Written and medically reviewed by Kwaku Osafo-Mensah, MD
Pulmonary Medicine | Sleep Medicine | Diplomate, American Board of Sleep Medicine (ABSM)
Medically reviewed: August 2026
Sleep-study reports contain an alphabet soup of abbreviations. Three of the most important are AHI, RDI, and REI.
They are related measurements of sleep-disordered breathing, but they are not necessarily interchangeable.
Understanding the difference is particularly important when comparing a laboratory polysomnogram with a home test for sleep apnea.
Quick Answer
AHI — Apnea-Hypopnea Index: the average number of apneas and hypopneas per hour of sleep.
RDI—Respiratory Disturbance Index: a respiratory event measure whose precise definition can vary; in some laboratory contexts it includes apneas, hypopneas, and respiratory effort-related arousals (RERAs).
REI — Respiratory Event Index: commonly used with home sleep apnea testing and generally represents qualifying respiratory events divided by monitoring or recording time rather than EEG-confirmed total sleep time.
This distinction matters because
AHI, RDI, and REI may produce different numbers in the same person.
What Is AHI?
AHI stands for Apnea-Hypopnea Index.
During polysomnography, it is generally calculated as:
AHI = Number of apneas + hypopneas ÷ hours of sleep
Because laboratory polysomnography records brain activity with EEG, the study can determine when a person is actually asleep.
For example, if 90 apneas and hypopneas occur during six hours of sleep:
90 ÷ 6 = AHI 15 events/hour
For a more detailed discussion, see AHI Explained: What Your Apnea-Hypopnea Index Means.
Link that article title to:
What Is RDI?
RDI stands for Respiratory Disturbance Index.
This term requires more caution because its definition has not always been used identically across laboratories, devices, reports, payers, and publications.
In an important sleep laboratory context, RDI may represent the following:
Apneas + hypopneas + RERAs ÷ hours of sleep
Therefore, RDI may be higher than AHI when clinically meaningful respiratory-effort-related arousals occur that do not meet apnea or hypopnea criteria.
When reading a report, it is useful to determine how that particular laboratory or system defines RDI rather than assuming the term always means exactly the same thing.
What Is a RERA?
RERA stands for Respiratory Effort-Related Arousal.
A RERA is a sequence of breaths characterized by increasing respiratory effort or airflow limitation that leads to an arousal from sleep but does not meet the scoring criteria for an apnea or hypopnea.
RERAs matter because breathing abnormalities can disrupt sleep even when they do not qualify as apneas or hypopneas.
A person may therefore experience respiratory sleep fragmentation that AHI alone does not completely represent.
What Is REI?
REI stands for Respiratory Event Index.
The term is commonly encountered with home sleep apnea testing (HSAT).
Many conventional HSAT devices measure airflow, respiratory effort, oxygen saturation, pulse, and related signals but do not record the full EEG-based sleep staging used during polysomnography.
Without EEG-confirmed sleep time, the denominator may be based on monitoring time or recording time rather than actual total sleep time.
That distinction can materially affect the resulting index.
Why REI Can Be Lower Than AHI
Consider a simplified example.
A person wears a home sleep apnea device for 8 hours but actually sleeps for only 6 hours.
Suppose 60 qualifying respiratory events occur.
If those events are divided by eight hours:
60 ÷ 8 = REI 7.5 events/hour
If the same 60 events were divided by six hours of confirmed sleep:
60 ÷ 6 = 10 events/hour
The respiratory events did not change.
The denominator changed.
This illustrates one reason a home sleep apnea test may sometimes underestimate respiratory-event frequency, particularly when substantial wakefulness is included in the monitoring period.
AHI vs REI
A simplified distinction is the following:
| Measurement | Common Context | Typical Denominator |
|---|---|---|
| AHI | Polysomnography | Hours of measured sleep |
| REI | Home sleep apnea testing | Monitoring/recording time, depending on device and methodology |
This distinction should not be applied blindly to every device. Newer technologies may use different signals and algorithms to estimate sleep or monitoring time.
For more about what home testing measures compared with polysomnography, see Home Sleep Apnea Test vs. Lab Sleep Study: Which One Do You Need?
Always interpret the number according to the methodology used by the specific study. For a broader overview of sleep-study types, respiratory measurements, oxygen data, and how sleep testing is interpreted, see Sleep Tests.
AHI vs RDI
AHI generally counts:
Apneas + hypopneas
RDI may, depending on the reporting definition, count the following:
Apneas + hypopneas + RERAs
This means:
RDI can sometimes be greater than AHI.
But because terminology has varied historically and across systems, the report’s own definitions matter.
Why Scoring Rules Matter
Even the word “hypopnea” is not completely independent of scoring methodology.
