AHI, the Apnea-Hypopnea Index, counts apneas and hypopneas per hour. RDI, the Respiratory Disturbance Index, adds RERAs, breathing efforts that end in an arousal without meeting the hypopnea threshold [S1]. RDI is always equal to or higher than AHI, so a sleep-study RDI and a CPAP AHI are not directly comparable.
Does it matter? Yes, if comparing a sleep-study number to your CPAP number: the study was scored with EEG before treatment, while your CPAP's AHI leaves RERAs out.
Medical disclaimer: AirwayLab is a data visualisation and analysis tool, not a medical device. The information it provides is for personal reference only and is not a substitute for professional clinical advice. Always discuss your sleep study results and therapy data with your prescribing clinician.
AHI: What It Actually Counts
AHI stands for Apnea-Hypopnea Index. It counts two types of breathing events per hour:
Apneas
Complete or near-complete airflow stoppages lasting ≥10 seconds. Obstructive apneas involve airway collapse; central apneas involve reduced breathing drive.
Hypopneas
Partial airflow reductions (≥30% for ≥10 seconds) associated with oxygen desaturation or an arousal, depending on the scoring criteria used.
AHI = (total apneas + hypopneas) ÷ hours of sleep. Your CPAP machine computes this from its internal flow sensor — no EEG or sleep technician required. For a deeper look at what AHI does and does not capture, see Why AHI Is Lying to You.
RDI: The Bigger Picture
RDI stands for Respiratory Disturbance Index. It counts everything in AHI, plus additional respiratory events that do not meet the full criteria for an apnea or hypopnea.
RDI = AHI + additional events
The difference between them is the count of RERA-type events included in RDI but not in AHI. RERAs (Respiratory Effort-Related Arousals) are sequences of flow-limited breaths that end in an arousal without a full apnea or hypopnea occurring. Learn more in What Are RERAs?
RDI is always ≥ AHI. When the two numbers are close, there are few RERA-type events. When they differ substantially, RERA-type events make up a significant portion of your respiratory disturbances.
The Gap That Confuses Everyone
Consider a concrete scenario:
Sleep study (PSG)
RDI 22
Includes apneas, hypopneas, and RERA-type events scored by a sleep technician with EEG data
CPAP data (nightly)
AHI 1.8
Most CPAP machines leave RERAs out of the AHI they report. Some log a separate flow-based RERA estimate, which is not scored from EEG like a sleep study.
Both numbers are correct. The sleep study measured more event types. The CPAP reports only what its flow sensor can detect without EEG data.
Why Your CPAP Only Reports AHI
Two reasons home CPAP machines do not report RDI:
- No EEG signal. Detecting arousals requires EEG data — brain activity monitoring. Home CPAP machines have only a flow sensor and a pressure sensor. They can detect breathing events but cannot directly detect arousals.
- Scoring rule variation. The AASM 2007 and 2012 hypopnea scoring rules differ in their arousal criteria, which affects what counts as a hypopnea vs a sub-threshold event. Your sleep study and your CPAP machine may apply different versions.
When Each Metric Applies
| Context | AHI | RDI |
|---|---|---|
| Sleep apnea diagnosis (PSG) | Yes | Yes |
| CPAP compliance reporting | Yes | Not available |
| Ongoing nightly monitoring | Yes | Not available |
| Evaluating UARS (symptoms disproportionate to AHI) | Incomplete | More informative |
| FAA / DOT medical certification | Varies | Often required |
| PSG report from sleep lab | Reported | Often reported |
Sleep physicians often look at RDI alongside AHI when symptoms are disproportionate to AHI alone. Your clinician can review both metrics together in the context of your clinical picture.
What AirwayLab Shows You
AirwayLab does not compute a clinical RDI — that requires EEG data that home devices do not have. From your SD card it computes several metrics related to the RERA-type patterns that drive the RDI-AHI gap:
NED (Negative Effort Dependence)
Estimates RERA-type patterns per hour using breath-shape analysis. Not equivalent to a clinical RERA count, but reflects the same flow-limited arousal patterns.
FL Score
An uncalibrated flow-shape flattening index (0-100), not a percentage of breaths. Elevated flow limitation is the airway mechanism underlying most RERA-type events.
Glasgow Index
A composite 9-component breath shape score (0–9). Captures overall airway quality across the night, not just event counts.
H1/H2 split
Compares metrics between the first and second halves of the night. Positional or REM-related patterns show up here.
These metrics are patterns to discuss with your clinician — not a substitute for a sleep study or a clinical RDI calculation. See Understanding Flow Limitation for more on how these metrics relate to what happens in your airway.
Practical Takeaways
AHI ≠ RDI
If your sleep study shows RDI and your CPAP shows AHI, the two numbers are not measuring the same thing. A low nightly AHI does not necessarily mean your CPAP data would match your sleep study RDI. Open your SD card in AirwayLab to see your own nightly AHI trend alongside AirwayLab's flow-pattern estimates.
Low AHI does not rule out sleep disturbance
If you have persistent symptoms with a low AHI, elevated RERA-type pattern estimates (NED, FL Score) in your AirwayLab data may be worth discussing with your clinician alongside your full sleep history. See Beyond AHI for the research context.
Bring data, not descriptions, to your appointment
If your FL Score and RERA-type pattern estimates look consistently elevated, export a summary and bring it to your next appointment. Your clinician can review it alongside your sleep study results and prescription history.
References
[S1] American Academy of Sleep Medicine. The AASM Manual for the Scoring of Sleep and Associated Events, Version 3 (2023). Grade: clinical scoring standard.
Berry RB et al. Rules for Scoring Respiratory Events in Sleep. J Clin Sleep Med. 2012;8(5):597-619.
Guilleminault C et al. Upper airway sleep-disordered breathing in women. Ann Intern Med. 1995;122(7):493-501.
Ruehland WR et al. The new AASM criteria for scoring hypopneas: impact on the apnea hypopnea index. Sleep. 2009;32(2):150-157.
Bonsignore MR et al. Obstructive sleep apnoea and comorbidities: a dangerous liaison. Multidiscip Respir Med. 2019;14:8.
The information on this page is for personal reference only. AHI and RDI are clinical metrics that should be interpreted by a qualified clinician in the context of your diagnosis, prescription, and symptoms. AirwayLab's pattern estimates are not equivalent to a clinical RDI calculation.
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