What objective values indicate the presence of obstructive sleep apnea (OSA) in a patient?

Objective Diagnostic Values for Obstructive Sleep Apnea

An Apnea-Hypopnea Index (AHI) ≥5 events per hour of sleep establishes the diagnosis of obstructive sleep apnea when accompanied by symptoms, or an AHI ≥15 events per hour alone is diagnostic even without symptoms. 1, 2

Primary Diagnostic Metric: Apnea-Hypopnea Index (AHI)

The AHI is calculated by dividing the total number of apneas and hypopneas by total sleep time in hours. 3 The diagnostic thresholds are:

• OSA Present: AHI ≥5/hour with symptoms (snoring, witnessed apneas, excessive sleepiness, choking/gasping) 1, 2, 4
• OSA Present: AHI ≥15/hour regardless of symptoms 4, 5

Severity Classification Based on AHI

Once OSA is diagnosed, severity is stratified as follows: 3, 6

• Normal: AHI 0-5 events/hour
• Mild OSA: AHI 5-14 events/hour
• Moderate OSA: AHI 15-29 events/hour
• Severe OSA: AHI ≥30 events/hour

Respiratory Event Definitions

For accurate AHI calculation, specific criteria must be met for each respiratory event type:

Hypopnea Criteria (AASM Recommended Definition)

A hypopnea is scored when ALL of the following occur: 1

• Peak airflow drops by ≥30% from baseline (measured via nasal pressure or PAP device flow)
• Duration of the ≥30% drop is ≥10 seconds
• Either ≥3% oxygen desaturation from baseline OR an EEG-documented arousal occurs

Critical caveat: The Centers for Medicare and Medicaid Services uses an "acceptable" alternative definition requiring ≥4% oxygen desaturation, which may underdiagnose OSA in symptomatic patients. 1 The AASM strongly recommends using the 3% desaturation or arousal-based criteria to avoid missing clinically significant disease. 1

Apnea Criteria

Complete cessation of airflow for ≥10 seconds despite ongoing respiratory effort (obstructive type). 7

Alternative Diagnostic Index: Respiratory Disturbance Index (RDI)

The RDI provides a more comprehensive assessment by including respiratory effort-related arousals (RERAs): 1

• RDI calculation: (# apneas + # hypopneas + # RERAs) × 60 / total sleep time in minutes
• RDI ≥5 events/hour is diagnostic for OSA 1

RERA Definition

A sequence of breaths lasting ≥10 seconds with increasing respiratory effort or flattening of the nasal pressure waveform that leads to arousal, but doesn't meet criteria for apnea or hypopnea. 1

Important distinction: Home sleep apnea testing (HSAT) typically cannot capture RERAs because it lacks EEG monitoring, potentially underestimating disease severity. 1, 2 If HSAT is negative in a patient with high clinical suspicion, in-laboratory polysomnography with full EEG is mandatory. 1

Supporting Objective Measures

While not diagnostic alone, these values support OSA severity assessment:

• Oxygen Desaturation Index (ODI): Frequency of ≥3% or ≥4% oxygen desaturations per hour 1, 8
• Minimum oxygen saturation: Lower nadir values correlate with worse outcomes 8
• Mean oxygen saturation during sleep: Lower values indicate more severe disease 8
• Arousal index: Frequency of EEG arousals per hour of sleep 1

Diagnostic Testing Requirements

Polysomnography (PSG) remains the gold standard and must include: 2

• EEG (electroencephalography) for sleep staging and arousal detection
• EOG (electrooculography)
• Chin EMG (electromyography)
• Airflow measurement
• Oxygen saturation
• Respiratory effort monitoring
• ECG (electrocardiography)

Home sleep apnea testing (Type III) may be used when: 2

• High pretest probability of moderate-to-severe OSA exists
• Manually scored respiratory event index ≥15/hour establishes diagnosis
• However, negative HSAT requires confirmatory in-laboratory PSG due to inability to detect arousal-based events 1, 2

Common Pitfalls to Avoid

Failing to recognize arousal-based respiratory events leads to misdiagnosis or underestimation of OSA severity. 1 Patients with frequent arousals but minimal desaturations may have significant symptoms and cardiovascular consequences despite a "normal" AHI using older criteria. 1

The absence of classic symptoms does not exclude OSA: 78% of patients with confirmed OSA denied snoring and sleepiness, and patients with severe OSA (AHI ≥30) often report normal sleepiness scores. 2 Therefore, objective testing is essential and cannot be replaced by symptom assessment alone. 2

Different hypopnea scoring criteria between centers can significantly alter AHI values and severity classification, so clinicians must verify which definition was used when interpreting results. 6