How to Calculate the Apnea-Hypopnea Index (AHI)

The AHI is calculated by dividing the total number of apneas plus hypopneas by the total sleep time in hours. 1

Basic Formula

AHI = (Total Apneas + Total Hypopneas) / Total Sleep Time (hours) 1

The calculation uses total sleep time (TST) in hours as the denominator, not total recording time 1
Each apnea and hypopnea event must meet specific duration and amplitude criteria to be counted 1

An apnea is scored when: 1

There is a ≥90% drop in airflow from baseline (measured by oronasal thermal sensor during diagnostic studies or PAP device flow during titration)
The event lasts ≥10 seconds in adults
No desaturation requirement is needed for apnea scoring 1

The American Academy of Sleep Medicine recommends the following hypopnea criteria (which significantly impacts your final AHI value): 1

A hypopnea is scored when ALL of the following are met:

Peak airflow drops by ≥30% from baseline (using nasal pressure for diagnostic studies or PAP device flow for titration) 1
The event duration is ≥10 seconds 1
The event is associated with either ≥3% oxygen desaturation OR an arousal 1

Different hypopnea definitions produce dramatically different AHI values—this is the single most important factor affecting your calculation. 1

Using the recommended AASM criteria (≥30% flow reduction + ≥4% desaturation) produces AHI values approximately 70% lower than older Chicago criteria 1
Using the alternative AASM criteria (≥50% flow reduction + ≥3% desaturation or arousal) produces AHI values approximately 40% lower than Chicago criteria 1
The same patient can have an AHI of 25/hr with one definition versus 8/hr with another—changing them from moderate OSA to borderline normal 1
You must know which hypopnea definition your sleep center uses, as this fundamentally changes the diagnostic threshold interpretation 1

RDI = AHI + RERA Index (where RERA Index = Respiratory Effort-Related Arousals per hour of sleep) 1, 2

RERAs are sequences of breaths ≥10 seconds with increasing respiratory effort or flow limitation leading to arousal, but not meeting apnea or hypopnea criteria 1
The American Academy of Sleep Medicine position statement emphasizes that arousal-based respiratory events should be captured because arousals (not just hypoxemia) predict hypersomnia and neurocognitive symptoms 1
If your AHI is ≥5/hr OR your RDI is ≥5/hr, treatment of symptomatic patients is recommended 1
Reporting RDI is optional but strongly encouraged, especially when using hypopnea definitions that include arousal 1

Count all apneas that meet the ≥90% flow reduction for ≥10 seconds 1
Count all hypopneas that meet your center's specific criteria (verify whether they use ≥30% or ≥50% flow reduction, and whether they require ≥3% or ≥4% desaturation, or accept arousal) 1
Add apneas + hypopneas = total respiratory events 1
Divide by total sleep time in hours (not total recording time) 1
Optionally calculate RDI by adding RERA index if RERAs were scored 1

Do not use total recording time—only actual sleep time counts in the denominator 1
Respiratory events occurring during wake epochs may be excluded from the AHI calculation even though they contribute to symptoms, so clinical interpretation should consider this 1, 3
Different oximeters can change your AHI by 3-7 events/hour due to varying signal averaging times, potentially changing whether a patient meets treatment thresholds 4
Events lasting 5-10 seconds with significant desaturation are excluded by standard definitions but may be clinically significant 5
The choice of hypopnea definition can cause 40% of patients to shift from "positive for OSA" to "negative" when using stricter criteria 1, 6

Once calculated, OSA severity is classified as: 3, 2, 7

The specific thresholds vary slightly between guidelines, but AHI >40/hr consistently defines severe OSA with high perioperative risk. 3, 2