What is the difference between the 3% and 4% hypopnea rules?

Both rules define a hypopnea as a drop in airflow during sleep. The AASM recommended rule requires association with a 3% oxygen desaturation or a cortical arousal.¹ The alternative rule requires association with a 4% desaturation and does not consider arousals.¹ Because the choice can change the AHI considerably, every report should state which rule it used.¹

By Wesper. Published November 2, 2026.

How does each rule define a hypopnea?

Each rule pairs a change in airflow with a second finding before the event counts as a hypopnea. As summarized in the 2017 AASM guideline on diagnostic testing for adult obstructive sleep apnea (OSA), the Scoring Manual's recommended definition requires that changes in flow be associated with a 3% oxygen desaturation or a cortical arousal.¹ Its alternative definition requires association with a 4% oxygen desaturation without consideration of cortical arousals.¹

In the guideline's summary, the two rules differ in what must accompany the change in flow. A 4% desaturation also clears a 3% threshold, so a hypopnea that qualifies under the alternative rule also meets the desaturation criterion of the recommended rule.

Why can the same night produce two different AHIs?

The same night can produce two AHIs because the recommended rule accepts more kinds of events. Any desaturation that meets the alternative rule's 4% criterion also meets the 3% criterion, and the recommended rule adds events with smaller desaturations or with a cortical arousal.¹ The guideline states that, depending on which definition is used, the AHI may be considerably different in a given individual.¹

Illustrative example: suppose a polysomnogram records 7 hours of sleep, with apneas plus hypopneas totaling 63 under the alternative rule and 112 under the recommended rule. The resulting AHIs are 9 and 16 events per hour, which fall in the mild and moderate ranges of the AASM severity measure.² For patients, our guide on how to read sleep study results is a plain-language starting point for those ranges.

Which rule does Medicare use?

Medicare's CPAP coverage policy uses a 4% desaturation threshold in its hypopnea definition. National Coverage Determination 240.4 defines a hypopnea as an abnormal respiratory event lasting at least 10 seconds with at least a 30% reduction in thoracoabdominal movement or airflow as compared to baseline, and with at least a 4% oxygen desaturation.³ It defines an apnea as a cessation of airflow for at least 10 seconds, and neither definition mentions cortical arousals.³

The same policy covers an initial 12-week period of CPAP in adults with OSA when the AHI or RDI is at least 15 events per hour.³ It also covers patients with an AHI or RDI of 5 to 14 events per hour who have documented excessive daytime sleepiness, impaired cognition, mood disorders or insomnia, or documented hypertension, ischemic heart disease or history of stroke.³

Because the policy shares the 4% threshold of the AASM alternative rule, an AHI scored with the recommended rule may not match the policy's definition. For other payers, the published policy is the place to confirm which definition applies.

How are arousals handled in home testing?

Home sleep apnea tests (HSATs) that use conventional sensors are unable to detect hypopneas associated only with cortical arousals, even though those events are included in the recommended AHI scoring rule.¹ The guideline notes that HSAT typically does not monitor EEG, EOG and EMG,¹ and EEG is the signal used to identify a cortical arousal.

On a conventional HSAT, the recommended rule therefore counts hypopneas through its desaturation criterion only. The guideline also names inconsistent definitions of respiratory events, such as hypopneas, between HSAT and polysomnography as one source of discordance between the two tests.¹

When a single HSAT is negative, inconclusive or technically inadequate, the guideline recommends polysomnography for the diagnosis of OSA.¹ Our patient guide on what happens after a negative home sleep test is written for patients facing that question.

Why report both values?

Reporting both values lets one study serve more than one purpose without rescoring. The 3% value follows the AASM Scoring Manual's recommended definition,¹ and the 4% value uses the same desaturation threshold as Medicare's CPAP policy.³ On an HSAT with conventional sensors, the 3% value reflects desaturation-linked hypopneas only, since arousal-only events cannot be detected.¹

Having both values also lets a later reader compare studies on a single definition, which addresses one source of discordance the guideline names, inconsistent hypopnea definitions between tests.¹ Oxygen indices raise the same threshold question, and our explainer on the oxygen desaturation index covers the 3% and 4% versions of ODI.

What does this mean for comparing studies over time?

Comparing studies over time works best when each AHI uses the same hypopnea rule. A change of rule between two studies can shift the AHI considerably on its own,¹ and that shift could be mistaken for a change in the patient's condition.

The index and its denominator matter too. AHI divides by total sleep time and REI divides by monitoring time,⁴ so a follow-up HSAT compared with a baseline polysomnogram can differ in both the hypopnea rule and the time base. Our explainer on the respiratory event index covers the denominator in detail.

The guideline also names night-to-night variability as a source of discordance between tests.¹ Our article on whether one night of home sleep testing is enough looks at that question for providers.

Wesper develops FDA-cleared home sleep testing devices, and multi-night testing comes standard with Wesper studies. The Wesper portal reports AHI at both 3% and 4% across nights, along with ODI and hypoxic burden, and clinicians have full access to the raw data for review. You can read more about home sleep testing for clinics on our professional page.

Frequently asked questions

Which hypopnea rule does the AASM recommend?

The AASM Scoring Manual's recommended definition requires that the change in flow be associated with a 3% oxygen desaturation or a cortical arousal.¹

What is the 4% hypopnea rule?

It is the alternative AASM definition, which requires the change in flow to be associated with a 4% oxygen desaturation and does not consider cortical arousals.¹

Does the 3% rule always give a higher AHI?

On the same recording it gives an equal or higher AHI, because it accepts every desaturation the alternative rule accepts and also counts smaller drops and arousals. The guideline notes that the difference can be considerable in a given individual.¹

Can a home sleep apnea test apply the 3% rule?

It can apply the desaturation part of the rule. HSAT devices with conventional sensors cannot detect hypopneas associated only with cortical arousals.¹

Which hypopnea definition does Medicare use for CPAP coverage?

Medicare's CPAP coverage policy defines a hypopnea with at least a 4% oxygen desaturation.³

To see AHI at 3% and 4% trended across nights, explore home sleep testing for clinics with Wesper.

This article is for general education and is not medical advice. Talk with a licensed medical provider about your symptoms and testing options.

Sources

1. Kapur VK, Auckley DH, Chowdhuri S, Kuhlmann DC, Mehra R, Ramar K, Harrod CG. Clinical practice guideline for diagnostic testing for adult obstructive sleep apnea: an American Academy of Sleep Medicine clinical practice guideline. J Clin Sleep Med. 2017;13(3):479-504. aasm.org
2. American Academy of Sleep Medicine. Quality Measure #277: Sleep Apnea Severity Assessment at Initial Diagnosis. Version 9.0, December 2024. aasm.org
3. Centers for Medicare & Medicaid Services. National Coverage Determination 240.4: CPAP Therapy for Obstructive Sleep Apnea (OSA). cms.gov
4. American Academy of Sleep Medicine. AASM style guide for sleep medicine terminology. Updated November 2015. aasm.org