Struggling to understand what your AHI score means? You’re not alone. The Apnea-Hypopnea Index (AHI) is the cornerstone of sleep apnea diagnosis and treatment, yet it’s often misunderstood.
The challenge? AHI sounds simple, just counting events per hour of sleep, but scoring rules, test types, and even your sleeping position can all change the number. This guide breaks it all down, offering clear explanations and expert-backed insights into:
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What AHI measures and why it matters
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How doctors calculate it during sleep studies
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Normal vs. mild, moderate, and severe AHI scores
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What to know about device-reported scores
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Practical FAQs about AHI, testing, and treatment
By the end, you’ll know exactly how to interpret your sleep study results, how clinicians use AHI in real-world care, and what steps you can take to improve your sleep and health.
What the Apnea-Hypopnea Index (AHI) Is
The Apnea-Hypopnea Index measures how many times per hour your breathing pauses or is partially reduced while you sleep. These interruptions are either apneas (complete pauses) or hypopneas (partial reductions in airflow).
Doctors use AHI to:
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Diagnose obstructive sleep apnea (OSA) and other breathing disorders
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Determine severity (mild, moderate, or severe)
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Decide on treatment, such as CPAP, oral appliances, or surgery
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Track how well therapy is working
The idea is simple: AHI gives a single number that captures the overall “burden” of breathing problems during sleep. But what counts as an event and how it’s measured depends on the testing method and scoring rules.
Apnea vs. Hypopnea
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Apnea: A complete stop in airflow for at least 10 seconds. It can be obstructive (airway collapse with continued breathing effort) or central (the brain temporarily stops sending the signal to breathe). According to the American Academy of Sleep Medicine (AASM), this is the standard definition in adults.
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Hypopnea: A partial reduction in airflow, usually at least 30% for 10 seconds or longer, paired with either a drop in oxygen (3–4%) or an arousal from sleep. The exact criteria matter: some labs use a 3% desaturation or arousal rule, while others require a 4% desaturation only. The Sleep Foundation explains these scoring differences in plain language.
Both apneas and hypopneas disrupt sleep and lower oxygen levels, although apneas tend to cause sharper drops. For example:
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An obstructive apnea might last 15 seconds and result in a 4% drop in oxygen levels.
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A hypopnea might reduce airflow by 50% for 12 seconds, accompanied by a 3% drop in oxygen saturation or an arousal.
Because scoring rules differ, two labs could report different AHI values on the same night. That’s why clinicians always interpret AHI in conjunction with symptoms, oxygen data, and overall health.
Obstructive vs. Central Events
Not all breathing pauses are the same. Doctors classify events based on whether the problem originates from a blocked airway or a pause in brain signalling.
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Obstructive apneas: Airflow stops, but the chest and abdomen still try to breathe. This is the hallmark of obstructive sleep apnea, which often improves with CPAP.
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Central apneas: Airflow stops because there’s no breathing effort at all. Common in central sleep apnea syndromes, sometimes linked to heart failure, stroke, or opioid use (StatPearls).
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Mixed events: Start central and end obstructive (or vice versa). These can indicate complex sleep apnea and require careful treatment choices.
Obstructive events are far more common in adults. Central events often prompt additional testing and may necessitate different therapies, such as addressing underlying heart conditions in addition to using CPAP.
How AHI Is Calculated and Measured
The formula for AHI is straightforward:
AHI = (Apneas + Hypopneas) ÷ Hours of Sleep
But the accuracy of that number depends heavily on how it’s measured.
In-Lab Polysomnography (PSG)
The gold standard for diagnosing sleep apnea is an overnight study in a lab. PSG uses multiple sensors to record:
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Brain waves (EEG) to track sleep stages
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Eye and chin movements for REM and arousals
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Airflow via nasal pressure and thermistor
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Chest and abdominal effort belts
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Oxygen levels (pulse oximetry)
Because it measures actual sleep time and EEG arousals, PSG provides the most accurate AHI. It can also distinguish obstructive from central events (AASM Scoring Manual).
Home Sleep Apnea Testing (HSAT)
Home tests (Type III devices) are more convenient and often used to diagnose moderate to severe OSA. They typically measure airflow, effort, and oxygen, but not brain waves. That means:
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They can’t detect arousal-based hypopneas.
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They often use total recording time instead of actual sleep time.
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As a result, HSAT tends to underestimate AHI, especially in milder cases (Canadian Thoracic Society).
CPAP and APAP Device Reports
Most CPAP/APAP machines report a nightly AHI based on airflow patterns. They detect:
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Apneas (near-absent flow)
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Hypopneas (reduced flow with flattening or snore signals)
However, devices don’t measure EEG arousals and may misclassify central vs. obstructive events. They’re best for tracking trends rather than making a fresh diagnosis. Studies in the Journal of Clinical Sleep Medicine show that device AHI correlates reasonably well with PSG, but discrepancies are common when leaks or complex breathing patterns are present.
