What is REI on a Sleep Study?
REI (Respiratory Event Index) is the number of respiratory events (apneas and hypopneas) per hour of monitoring time, used primarily in home sleep apnea tests (HSAT) that lack EEG monitoring to determine actual sleep time. 1
Key Distinction from AHI
REI differs fundamentally from the Apnea-Hypopnea Index (AHI) in its denominator:
- REI = Total respiratory events ÷ Total monitoring/recording time 1
- AHI = Total respiratory events ÷ Total sleep time (requires polysomnography with EEG) 1
Because patients are awake for portions of their monitoring time, REI typically underestimates the true severity of obstructive sleep apnea compared to AHI 1, 2. Research demonstrates that REI has only 42-73% accuracy in correctly classifying OSA severity when compared to the gold standard AHI 2.
Clinical Context and Limitations
Why REI is Used
Home sleep apnea tests cannot measure actual sleep because they lack electroencephalography (EEG), electrooculography (EOG), and electromyography (EMG) sensors required to distinguish sleep from wakefulness 1. Therefore, HSAT devices calculate REI based on the total time the device is recording, not actual sleep time 1.
Important Caveats
- REI underestimates OSA severity because the denominator (monitoring time) is larger than actual sleep time 1, 2
- HSAT devices cannot detect hypopneas associated only with cortical arousals (no EEG to score arousals) 1
- The mean absolute error between REI and AHI ranges from 6.7 to 13.9 events per hour 2
- REI shows lower sensitivity (42-73%) and specificity (81-91%) compared to AHI for diagnosing OSA 2
Confusion with RDI
Do not confuse REI with RDI (Respiratory Disturbance Index). According to the American Academy of Sleep Medicine consensus definition 3:
- RDI = AHI + RERA index (Respiratory Effort-Related Arousals per hour of sleep)
- RDI requires polysomnography with EEG to score arousals 3
- RDI reporting is optional, not required 3
Note that the Centers for Medicare and Medicaid has historically defined RDI differently (as apneas and hypopneas per hour of monitoring), adding to the confusion 3.
Clinical Interpretation
When reviewing HSAT results with REI values:
- A positive REI (≥5-15 events/hour depending on threshold) supports OSA diagnosis in patients with appropriate symptoms and risk factors 1
- A negative or low REI does not rule out OSA due to the inherent underestimation 1, 2
- Raw HSAT data must be reviewed by a board-certified sleep medicine physician or under their supervision 1
- Consider full polysomnography if clinical suspicion remains high despite low REI 1
The physician's diagnosis must integrate the REI value with the patient's medical history, symptoms, and clinical evaluation—not rely on the REI number alone 1.