What Is the Apnea–Hypopnea Index, and What Do the Numbers Mean?

The apnea–hypopnea index (AHI) is the number of times an hour of sleep your breathing stops (an apnea) or becomes shallow enough to lower your oxygen or wake you briefly (a hypopnea). It’s the number used to diagnose obstructive sleep apnea and grade how severe it is. An AHI under 5 is normal for adults; 5 to 15 is mild, 15 to 30 moderate, and 30 or more severe.

This article is general information, not medical advice.

The Thresholds

The American Academy of Sleep Medicine’s (AASM) task force on sleep-related breathing disorders set the severity grades still used today:

AHI (events per hour)	Severity
Under 5	Normal
5 to under 15	Mild
15 to under 30	Moderate
30 or more	Severe

Under the AASM’s International Classification of Sleep Disorders, obstructive sleep apnea is diagnosed with an AHI of 15 or more, or 5 or more with symptoms such as daytime sleepiness, loud snoring, or witnessed pauses in breathing, or with related conditions such as high blood pressure. It’s common: Benjafield and colleagues estimated that 936 million adults aged 30 to 69 worldwide have at least mild sleep apnea, and 425 million moderate to severe.

Three Indices, Three Tests

• AHI, from a laboratory sleep study (polysomnography). Brain waves show when you’re actually asleep, so events are counted per hour of sleep.
• Respiratory event index (REI), from a home sleep apnea test. The device records breathing and oxygen but not sleep, so events are divided by the hours of recording, including time awake. The REI therefore tends to read lower than a laboratory AHI. The AASM’s 2017 guideline (Kapur and colleagues) accepts home testing for uncomplicated adults likely to have moderate to severe sleep apnea, and advises a laboratory study if a home test is negative or inconclusive but suspicion remains.
• Residual AHI, from a CPAP machine. An estimate of events remaining during treatment; see What Does the AHI on Your CPAP Machine Mean?

These are different measurements and aren’t directly comparable.

What a Watch Can Tell You

Some watches and rings now flag possible breathing disturbances or show overnight oxygen dips. They can prompt a conversation with a doctor, but they don’t measure AHI and can’t diagnose or rule out sleep apnea. If you snore loudly, someone has seen you stop breathing, or you’re sleepy during the day despite enough sleep, ask about a sleep study.

Following It in Kaizora

Kaizora keeps the three as separate metrics on the Sleep page: Apnea–Hypopnea Index from a laboratory study, Respiratory Event Index from a home test, and Residual Apnea–Hypopnea Index from a CPAP machine. Log a study’s result with Log at the date of the study, with the lab or device as its device. With several studies over the years, the chart shows how your sleep apnea has changed, such as after weight loss.

The Short Version

• AHI counts pauses and shallow breaths per hour of sleep.
• Under 5 is normal; 5 to 15 mild, 15 to 30 moderate, 30 or more severe.
• A home test’s REI counts per hour of recording and reads lower than a lab’s AHI.
• Watches can’t diagnose sleep apnea; a sleep study can.

Sources

• American Academy of Sleep Medicine Task Force, “Sleep-related breathing disorders in adults: recommendations for syndrome definition and measurement techniques in clinical research,” Sleep 22, no. 5 (1999): 667–689. doi:10.1093/sleep/22.5.667
• Kapur VK et al., “Clinical practice guideline for diagnostic testing for adult obstructive sleep apnea: an American Academy of Sleep Medicine clinical practice guideline,” Journal of Clinical Sleep Medicine 13, no. 3 (2017): 479–504. doi:10.5664/jcsm.6506
• Benjafield AV et al., “Estimation of the global prevalence and burden of obstructive sleep apnoea: a literature-based analysis,” The Lancet Respiratory Medicine 7, no. 8 (2019): 687–698. doi:10.1016/S2213-2600(19)30198-5