How does a sleep doctor distinguish between persistent sleepiness even after cpap use and narcolepsy?

MSLT. A multiple sleep latency test could be performed after CPAP use if a patient were still persistently sleepy. This would help to differentiate hypersomnia vs narcolepsy.
Doing further "sleep. If one has a normal sleep study with the use of c-pap, one can do testing during the day to determine how fast the patient falls asleep in a darkened room and whether the subject enters rem sleep quickly. If this occurs, the subject probably has narcolepsy ( assuming the patient has further symptoms of the disorder).
By taking a history. ...And reviewing the results of your sleep test. There are specific findings on a sleep test, and a multiple sleep latency test, than can fairly easily distinguish among the options you refer to.

Related Questions

If you're sleepy all of the time, but your sleep doctor doesn't find evidence of narcolepsy, what other conditions might you suspect?

Depends. 1) depression 2) idiopathic hypersomnolence 3) chronic insufficient sleep 4) medication side effect (you mention you are on 3mg/d of klonopin (clonazepam)... That will certainly cause sleepiness) 5) endocrine (e.g. Thyroid) issue this is by no means an exhaustive list, but it's a start... Talk with your primary doc.... Read more...