REI and AHI Are NOT the Same
No, REI (Respiratory Event Index) and AHI (Apnea-Hypopnea Index) are fundamentally different metrics that should not be used interchangeably. According to the American Academy of Sleep Medicine (AASM) guidelines, RDI (Respiratory Disturbance Index) = AHI + RERA index, where AHI represents apneas plus hypopneas per hour of sleep 1.
Key Differences
What Each Metric Measures
- AHI: Number of apneas + hypopneas per hour of sleep time (requires EEG to determine actual sleep)
- REI: Number of respiratory events per hour of total recording time (does not account for wake periods)
Why This Matters Clinically
REI systematically underestimates disease severity compared to AHI 2. In a comprehensive analysis of 1,561 patients, REI had only 42.1-72.8% accuracy in correctly classifying OSA severity when compared to the gold standard AHI 2. The mean absolute error between REI and AHI ranged from 6.7 to 13.9 events per hour 2.
The underestimation occurs because:
- REI uses total recording time as the denominator, which includes wake periods
- REI cannot detect arousal-related hypopneas (requires EEG)
- Patients with high sleep fragmentation or prolonged wake periods have dramatically lower REI values despite severe OSA 3
When Each Is Used
AHI (Gold Standard)
- Requires in-laboratory polysomnography (PSG) with EEG
- Provides accurate severity classification
- Detects arousal-related respiratory events
- Based on actual sleep time
REI (Home Testing)
- Used in home sleep apnea testing (HSAT) devices that lack EEG
- More practical and accessible
- Acceptable for screening but may miss cases
- Tends to underdiagnose severity, particularly in patients with poor sleep efficiency
Clinical Implications
When interpreting sleep studies, always verify which metric is being reported. A patient with REI of 10 events/hour might actually have moderate-to-severe OSA (AHI ≥15) that is being missed 2. Recent validation studies show REI correlates strongly with AHI (r=0.89-0.98) but consistently underestimates absolute values 4, 5, 6.
Common Pitfall
The most dangerous error is assuming REI = AHI when making treatment decisions. This leads to:
- Undertreatment of patients with fragmented sleep
- False negatives in patients with high wake time during testing
- Missed diagnoses when arousal-related hypopneas are the predominant event type
Bottom line: If clinical suspicion remains high despite a low REI on home testing, proceed to full PSG with AHI calculation 2, 7.