AHI, RDI & REI Explained

Three related indexes appear frequently on sleep study reports. Each expresses a rate — the average number of breathing events per hour — but they differ in exactly which events are included and what time period is used as the denominator.

What this means: You can use this information to understand why your report shows a particular number and why it may differ from another test's result.

What this does not mean: This page cannot interpret your individual report or determine your severity. That requires review by the diagnosing provider in the context of your full clinical picture. For an overview of how severity is classified, see the mild, moderate, and severe sleep apnea guide.

An apnea is a complete or near-complete pause in airflow that lasts at least 10 seconds during sleep. A hypopnea is a partial reduction in airflow that lasts at least 10 seconds and is accompanied by a drop in blood oxygen or an arousal from sleep.

AHI (Apnea-Hypopnea Index) counts apneas plus hypopneas and divides by the hours of measured sleep. It is the most widely used index for classifying adult obstructive sleep apnea severity.

Both apneas and hypopneas must meet specific scoring criteria defined by published guidelines. Different scoring rules — for example, whether a hypopnea requires a 3% or 4% oxygen drop — can change the count, which means the same raw data can produce slightly different AHI values depending on the scoring standard applied.

A respiratory effort-related arousal (RERA) is a sequence of increasingly difficult breathing that does not meet the criteria for an apnea or hypopnea but still causes an arousal from sleep. RERAs can fragment sleep and contribute to daytime symptoms even when they are not counted in the AHI.

RDI (Respiratory Disturbance Index) includes apneas, hypopneas, and RERAs. Because it captures more events, RDI can be higher than AHI for the same person. The two terms are sometimes used interchangeably in casual conversation, but they are technically distinct — RDI is the broader measure.

If your report shows both numbers, the difference between them reflects events that disturb sleep without meeting the threshold for a scored apnea or hypopnea.

In-lab polysomnography measures sleep directly using brain wave activity (EEG), so the AHI denominator is actual sleep time. Home sleep apnea tests typically do not measure brain activity and cannot confirm when the person was asleep versus merely in bed with the device on.

REI (Respiratory Event Index) is used with home tests because the denominator is monitoring time — the total time the device was recording — rather than confirmed sleep time. If a person lies awake for an hour before falling asleep, that awake time is included in the monitoring period but would not be part of sleep time in an in-lab study.

This means that for the same actual events, a home test REI can be lower than an in-lab AHI, because the denominator (monitoring time) is larger. Conversely, if sleep time is much shorter than monitoring time, the relationship can change. This is not a flaw — it is a consequence of what each test measures.

Learn more about the differences between test types in the sleep study vs. home sleep test guide.

Several factors can affect the number on a sleep study report:

• Scoring rules: Published guidelines define how events are counted. Different versions of these rules can produce different counts from similar data.
• Equipment sensitivity: In-lab studies use more sensors (including EEG, effort belts, and nasal pressure) and can detect events that a simplified home device may miss.
• Denominator choice: Sleep time (in-lab) versus monitoring time (home) changes the rate calculation.
• Night-to-night variation: Sleep can vary from night to night, and a single study is a snapshot, not a long-term average.

None of these factors makes a test result wrong. They mean that the number should be understood in the context of how it was obtained, which is why professional interpretation matters.

Sleep study reports contain more than a single index. They typically include oxygen data, sleep stage information (for in-lab studies), event types, body position, and clinical observations. The index is one piece of a larger picture.

When reviewing results with the diagnosing provider, it may help to ask:

• What type of test was performed, and how might that affect the numbers?
• Which scoring rules were applied?
• What do the oxygen and symptom data show alongside the index?
• What are the treatment options to discuss, and what follow-up is recommended?

For a broader discussion of how severity classifications are used and their limits, see the severity and treatment decisions hub.