AHI, REI and RDI Explained

AHI is calculated by dividing the total number of apneas and hypopneas recorded during a sleep study by the total measured sleep time, then expressing the result as events per hour.

For example, if a study records 30 apneas and hypopneas over 5 hours of measured sleep, the AHI would be 6 events per hour.

AHI is most commonly reported on in-lab polysomnography, where sleep time is measured using brain activity (EEG). This allows the denominator to be actual sleep time rather than total recording time.

REI is a metric commonly used in home sleep apnea testing. Because most home devices do not measure brain activity to determine actual sleep time, the index is generally calculated using recording or monitoring time rather than measured sleep time.

This means the denominator includes time spent awake in bed, which can make the REI lower than the AHI would be if measured sleep time were used. Alternatively, if a patient sleeps for most of the recording time, the REI and AHI may be similar.

REI is sometimes also called the Respiratory Event Index or referred to by other names depending on the device and reporting standard. The key point is that the denominator differs from AHI, so direct comparison between the two numbers requires caution.

RDI is a broader measure that may include respiratory events beyond apneas and hypopneas — such as respiratory effort-related arousals (RERAs) — depending on how the reporting laboratory defines and calculates it.

Because RDI may include additional events, it can be higher than AHI for the same patient. Not all laboratories report RDI, and those that do may use different definitions, making cross-laboratory comparison complex.

When reviewing a sleep study report, it is important to know which metric is being reported and how it was calculated. This information is typically noted in the report methodology.

The most common reason AHI, REI, and RDI differ is the denominator. AHI uses measured sleep time; REI uses recording or monitoring time; RDI may use either, plus additional events in the numerator. The same patient can have a lower REI than AHI simply because the recording time denominator is larger.

This is not an error — it reflects a difference in methodology. Understanding this helps avoid confusion when comparing results from different types of tests or different laboratories.

Home sleep tests and in-lab polysomnography use different sensors and may detect events differently. A home test may miss some events that an in-lab study would capture, or vice versa. The device manufacturer, scoring software, and laboratory protocols can all affect the final number.

This is one reason why the treating clinician — not the patient — should interpret sleep study results. The clinician understands the limitations of the specific test and can contextualize the numbers.

Sleep apnea severity can vary from night to night due to factors such as sleep position, alcohol consumption, nasal congestion, medication changes, and natural biological variation. A single night’s study is a sample, not a definitive lifelong measurement.

Follow-up studies measure the appliance’s effect at a point in time. If significant changes occur later — weight gain, health changes — reassessment may be needed.

AHI, REI, and RDI are quantitative metrics, but they do not by themselves determine a patient’s symptoms, health risk, or treatment needs. A patient with a moderate AHI may have severe daytime symptoms, while another with the same AHI may feel fine. A patient with a low AHI may still have significant oxygen desaturation.

Clinical decision-making considers the full report — respiratory events, oxygen levels, arousals, sleep stages, position data — alongside the patient’s symptoms, medical history, comorbidities, and treatment goals.

This is why these metrics must be interpreted by a qualified medical provider who can place them in clinical context, rather than being used as standalone indicators for self-diagnosis or treatment decisions.

Clinical guidelines commonly categorize obstructive sleep apnea severity using AHI ranges. These categories are widely used as a starting point for clinical discussion, but they do not independently determine symptoms, risk, treatment choice, or treatment success.

The commonly used categories are: normal (below a certain threshold), mild, moderate, and severe. The exact thresholds may vary slightly by guideline, but the general framework is consistent across clinical practice.

It is important to understand that a severity category is a descriptive label, not a complete clinical assessment. Two patients in the same category may have very different experiences and treatment needs. The interpreting provider considers the category alongside the full clinical picture.

American Academy of Sleep Medicine (AASM) — scoring manual and clinical practice guidelines defining AHI, REI, and RDI calculation methods.

American Academy of Dental Sleep Medicine (AADSM) — clinical guidelines referencing respiratory event indices in oral appliance therapy titration and follow-up.

Peer-reviewed literature on the variability of respiratory event indices across test types, devices, and scoring methodologies.