Nocturnal Oximetry, sometimes called a "Level IV" study, is also a study performed outside of the sleep lab at home or on the hospital ward. It monitors blood oxygen levels only. This information is obtained from a probe attached to a finger. It is useful for monitoring baseline oxygen levels in patients suffering from sustained low oxygen levels. Sustained low oxygen levels may be found in patients with underlying lung disease such as Asthma or COPD. It is also seen in patients with hypoventilation secondary to significant obesity or neurological conditions such as Amyotrophic Lateral Sclerosis (ALS) and Muscular Dystrophy (MD).
Nocturnal Oximetry can also be used to diagnose sleep apnea when it is in the moderate to severe range and shows the classic "saw-tooth" fluctuations. However, because it is not a sensitive test, it cannot be relied on to rule out significant sleep, nor can it differentiate between obstructive or central sleep apnea. If sleep apnea is suspected in a symptomatic patient and Nocturnal Oximetry is found to be negative or inconclusive, the patient requires a Level III Study or a Nocturnal Polysomnogram for more accurate testing.