What Is the Oxygen Desaturation Index (ODI) Measured on a Sleep Study?

Measurement Reveals Degree of Oxygen Levels Drop in Sleep Apnea

The oxygen desaturation index (ODI) is a measurement in a sleep study used to identify sleep apnea that causes oxygen levels to drop
The oxygen desaturation index (ODI) is a measurement in a sleep study used to identify sleep apnea that causes oxygen levels to drop. Images Bazaar/Getty Images

If you have had a sleep study to evaluate for obstructive sleep apnea, you no doubt have some questions about some of the terminology used in the report describing the test results. One possible measurement that can be included, called the oxygen desaturation index (ODI), may be particularly perplexing. What is the oxygen desaturation index? Learn how this measure may be helpful to identify more severe sleep apnea that may be associated with oxygen level drops and other long-term health consequences like cardiac disease and dementia.

What Is the Oxygen Desaturation Index (ODI)?

The oxygen desaturation index (ODI) is the number of times per hour of sleep that the blood's oxygen level drop by a certain degree from baseline. The ODI is typically measured as part of standard sleep studies, such as a diagnostic polysomnogram, home sleep apnea testing, or with overnight oximetry.

The degree of change from baseline can be measured in two different ways. The criteria used to determine the index may vary depending on the scoring rules used. According to the 2007 guidelines from the American Academy of Sleep Medicine, any event with a 3 percent drop in blood oxygen levels is counted towards the total. For example, a change from 95 percent to 92 percent would be an event that is counted toward the index's total. However, Medicare and some other insurances still rely on older scoring rules and require a 4 percent change for an event to be counted toward the index.

These drops in oxygen levels are called desaturations. ODI is measured by an oximeter, which is a device typically placed on the fingertip that shines a red light through the skin and can estimate the amount of oxygen in the peripheral blood.

When breathing becomes disrupted during sleep, as may occur in obstructive sleep apnea, the oxygen levels of the blood may repeatedly fall.

These drops are typically associated with collapses of the upper airway, events called either an apnea or hypopnea. (A hypopnea represents a partial collapse of the airway.) Drops occur less frequently in snoring or upper airway resistance syndrome (UARS), two conditions in which breathing is disturbed, yet to a lesser degree.

It is important to understand that the ODI differs from another measurement called the apnea-hypopnea index (AHI). The AHI also includes events that may cause arousals or awakenings from sleep without affecting oxygen levels. The ODI also does not reflect the absolute minimum blood oxygen level measured, which may be called the minimum oxygen saturation or the oxygen nadir of the study. If the oxygen levels are low enough (often less than 88 percent is the threshold) and sustained for more than 5 minutes, hypoxemia may be diagnosed.

What Contributes to a Worsened ODI and What Are the Consequences?

ODI may be worsened in people with underlying lung disease, including chronic obstructive pulmonary disease (COPD), and congestive heart failure. With decreased reserves, the collapse of the upper airway may lead the blood's oxygen levels to more quickly drop. This may also be associated with an increased level of carbon dioxide, such as in obesity hypoventilation syndrome.

It is believed that an elevation in ODI may lead to increased oxidative stress and free radicals in the body that may predispose people to long-term cardiovascular risks, including high blood pressure (hypertension), heart attack, stroke, and memory loss associated with dementia. These consequences are an active area of sleep research.

Fortunately, effective treatment of obstructive sleep apnea with continuous positive airway pressure (CPAP) can normalize breathing and reduce the long-term risks associated with untreated sleep apnea. As part of reviewing your sleep study results, speak with your sleep doctor about the best treatments for your condition.


Kryger, M.H. et al. "Principles and Practice of Sleep Medicine." ExpertConsult, 6th edition, 2017.