Undergoing a sleep study, such as a Polysomnography (PSG) or a Home Sleep Apnea Test (HSAT), can be overwhelming when the results arrive filled with medical acronyms and numbers. These diagnostic reports map your sleep health, but their technical language often obscures the simple story they tell. This guide translates the essential metrics on your sleep report, focusing on the core indices that determine a sleep apnea diagnosis and its severity. Understanding these key numbers prepares you to discuss your results confidently with your healthcare provider and take steps toward effective treatment.
Decoding the Primary Diagnostic Metrics
The most revealing numbers quantify the frequency and impact of breathing interruptions during sleep. The Apnea-Hypopnea Index (AHI) is the foundational metric, calculated by dividing the total number of apneas and hypopneas by the total hours of sleep recorded. An apnea is a complete cessation of airflow for at least ten seconds, while a hypopnea is a partial reduction in breathing lasting ten seconds or longer. The resulting AHI score represents the average number of these breathing events experienced every hour of sleep.
A closely related number is the Respiratory Disturbance Index (RDI), which provides a broader view of sleep-disordered breathing. The RDI includes all the events counted in the AHI—apneas and hypopneas—but also incorporates Respiratory Effort-Related Arousals (RERAs). An RERA is a period of increasing breathing effort that causes a brief, subtle awakening in the brain, known as an arousal. Because RDI captures these subtle disturbances, it is always equal to or higher than the AHI and can sometimes indicate a problem even when the AHI is relatively low.
The Oxygen Desaturation Index (ODI) focuses on the physiological consequence of these breathing events. The ODI measures the average number of times per hour that blood oxygen saturation drops significantly, typically by three or four percent, from the baseline level. Since each apnea or hypopnea often triggers this drop, a high ODI is an indicator of the strain placed on the cardiovascular system during sleep. Together, the AHI, RDI, and ODI metrics form the basis for objectively diagnosing the full scope of sleep-disordered breathing.
Translating AHI into Severity Levels
The AHI score is the primary scale used by sleep specialists to standardize the diagnosis and classify the severity of sleep apnea in adults. A score of fewer than five events per hour is generally considered within the normal range for healthy adults. Once the AHI reaches five events per hour, a clinical diagnosis is typically made, and the severity is categorized into one of three levels.
Severity Classifications
A score between five and fewer than fifteen events per hour is classified as mild sleep apnea. Moderate sleep apnea occurs when the AHI falls between fifteen and fewer than thirty events per hour. An AHI of thirty or more events per hour indicates severe sleep apnea, where breathing interruptions happen frequently throughout the night.
Your report will also show the lowest blood oxygen saturation level recorded during the night, often called the nadir. This single number is highly informative because a lower nadir indicates greater physiological stress and risk, even if the AHI is borderline. For instance, a patient with a mild AHI but a nadir oxygen saturation that dips below eighty percent may be considered at higher risk than a patient with a moderate AHI whose oxygen levels remain higher.
Understanding Sleep Architecture and Efficiency Data
Beyond the raw count of breathing events, a comprehensive sleep report details the quality and structure of your sleep, known as sleep architecture. Sleep efficiency is a key measure, calculated as the percentage of time spent actually sleeping compared to the total time spent lying in bed during the study. A normal, efficient night of sleep typically results in a sleep efficiency of eighty-five percent or higher. A lower percentage suggests fragmented or poor-quality sleep.
The report breaks down the time spent in different sleep stages, which fall into two main categories: Non-Rapid Eye Movement (NREM) and Rapid Eye Movement (REM) sleep. NREM sleep includes Stage N3, or deep sleep, which is the most physically restorative stage. REM sleep, where most dreaming occurs, is also psychologically important, generally accounting for twenty to twenty-five percent of total sleep time. Sleep apnea fragments the natural progression through these stages, preventing adequate time in the restorative N3 and REM phases.
Arousal Index
A high Arousal Index is a direct measure of this fragmentation, counting the total number of brief awakenings per hour of sleep, regardless of the cause. These microarousals are the brain’s response to breathing difficulty and can be triggered by apneas or RERAs. A score below twenty to twenty-five per hour is generally viewed as normal.
Limb Movement Index (LMI)
The Limb Movement Index (LMI) quantifies the number of periodic limb movements, such as repetitive leg jerks, that occur per hour of sleep. An elevated LMI may indicate a co-occurring condition like Periodic Limb Movement Disorder (PLMD). This condition also contributes to sleep fragmentation and daytime fatigue.
What Happens After Receiving the Results
Once you understand the metrics in your report, the next step is to schedule a consultation with a sleep specialist. The sleep study report requires professional interpretation that considers your individual symptoms and medical history. The severity classification based on your AHI, RDI, and ODI scores directly influences the recommended treatment pathway.
Mild sleep apnea might be managed initially with lifestyle modifications, such as weight loss or positional therapy. Moderate or severe sleep apnea often necessitates mechanical support like Continuous Positive Airway Pressure (CPAP) therapy. Other options, including oral appliance therapy or surgical interventions, are considered based on the specific data points. Your specialist uses architectural data, like low sleep efficiency or high LMI, to ensure the treatment plan addresses all factors contributing to poor sleep quality.