Sleep Apnea can be defined in three levels of severity...
AHI Rating: <5... Normal; 5-15... Mild; 15-30... Moderate; >30... Severe
The apnea-hypopnea index (AHI) is an index of severity that combines apneas and hypopneas. Combining them gives an overall severity of sleep apnea including sleep disruptions and desaturations (a low level of oxygen in the blood). The apnea-hypopnea index, like the apnea index and hypopnea index, is calculated by dividing the number of apneas and hypopneas by the number of hours of sleep.
Another index that is used to measure sleep apnea is the respiratory disturbance index (RDI). The respiratory disturbance index is similar to the apnea-hypopnea index; however, it also includes respiratory events that do not technically meet the definitions of apneas or hypopneas, but do disrupt sleep.
Sleep apnea is formally defined as an apnea-hypopnea index of at least 15 episodes/hour in a patient if they do not have medical problems that are believed to be caused by the sleep apnea. This is the equivalent of approximately one episode of apnea or hypopnea every 4 minutes.