muscle tone, airflow and oximetry, so it can register those events. A home study cannot. That is a second reason the home number can sit low.

Real reports mix these acronyms, so the table below sets out REI vs AHI and the RDI side by side.

Index Full name Events counted Divided by Typical test
REI Respiratory event index Apneas and hypopneas the device detects Monitoring time Home sleep apnea test (Type III portable monitor)
AHI Apnea-hypopnea index Apneas and hypopneas, arousal-based ones included Total sleep time (EEG) In-lab polysomnography (PSG)
RDI Respiratory disturbance index Apneas, hypopneas and RERAs Total sleep time (EEG) In-lab polysomnography
Key Takeaway
  • REI, AHI and RDI are related scores, but they are not interchangeable.
  • Home polygraphy cannot score RERAs or arousal-only hypopneas, because those need EEG.
The events both scores count

Why home tests can underestimate severity

A home REI can understate how severe sleep apnea is, and in some patients the gap is wide enough to change the grade. Pitkanen and colleagues compared both scores across two datasets in Sleep Advances (2023). In both, the recording-time REI placed people in the right severity class less often than the sleep-time AHI. The mechanism is simple. Without EEG the device cannot reliably spot quiet wakefulness, so those awake minutes with normal breathing often stay in the denominator and pull the average down.

European clinical literature makes the same point, stating that the apnea-hypopnea index should be calculated on sleep hours and not recording hours. The Sleep Foundation (2024) also notes that home tests tend to underestimate the AHI. None of this makes a home test useless. It means a mild home reading needs context when symptoms are clear.

Where common guides fall short Many AHI explainers skip the REI entirely, so a home report stays hard to match. Many also say brain activity is not recorded without naming the events that go missing, which are RERAs and arousal-only hypopneas.
Key Takeaway
  • The recording-time denominator can push a home score below the matching sleep-time AHI.
  • Treat a borderline home result as a question, not a final answer.

REI vs AHI severity bands and the European names on your report

Both scores are read against the same adult severity bands, so the REI vs AHI difference matters most near a cut point. The bands on the apnea-hypopnea index reference page are:

  • under 5 events per hour is considered normal
  • 5 to under 15 is mild
  • 15 to under 30 is moderate
  • 30 or more is severe

Your report may not say REI at all. In France, Belgium and other French-speaking countries, the home study is a polygraphie ventilatoire (or respiratoire) and the index is printed as the IAH, the indice d'apnées-hypopnées. It uses the same 5, 15 and 30 cut points. On a home polygraphy report, the IAH plays the role of the English REI, because both count events over recording time.

In the UK, NICE guideline NG202 (2021) makes home respiratory polygraphy the first-line test for suspected obstructive sleep apnoea in adults over 16. Most UK patients therefore get a recording-time index by default.

Key Takeaway
  • The IAH on a French home polygraphy and the English REI describe the same recording-time index.
  • In the UK, and routinely in France, home polygraphy is often the first test offered.

What to do when a home result is borderline or normal

A borderline or normal home REI does not rule sleep apnea out when symptoms continue. The American Academy of Sleep Medicine guideline (2017) says a home test that is negative, inconclusive or technically inadequate should be followed by in-lab polysomnography, or PSG. A technically adequate recording needs at least 4 hours of oximetry and airflow data. A 2024 study in Sleep and Breathing suggests severity grading becomes more reliable once monitoring time reaches about 6 hours.

Use this sequence to read your own result:

  1. Find the monitoring time or total analyzed time on your report and check it covers at least 4 hours.
  2. Place your REI or IAH in its severity band, remembering the score may sit low.
  3. Note your symptoms for your clinician, including witnessed pauses and daytime sleepiness.
  4. If the result is borderline or normal but symptoms persist, ask whether an in-lab sleep study is warranted.

Our comparison of a home sleep test and a lab sleep study explains when each fits. It also helps to know why one night of testing can misclassify severity before you accept a mild label.

What comes next depends on the confirmed result. If it is severe, or the pattern is central, a doctor may prescribe a machine-based treatment, and a nasal device is no substitute for it. For snoring and clinician-confirmed mild to moderate obstructive sleep apnea, the goal is keeping the upper airway open so enough air keeps flowing during sleep. Back2Sleep, a soft silicone intranasal stent sold without prescription, is one conservative option to raise with your physician.

Key Takeaway
  • A normal or borderline home score with ongoing symptoms warrants an in-lab study.
  • Confirm a mild to moderate result with a clinician before choosing how to manage it.
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Severity bands by events per hour
Infographic: Severity bands by events per hour
Shows the severity bands by events per hour, with the thresholds marked at 5, 15 and 30 for mild, moderate and severe.

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Frequently Asked Questions

Why does my home sleep test say REI instead of AHI?

A home sleep apnea test records airflow, breathing effort and oxygen, but it has no EEG to confirm when you are asleep. Without true sleep time, it divides your breathing events by monitoring time and reports a respiratory event index. An apnea-hypopnea index needs an in-lab study that records brain waves.

Is REI the same as AHI?

No. Both count apneas and hypopneas per hour, but the REI divides them by monitoring time while the AHI divides them by EEG-confirmed sleep time. Quiet awake minutes the device cannot detect without EEG can stay inside the REI denominator, and a home device cannot score arousal-only events. The two scores are related but not interchangeable, and the REI can read lower.

Can a home sleep test underestimate sleep apnea?

It can. Counting events over recording time rather than sleep time can dilute the score and under-grade severity. A 2023 comparison in Sleep Advances by Pitkanen and colleagues found the recording-time index classified severity correctly less often than the sleep-time AHI. A low result with clear symptoms still deserves a review with your clinician.

What is a normal REI score?

An REI under 5 events per hour is generally read as normal in adults, using the same bands as the AHI. A score of 5 to under 15 is mild, 15 to under 30 is moderate, and 30 or more is severe. A normal REI with ongoing symptoms does not rule sleep apnea out.

How many hours does a home sleep test need to record to be valid?

The American Academy of Sleep Medicine guideline (2017) treats at least 4 hours of usable oximetry and airflow data as the minimum for a technically adequate home study. A 2024 study in Sleep and Breathing suggests severity grading becomes more reliable once monitoring time reaches about 6 hours. Check the monitoring time printed on your report.

Do I need an in-lab sleep study if my home test is normal?

Possibly. The American Academy of Sleep Medicine guideline (2017) advises that a home test which is negative, inconclusive or technically inadequate should be followed by in-lab polysomnography. In the UK, NICE guideline NG202 (2021) also keeps polysomnography for these cases. Talk to your clinician if breathing pauses, loud snoring or daytime sleepiness continue.

What does IAH mean on a polygraphie ventilatoire report?

IAH stands for indice d'apnées-hypopnées. On a polygraphie ventilatoire, the home respiratory polygraphy used in France and other French-speaking countries, it counts breathing events over recording time, like the English REI. It uses the same cut points of 5, 15 and 30 events per hour for mild, moderate and severe.

Medical Disclaimer: This article is for informational purposes only and does not replace professional medical advice. Snoring can be a symptom of obstructive sleep apnea, a serious medical condition. If you suspect sleep apnea, consult a healthcare professional. Back2Sleep is a CE-certified Class I medical device intended for snoring and sleep apnea. If you have severe sleep apnea, talk to your doctor.
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