What is AHI?

Sleep apnea can be difficult to detect and diagnose, but the key to unlocking the presence and severity of this condition lies in one simple measurement. The apnea-hypopnea index (AHI) is the biggest determining factor when it comes to sleep apnea, and the most important metric measured by sleep tests. So, what exactly is AHI, how is it measured, and why does it matter?

To put it simply, AHI is the average number of apneas and hypopneas that occur per hour during sleep. Hypopnea may be a new word in your study of sleep apnea, so let’s break it down with sleep neurologist, Dr. Guy Leschziner.

• Apnea - “Apneas are defined as complete obstructions of the airway.” This is a full stop of airflow for at least 10 seconds.
• Hypopnea - “Hypopneas are defined as a narrowing of the airway associated with a drop in oxygen levels.” This partial obstruction or narrowing of the airway must also last at least 10 seconds to be classified as hypopnea.

Both of these occurrences contribute to obstructive sleep apnea (OSA) and can only be accurately measured through a sleep study with specialized equipment, either at home or in a clinical setting.

How is it measured?

According to Leschziner, “the index counts the number of apneas and hypopneas over the course of the night and averages per hour of sleep. Typically, we characterise an AHI of 0 to 4 as being normal, 5 to 15 as being mild, 15 to 30 as being moderate, and 30 plus as being severe.” So, the metric seems simple enough, but how do you know how often your breathing is being obstructed during sleep? That’s where sleep tests come in.

Use our calculator to estimate your AHI based on sleep study data or monitoring results.

Polysomnography (PSG)

Dr. Lezschiner refers to polysomnography as the gold standard when it comes to sleep studies. “In polysomnography, we are measuring brain waves, so we can see the impact of any breathing disturbance on sleep. We also have video and other electrobes that, for example, monitor leg movement as well. So, it gives us a much more complete picture of sleep.” This test is usually conducted overnight in a clinical setting and is the most expensive option, but also the most thorough.

Home sleep apnea test (HSAT)

These “take-home tests” are becoming an increasingly popular choice for patients, due to their affordability, convenience, and reliability. “At-home tests are used routinely by clinical services because they are generally very accurate. What a lot of the home devices don't do is measure the direct impact of the respiratory events on your brain waves and on the sleep architecture itself.” So, if you’re presenting with classic symptoms of OSA such as snoring, this may be a good place to start.

What’s a normal AHI?

AHI exists on a sliding scale where the more breathing interruptions per hour, the more severe your OSA is. Knowing the severity of the condition is essential because it’s necessary for planning treatment, and a higher AHI is linked to long-term health risks. For example, moderate to severe AHI levels are associated with cardiovascular and metabolic risks.

• Normal: AHI < 4
• Mild sleep apnea: AHI 5–14
• Moderate sleep apnea: AHI 15–29
• Severe sleep apnea: AHI ≥ 30

Wondering if sleep apnea is affecting you? Take a few seconds to check out the Dumbo Health Sleep Quiz, and with a few simple questions, you can begin your treatment journey today.

Frequently Asked Questions

Does AHI apply to children as well as adults?
No. Children have different diagnostic thresholds. An AHI of one or more events per hour can be considered abnormal in pediatric sleep studies, while in adults, fewer than five events per hour is typically considered normal.

Can lifestyle changes reduce AHI?
Yes. Weight loss, positional therapy (avoiding supine sleep), and reducing alcohol intake have all been shown to lower event frequency in some patients, though they may not eliminate sleep apnea entirely.

Why might my AHI look different on CPAP vs. diagnostic testing?
When using CPAP or another therapy, the machine often reports a residual AHI based on airflow data. This isn’t identical to a lab-calculated AHI but helps track treatment effectiveness.

Can wearable devices measure AHI accurately?
Not yet. Most consumer wearables track oxygen trends or sleep stages, but cannot measure airflow and respiratory effort simultaneously. Therefore, they estimate sleep disturbance rather than calculate true AHI.

Is AHI the only number that matters in a sleep study?
No. Other indices, like the oxygen desaturation index (ODI) or hypoxic burden, provide additional insights into how severely sleep apnea affects oxygenation and cardiovascular risk. AHI is important, but not the full picture.

Wondering if sleep apnea is affecting you? Take a few seconds to check out the Dumbo Health Sleep Quiz, and with a few simple questions, you can begin your treatment journey today.