Laboratory Workup for Excessive Daytime Sleepiness
Order thyroid stimulating hormone (TSH), complete blood count (CBC), comprehensive metabolic panel (CMP) with liver function tests, and ferritin as your initial laboratory screening for any patient presenting with constant sleepiness. 1
Essential First-Line Laboratory Tests
The American Geriatrics Society provides clear guidance on the core laboratory panel needed to evaluate excessive daytime sleepiness:
Thyroid function (TSH): Hypothyroidism is a treatable metabolic cause that commonly presents with hypersomnia and must be excluded early 1
Complete blood count (CBC): Identifies anemia and other hematologic conditions that contribute to fatigue and sleepiness 1
Comprehensive metabolic panel/serum chemistry: Detects metabolic derangements, renal dysfunction, and electrolyte abnormalities that can manifest as excessive sleepiness 1
Liver function tests: Critical for identifying hepatic encephalopathy, which can present primarily as hypersomnia 1
Ferritin level: Essential when restless legs syndrome is suspected; levels <45-50 ng/mL indicate treatable RLS that disrupts nocturnal sleep and causes daytime sleepiness 1
Clinical Context That Guides Laboratory Selection
Before ordering extensive testing, conduct a thorough medication review, as sedating medications (benzodiazepines, opioids, antihistamines, certain antidepressants) are among the most common and overlooked causes of hypersomnia, particularly in older adults taking multiple medications 2, 1
When to Expand Beyond Basic Labs
Brain MRI: Order when structural causes are suspected, including tumors, multiple sclerosis, strokes, intracranial hemorrhage, or neurodegenerative diseases like Parkinson's disease or Alzheimer's disease 2, 1
Polysomnography (PSG): The gold standard for diagnosing obstructive sleep apnea in patients with snoring, witnessed apneas, or obesity (BMI ≥33 kg/m²) 1
Multiple Sleep Latency Test (MSLT): Required after PSG when central disorders of hypersomnolence (narcolepsy, idiopathic hypersomnia) are suspected after excluding other causes; mean sleep latency ≤8 minutes with ≥2 sleep-onset REM periods confirms narcolepsy 2, 1
CSF hypocretin-1 levels: Can definitively confirm narcolepsy type 1 when levels are ≤110 pg/mL, particularly useful when cataplexy is present 2, 1
Common Pitfalls to Avoid
The most frequent error is failing to recognize medication-induced sedation as the culprit before pursuing expensive sleep studies 2. Sedating medications should be reduced or removed under guidance of both a sleep specialist and the patient's primary care physician 3
Another critical mistake is ordering MSLT without ensuring adequate sleep duration (documented by sleep diary for 1-2 weeks) and overnight PSG immediately before testing, as sleep deprivation can produce false-positive results 2
Patients with BMI ≥40 kg/m² or BMI ≥33 kg/m² plus resistant hypertension or type 2 diabetes warrant comprehensive sleep evaluation for obstructive sleep apnea, which affects an estimated 26-32% of adults and is a particularly significant cause of excessive daytime sleepiness 4