Difference Between AHI and RDI on a Sleep Study Report

Looking at a sleep study report for the first time can be overwhelming. Seeing multiple different acronyms and scores might leave you wondering exactly how severe your condition is. The Apnea-Hypopnea Index (AHI) and the Respiratory Disturbance Index (RDI) are two different scores used to measure how often your breathing is disrupted while you sleep. The main difference between them is what they count. AHI counts the times your breathing stops completely or drops significantly enough to lower your oxygen levels or briefly wake your brain ^[1]. RDI includes everything in your AHI, but it also counts subtler breathing struggles called Respiratory Effort-Related Arousals (RERAs) ^[2]. For example, an AHI of 15 means you have 15 major breathing pauses per hour, but an RDI of 25 means you have an additional 10 events per hour where you struggle to breathe just enough to wake your brain, even if your oxygen didn’t drop.

What Makes Up Your AHI?

The Apnea-Hypopnea Index (AHI) measures the more severe breathing disruptions during sleep ^[3]. It is the most common number used by doctors to diagnose the severity of sleep apnea ^[4]. AHI adds up two types of events:

• Apneas: A near-complete pause in breathing (airflow drops by 90% or more) that lasts for at least 10 seconds ^[1].
• Hypopneas: A partial pause in breathing (airflow drops by at least 30%) that causes your blood oxygen levels to drop or briefly wakes your brain ^[2].

Doctors categorize sleep apnea severity based on the AHI score:

• Normal: Less than 5 events per hour
• Mild: 5 to 14.9 events per hour
• Moderate: 15 to 29.9 events per hour
• Severe: 30 or more events per hour

While AHI is the standard measurement for diagnosing sleep apnea severity, experts note that relying solely on AHI can miss the full picture of a patient’s sleep quality and daytime symptoms ^[5][6]. Furthermore, sleep labs sometimes use different scoring rules for hypopneas—for example, requiring either a 3% or a 4% drop in oxygen ^[2]. This means your AHI could technically change slightly depending on the lab’s criteria.

Why RDI Gives a More Complete Picture

The Respiratory Disturbance Index (RDI) provides a wider view of sleep disruption because it adds a third type of event to the apneas and hypopneas ^[7]. This event is called a Respiratory Effort-Related Arousal (RERA). RDI uses the exact same severity scale (Normal, Mild, Moderate, Severe) as the AHI.

A RERA happens when your airway narrows slightly, and you have to work harder and harder to pull air into your lungs ^[2]. Eventually, this physical struggle causes a micro-awakening in your brain (an arousal) so you can take a normal breath ^[2]. Because your oxygen levels don’t drop enough, and your airflow doesn’t decrease to the 30% threshold, a RERA doesn’t qualify as an apnea or a hypopnea ^[2].

However, these continuous micro-awakenings severely fragment your sleep ^[8]. A person with a moderate AHI might actually feel exhausted during the day because their RDI is much higher, showing that their brain is constantly being woken up by the sheer physical effort of breathing ^[5]. Because it captures these subtle disruptions, the RDI gives a much better picture of overall sleep fragmentation.

What is the ODI?

You will also likely see the Oxygen Desaturation Index (ODI) on your report. While AHI and RDI measure how often your sleep is physically interrupted, the ODI measures how often your oxygen levels suffer as a result ^[9].

The ODI counts the number of times per hour your blood oxygen level drops by a certain percentage (usually 3% or 4%) ^[10]. High ODI levels are strongly linked to cardiovascular risks, such as high blood pressure and heart disease, and are a strong predictor of daytime sleepiness ^[11][12]. Because the RDI includes RERAs (which don’t necessarily cause oxygen drops), your RDI will almost always be higher than your ODI ^[13].

Types of Sleep Apnea on Your Report

Understanding your AHI, RDI, and ODI helps measure how often your breathing is disrupted, but it is also important to know why your breathing is disrupted. Your sleep study will categorize your breathing pauses into three distinct types:

• Obstructive Sleep Apnea (OSA): This is a mechanical problem. Your brain tells your body to breathe, and your chest makes the effort, but a physical blockage in your upper airway stops the air from passing through ^[14]. OSA is strongly tied to symptoms like loud snoring and daytime sleepiness ^[15]. Treatment options for OSA typically include Continuous Positive Airway Pressure (CPAP) therapy, oral appliances (mouthguards that hold the jaw forward), or lifestyle modifications ^[16].
• Central Sleep Apnea (CSA): This is a neurological communication problem. Your airway is completely open, but the respiratory control center in your brain temporarily fails to send the signal to your breathing muscles ^[17]. Your body simply makes no effort to breathe for a short period ^[18]. CSA requires different management than OSA, sometimes involving specialized devices that stimulate the breathing nerves ^[16][19].
• Mixed Sleep Apnea: This is a combination of both central and obstructive issues in a single event. The pause in breathing begins as central (your body makes no effort to breathe) but ends as obstructive (your body starts trying to breathe, but your airway has collapsed or blocked) ^[20][21].

Accurately categorizing these events is vital for treatment. For example, treating obstructive events with standard CPAP machines can sometimes unmask or worsen central events, leading to what is called Complex Sleep Apnea, which requires different machine settings ^[22][20].