OSA Severity Classification
Obstructive Sleep Apnea severity is classified based on the Apnea-Hypopnea Index (AHI) or Respiratory Disturbance Index (RDI), with mild OSA defined as 5-14 events/hour, moderate OSA as 15-29 events/hour, and severe OSA as ≥30 events/hour in adults. 1, 2
Adult Classification Criteria
The standard severity classification for adults uses the following AHI thresholds 1, 2:
- No OSA: AHI <5 events/hour 1
- Mild OSA: AHI 5 to <15 events/hour (or 5-14 events/hour) 1, 2
- Moderate OSA: AHI 15 to <30 events/hour (or 15-29 events/hour) 1, 2
- Severe OSA: AHI ≥30 events/hour 1, 2
Pediatric Classification Criteria
Children have different thresholds due to their distinct physiology 1:
- No OSA: AHI = 0 events/hour 1
- Mild OSA: AHI 1-5 events/hour 1
- Moderate OSA: AHI 6-10 events/hour 1
- Severe OSA: AHI >10 events/hour 1
Alternative Perioperative Classification
The American Society of Anesthesiologists provides a slightly different classification system for perioperative risk assessment 1:
- No OSA: AHI 0-5 events/hour 1
- Mild OSA: AHI 6-20 events/hour 1
- Moderate OSA: AHI 21-40 events/hour 1
- Severe OSA: AHI >40 events/hour 1
This classification is specifically designed for surgical risk stratification and differs from the standard diagnostic criteria 1.
Measurement Indices
Both AHI and RDI can be used for severity classification 1:
- AHI measures the average number of apneas and hypopneas per hour of sleep 1
- RDI includes apneas, hypopneas, and respiratory effort-related arousals (RERAs) per hour of sleep 1
- RDI uses the same severity thresholds as AHI (RDI <5 = no OSA; 5-14 = mild; 15-29 = moderate; ≥30 = severe) 1
Critical Clinical Considerations
The sleep laboratory's overall severity assessment should take precedence over the raw AHI number when available 1. This is because different sleep laboratories may use varying criteria for detecting apneas and hypopneas, leading to inconsistent AHI values across facilities 1.
Hypopnea scoring definitions significantly impact AHI values and severity classification 1. Studies show that using different hypopnea criteria (30% reduction with 4% desaturation versus 50% reduction with 3% desaturation or arousal) can result in 40-70% lower AHI values, potentially reclassifying 36-48% of patients from OSA-positive to OSA-negative 1.
Beyond AHI: Comprehensive Severity Assessment
AHI alone does not fully capture OSA severity or predict clinical outcomes 1, 3. Consider these additional factors:
- Oxygen desaturation patterns and hypoxic burden 1, 3
- Severity of individual obstruction events 3
- Symptom burden and daytime consequences 1, 4
- Associated comorbidities (cardiovascular disease, metabolic dysfunction) 1, 4
Research demonstrates that incorporating the severity of individual obstruction events into AHI calculations better identifies patients at highest risk for mortality and cardiovascular morbidity compared to conventional AHI alone 3.
Diagnostic Requirements
Formal OSA diagnosis requires polysomnography (Type 1 or 2) or home sleep apnea testing (Type 3) demonstrating the specified AHI thresholds 1, 2. Type 4 studies (1-2 channels) are not considered diagnostic for OSA 1.