Your doctor will diagnose narcolepsy based on your signs and symptoms, your medical and family histories, a physical exam, and test results. If your doctor thinks you have narcolepsy, he or she will likely suggest that you see a sleep specialist. That sleep specialist can do the following diagnostic tests to determine if you have narcolepsy.

Polysomnogram. You either stay overnight at a sleep center or you get an ambulant PSG where you can sleep at home. The test records brain activity, eye movements, heart rate, and blood pressure. A PSG can help find out whether you:

  • Fall asleep quickly
  • Go into rapid eye movement (REM) sleep soon after falling asleep
  • Wake up often during the night

Multiple sleep latency test. This daytime sleep study measures how sleepy you are.

It’s often done the day after a PSG. The MSLT is a full-day test that consists of five scheduled naps separated by two-hour breaks. During each nap trial, you will lie quietly in bed and try to go to sleep. Once the lights go off, the test will measure how long it takes for you to fall asleep. You will be awakened after sleeping 15 minutes. If you do not fall asleep within 20 minutes, the nap trial will end.

Each nap will be taken in a dark and quiet sleep environment that is intended for your comfort and to isolate any external factors that may affect your ability to fall asleep. A series of sensors will measure whether you are asleep. The sensors also determine your sleep stage.

Hypocretin test. This test measures the level of hypocretin in the fluid that surrounds your spinal cord. Most people who have narcolepsy have low levels of hypocretin. Hypocretin is a chemical that helps promote wakefulness.

To get a sample of spinal cord fluid, a spinal tap (also called a lumbar puncture) is done. For this procedure, your doctor inserts a needle into your lower back area and then withdraws a sample of your spinal fluid. The procedure is a little similar to an epidural anesthesia.

HLA blood test. The best HLA marker for narcolepsy is HLA-DQB1*0602 and can be found in your blood. Over 90% of patients with narcolepsy-cataplexy carry HLA-DQB1*0602.
Still it’s controversial to be used as diagnostic test since About 20% of the general population carry the exact same HLA subtypes (HLA-DR2, DQB1*0602, etc.). Furthermore, many patients without cataplexy do not have HLA-DQB1*0602. The HLA subtypes are only predisposing factors but are not sufficient by themselves to cause narcolepsy.
(source: Stanford Medicine)