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CPAP Clarity

AHI Score Lookup

Tell us where your number came from, a sleep study or your CPAP machine, and see it read against the right reference for each.

Where is this number from?

The same number means different things before treatment and on CPAP.

What This Tool Does

The tool first asks where your Apnea-Hypopnea Index came from, because the same number means different things in two places. From a sleep study, it shows the AASM category the number falls in, how common that range is among untreated adults, and the options clinical guidelines describe at that level. From your CPAP machine, it reads the number as a residual AHI against the under-5 mark the AASM associates with well-controlled therapy, explains how a machine scores events, and lists what to bring to your provider if it is above the mark.

If you have imported your CPAP data into CPAP Clarity, it can read your last night or your 7- or 30-night average for you, the same numbers your dashboard and History show. Everything happens in your browser. Your CPAP data is never sent to any server.

How AHI Is Measured

AHI is measured during a sleep study, either an in-lab polysomnography (PSG) or a home sleep apnea test (HSAT). During polysomnography, technicians monitor brain activity (EEG), airflow, chest and abdominal movement, blood oxygen levels, and other signals throughout the night. A trained sleep technologist then manually scores each respiratory event according to AASM criteria.

Home sleep tests use a simplified sensor set (typically nasal airflow, pulse oximetry, and chest effort) and calculate AHI using recording time rather than actual sleep time, since they cannot measure brain waves. This means HSAT-derived AHI can underestimate severity compared to PSG in some patients, because time spent awake during the recording is included in the denominator.

CPAP machines also report a nightly AHI using their built-in flow sensors. This "residual AHI" reflects how many events your machine detected while you were on therapy. It is useful for tracking treatment effectiveness night to night, but it is calculated using device-specific algorithms, not the same manual scoring criteria used in a sleep lab.

Severity Classification

The four severity levels below are the 1999 AASM Task Force categories (American Academy of Sleep Medicine, Sleep, 1999). That report, like the AASM's 2009 guideline, puts severe at above 30; this tool also places a reading of exactly 30 there.

  • Normal (AHI under 5). Fewer than 5 events per hour. Within the range seen in healthy adults without clinically significant sleep-disordered breathing.
  • Mild (AHI 5 to under 15). 5 to under 15 events per hour. May or may not produce noticeable symptoms. Treatment decisions at this level often depend on the presence of daytime sleepiness, cardiovascular risk factors, or occupational hazards.
  • Moderate (AHI 15 to under 30). 15 to under 30 events per hour. Associated with increased cardiovascular risk and more pronounced daytime symptoms.
  • Severe (AHI 30 or more). 30 or more events per hour. Strongly associated with significant oxygen desaturation, cardiovascular morbidity, and impaired daytime functioning.

These are the categories for a diagnostic sleep study. A diagnosis also weighs symptoms and other conditions, so a category is not a diagnosis on its own; only a clinician can make one.

Reading an AHI From Your CPAP Machine

Once you are on CPAP, the number your machine reports each morning is a residual AHI, the count of events still happening with therapy on. It is read against a different reference. Under 5 is the mark the AASM associates with well-controlled therapy. That mark comes from guidance on titrating pressure in a sleep lab (Kushida et al., 2008), which the site uses as the reference line for a nightly machine reading. A reading under it is not a reason to stop using CPAP.

The severity words used for a sleep study (mild, moderate, severe) describe untreated sleep apnea, so this tool does not apply them to a CPAP number. It tells you how far the reading is from the mark instead, and the trend over several nights says more than any one night. If your residual AHI stays above the mark, that is worth reviewing with your sleep provider.

AHI and Treatment Decisions

AASM guidelines (Patil et al., 2019) recommend CPAP or other positive airway pressure therapy for adults with obstructive sleep apnea and excessive daytime sleepiness, and suggest it when sleep apnea comes with poor sleep-related quality of life or high blood pressure. For mild cases, CPAP is one option, but oral appliances (mandibular advancement devices), positional therapy (avoiding supine sleep), and weight management may also be appropriate depending on individual factors. Consider discussing the full range of options with your sleep provider.

Weight is linked to AHI. In one long-running study, a 10% drop in body weight was associated with a roughly 26% drop in AHI (Peppard et al., 2000, JAMA). Weight loss alone may not be enough at higher levels, and the effect varies from person to person. These options apply to a diagnosis; on CPAP, keep using it as prescribed and talk to your provider before changing anything. Your provider can help you decide which combination of therapies fits your situation.

Frequently Asked Questions

What is AHI?

AHI stands for Apnea-Hypopnea Index. It is the number of apneas (complete airway blockages) and hypopneas (partial airway blockages) you experience per hour of sleep. A sleep study or your CPAP machine calculates this by dividing the total number of respiratory events by total sleep time (or recording time for home studies and CPAP devices). AHI is the primary metric used to diagnose sleep apnea severity and to evaluate whether treatment is effective.

What events are counted in AHI?

AHI counts two types of respiratory events. An apnea is a pause in airflow lasting at least 10 seconds. A hypopnea is a reduction in airflow of at least 30% lasting at least 10 seconds, accompanied by a 3% or greater drop in blood oxygen saturation or an arousal from sleep (per the AASM 2012 recommended scoring criteria). Some labs use an older 4% desaturation rule, which can produce a lower AHI for the same night of sleep. Obstructive, central, and mixed apneas are all included in the total count.

How does AHI change with CPAP treatment?

On CPAP, the AHI your machine reports each night is a residual AHI, the count of events still happening with therapy on. It is read against the under-5 mark the AASM associates with well-controlled therapy, not against the sleep-study categories. A reading under that mark is not a reason to stop or change your CPAP. A residual AHI that stays above the mark is worth reviewing with your sleep provider, who can look at leak, sleep position and your prescription to sort out why.

Why is my CPAP AHI different from my sleep study AHI?

They are different measurements. A sleep lab scores events by hand from brain waves, breathing effort, airflow and oxygen, and the result describes your breathing without treatment. Your CPAP machine counts events from the airflow through the mask using its manufacturer's own detection rules, while therapy is on. So the machine's number is an estimate of what is left on treatment, and it can differ from what a lab would score on the same night.

What is the difference between AHI and RDI?

AHI counts apneas and hypopneas only. RDI (Respiratory Disturbance Index) includes apneas, hypopneas, and respiratory effort-related arousals (RERAs). RERAs are brief arousals caused by increased breathing effort that do not meet the full criteria for an apnea or hypopnea. Because RDI includes more event types, it is always equal to or higher than AHI for the same night. Some insurance companies use RDI for CPAP qualification, which can help patients with upper airway resistance syndrome who have a low AHI but frequent RERAs.

Can AHI change over time without treatment changes?

Yes. Several factors can cause AHI to change over months or years. Weight gain tends to increase AHI because excess tissue around the airway narrows the pharyngeal space. Weight loss can reduce AHI, sometimes substantially. Aging, alcohol consumption, sedative medications, sleeping on your back, nasal congestion, and hormonal changes (such as menopause) can all influence AHI. Periodic re-evaluation with your sleep provider is worthwhile, especially if your symptoms change or you have gained or lost a significant amount of weight.

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