Understanding AHI and REI

The two numbers at the top of most sleep apnea reports, what each one counts, and what neither can tell you.

The apnea-hypopnea index (AHI) is the number of breathing pauses and shallow-breathing episodes per hour of sleep, measured in a lab study. The respiratory event index (REI) is the home-test version, counted per hour of recording time.¹ Both are read against the same bands: under 5 is normal, 5 to 14 mild, 15 to 29 moderate, and 30 or more severe.² For the same person the REI often runs lower, and neither number captures everything that matters about a night’s breathing.^1,3

What Counts as an Event

An apnea is a drop in airflow of 90% or more lasting at least 10 seconds.³

A hypopnea is a partial drop, of 30% or more for at least 10 seconds, that comes with a fall in blood oxygen or a brief arousal from sleep. Under the AASM’s recommended rule, the oxygen fall must be at least 3%.³

Each index is a count of these events divided by time. The difference between the two lies in which events can be seen and which hours are counted.

AHI: Events per Hour of Sleep

An in-lab sleep study records brain waves, so it knows exactly when you were asleep. Its AHI divides events by hours of sleep, and it can count hypopneas that cause an arousal without an oxygen dip.^1,3

REI: Events per Hour of Recording

A home sleep apnea test has no brain-wave channels. It divides events by total recording time, including time lying awake, and it cannot detect hypopneas that only cause an arousal.¹ Both differences push the REI down, which is why a home result can understate severity.¹

The Scoring Rule Changes the Number

Labs have not always defined a hypopnea the same way, and the choice matters. When researchers rescored 328 sleep studies under three definitions in use at the time, the strictest rule produced a median AHI about 30% of the most lenient one. Around 40% of patients diagnosed under the lenient rule would have been classed as negative under the strict one.⁴ A report should state which rule was used, and two reports scored differently are not directly comparable.

Other Numbers on the Report

Respiratory disturbance index (RDI). Some labs also report this broader count, which adds breathing events that disturb sleep without meeting the definition of an apnea or hypopnea.³

Oxygen measures. These usually include the lowest oxygen level reached during the night and how often oxygen dipped. They show how deep the events went, which an event count alone does not.³

What the Number Leaves Out

How long and how deep. Two people with the same AHI can have very different events: brief and shallow, or long with deep drops in oxygen. The oxygen measures listed alongside the index help show which.

Night-to-night change. Sleep apnea varies. An analysis of more than 67,000 people monitored at home for months estimated that a single night misclassifies 20% to 50% of people with the condition.⁵

How you feel. Many people with a high AHI report little sleepiness, and some with a mild AHI feel the effects keenly.² That is why the AASM’s testing guideline calls for any sleep study to be paired with a full sleep evaluation.¹

Reading Your Own Report

Look for whether the number is an AHI or an REI, which hypopnea rule was applied, how much of the night was recorded or slept, and the oxygen measures alongside the index. Bring the full report, not only the summary page, when you discuss it with a clinician. Obstructive sleep apnea covers what the condition involves.