Different scoring rules may use different requirements concerning airflow reduction, oxygen desaturation, and arousal.
Therefore, two studies performed at different times or laboratories may produce somewhat different respiratory indices even when the underlying physiology is similar.
When comparing studies, ask whether the same scoring definitions and methodology were used.
Why a Negative Home Sleep Test May Not End the Evaluation
A sleep apnea test conducted at home can be extremely useful when used in appropriately selected patients.
However, a negative or inconclusive HSAT does not always exclude clinically important obstructive sleep apnea.
Potential reasons include the following:
- Substantial wakefulness during the monitoring period
- Technical signal loss
- Night-to-night variability
- Limited time sleeping on the back
- Limited REM sleep
- Respiratory abnormalities not fully characterized by the device
- Another sleep disorder requiring different measurements
When clinical suspicion remains significant despite a negative or inconclusive home test, additional evaluation may be appropriate.
Learn more in Sleep Tests.
Which Number Determines Sleep Apnea Severity?
For adult obstructive sleep apnea, severity classifications are commonly based on AHI or an appropriate respiratory-event index in the context of the study being interpreted.
But severity should not be reduced to one number.
Important additional information may include:
- Oxygen desaturation
- Duration of respiratory events
- REM-related disease
- Positional disease
- Symptoms
- Daytime sleepiness
- Cardiovascular and pulmonary conditions
- Event type
- Sleep fragmentation
- Technical quality of the study
This is why the complete report and clinical context matter.
Can You Compare AHI From One Study With REI From Another?
You can compare them cautiously, but they should not automatically be treated as equivalent measurements.
Before concluding that sleep apnea has improved or worsened, consider:
- Was one study performed at home and the other in a laboratory?
- Was actual sleep time measured?
- What denominator was used?
- Were the same hypopnea criteria used?
- How much REM sleep occurred?
- How much supine sleep occurred?
- Was there significant signal loss?
- Were RERAs included?
- Were treatment, body weight, medications, alcohol exposure, or health conditions different?
A numerical change may reflect genuine physiologic change, methodological differences, or both.
Questions to Ask About Your Sleep Report
When you see AHI, RDI, or REI on your report, consider asking the following:
- What does this index represent, exactly?
- What events were included?
- What denominator was used?
- Was actual sleep measured?
- Were RERAs scored?
- What was my oxygen saturation?
- Was apnea worse during REM sleep?
- Was apnea worse while sleeping on my back?
- Was the study technically adequate?
- Does this result fit my symptoms and clinical history?
These questions often provide more useful information than focusing on the index alone.
The Bottom Line
AHI, RDI, and REI describe related—but not necessarily identical—aspects of sleep-disordered breathing.
AHI generally describes apneas and hypopneas per hour of measured sleep.
RDI may include additional respiratory disturbances such as RERAs, depending on how it is defined.
REI is commonly used in home testing for sleep apnea and may use monitoring or recording time rather than EEG-confirmed total sleep time.
The most important question is therefore not simply
“What is my number?”
The more useful question is
What was measured, how was the index calculated, and what does it mean for my sleep and health?
References & Further Reading
- Berry RB, Quan SF, Abreu AR, et al. The AASM Manual for the Scoring of Sleep and Associated Events: Rules, Terminology, and Technical Specifications. American Academy of Sleep Medicine.
- Kapur VK, Auckley DH, Chowdhuri S, et al. Clinical practice guideline for diagnostic testing for adult obstructive sleep apnea. Journal of Clinical Sleep Medicine. 2017;13(3):479–504.
https://doi.org/10.5664/jcsm.6506 - American Academy of Sleep Medicine (AASM). International Classification of Sleep Disorders, Third Edition, Text Revision (ICSD-3-TR). American Academy of Sleep Medicine; 2023.
- Berry RB, Budhiraja R, Gottlieb DJ, et al. Rules for scoring respiratory events in sleep: update of the 2007 AASM Manual for the Scoring of Sleep and Associated Events. Journal of Clinical Sleep Medicine. 2012;8(5):597–619.
https://doi.org/10.5664/jcsm.2172 - Gottlieb DJ, Punjabi NM. Diagnosis and management of obstructive sleep apnea: a review. JAMA. 2020;323(14):1389–1400.
https://doi.org/10.1001/jama.2020.3514
Medical Author & Reviewer
Kwaku Osafo-Mensah, MD
Pulmonary Medicine | Sleep Medicine
Diplomate, American Board of Sleep Medicine (ABSM)
More than 20 years of experience in sleep medicineMedically reviewed: August 2026
About the medical reviewer · Editorial Policy
Medical Disclaimer: This information is provided for general educational purposes and is not a substitute for individualized medical evaluation, diagnosis, or treatment. Discuss symptoms and treatment decisions with a qualified healthcare professional.