What AHI Scores Mean
AHI is reported as “events per hour,” with standard cutoffs for adults:
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Normal: < 5
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Mild OSA: 5–14.9
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Moderate OSA: 15–29.9
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Severe OSA: ≥ 30
In children, even a single event per hour may be abnormal. Pediatric thresholds are stricter:
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Normal: < 1
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Mild: 1–4.9
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Moderate: 5–9.9
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Severe: ≥ 10
Why the difference? Children are more sensitive to sleep disruption and oxygen drops, so doctors intervene earlier (American Academy of Pediatrics).
Related Metrics
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RDI (Respiratory Disturbance Index): Adds respiratory effort–related arousals (RERAs) to apneas and hypopneas. Useful when AHI is low but symptoms persist.
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ODI (Oxygen Desaturation Index): Counts oxygen drops per hour. Often reported in home oximetry or when airflow data is missing. ODI can sometimes better predict cardiovascular risk (CMAJ review).
Why AHI Has Limits
Despite its usefulness, AHI isn’t perfect. It only counts events per hour and ignores:
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Event duration: A 90-second apnea and a 10-second apnea both count as “1 event.”
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Oxygen depth: AHI doesn’t directly measure how low oxygen levels go.
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Sleep fragmentation: Many symptoms come from arousals, not just oxygen drops.
Sources of Variability
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Scoring rules: Different labs may use different desaturation criteria, such as 3% vs. 4%.
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Sleep stage: Events often cluster during REM sleep.
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Body position: Supine (on your back) usually produces a higher AHI (Journal of Clinical Sleep Medicine).
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Night-to-night differences: Studies show AHI can shift enough between nights to change the severity category. Multi-night testing reduces this risk.
Modifiable Factors That Raise AHI
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Weight gain (even 10%) can significantly increase event frequency (Wisconsin Sleep Cohort).
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Alcohol or sedatives before bed relax the airway muscles.
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Nasal congestion, irregular sleep timing, or untreated heart failure can all worsen AHI.
How Treatment Affects AHI
CPAP and APAP
Continuous positive airway pressure (CPAP) is the gold standard treatment for obstructive sleep apnea. When properly fitted and used, it can reduce AHI from severe levels (≥30) down to fewer than five events per hour. A systematic review in JCSM found an average 86% reduction in AHI with positive airway pressure therapy.
Other Therapies
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Oral appliances Are Effective in mild to moderate OSA, although results are more variable.
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Weight loss can significantly improve AHI, although changes may take months.
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Positional therapy is most effective for patients whose events typically occur when lying on their back.
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Surgery or hypoglossal nerve stimulation: Reserved for select cases where CPAP fails.
Device Reports and Residual AHI
If your CPAP machine shows an AHI above five despite good adherence:
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First, check for mask leaks or mouth breathing.
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Next, review whether events are obstructive or central.
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If problems persist, clinicians often retitrate pressure settings or repeat in-lab testing.
The bottom line: device-reported AHI helps monitor therapy trends, but significant clinical decisions still rely on validated testing.
How Clinicians Use AHI in Care
Doctors use AHI and related metrics to:
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Decide therapy intensity (e.g., mild cases may start with conservative measures, severe cases almost always need CPAP).
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Monitor adherence (insurers often require ≥4 hours of use per night on ≥70% of nights for continued CPAP coverage, according to Aetna’s medical policy).
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Guide referrals: Patients with severe desaturation or comorbidities are often referred to cardiology or endocrinology.
Clinical follow-up typically includes:
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An early review (2–6 weeks) to check mask fit and device data.
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A more comprehensive review will be conducted in 3 months.
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Ongoing checks every 6–12 months for stable patients.
FAQs
What’s a normal AHI?
For adults, fewer than five events per hour is considered normal. For children, the cutoff is <1.
Why might home sleep tests underestimate AHI?
They don’t measure brain waves (EEG), so they miss arousal-based hypopneas and often use total recording time instead of actual sleep. This can underestimate event frequency (Canadian Thoracic Society).
Why does my AHI vary from night to night?
Factors like sleep stage, body position, alcohol use, and scoring rules can all change nightly AHI. Studies show variability can be large enough to shift severity categories.
How accurate are CPAP machine AHI reports?
Device-reported AHI correlates with lab studies but isn’t perfect. Leaks, mouth breathing, and central events can skew results. Use them for tracking trends, not for fresh diagnosis.
How much can treatment lower AHI?
CPAP typically lowers AHI by 70–90%. Oral appliances and weight loss also help, but results vary more.
Take the Next Step
Understanding AHI is the first step in managing sleep apnea, but numbers alone don’t tell the whole story. Symptoms, oxygen levels, and comorbidities all play a role.
If you’re starting therapy or want to get more from your current device, our team at YourCPAPShop can help. With over 26 years of sleep-care experience, we specialize in:
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Interpreting CPAP and APAP reports
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Troubleshooting mask leaks and comfort issues
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Guiding next steps, from retitration to follow-up testing
Browse CPAP machines, CPAP masks, and CPAP supplies/accessories, or book an online consultation for personalized guidance.
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